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Keywords:

  • caspase;
  • cell death;
  • cell survival;
  • nerve growth factor;
  • neuroprotection;
  • pituitary adenylate cyclase-activating polypeptide

Abstract

  1. Top of page
  2. Abstract
  3. Materials and methods
  4. Results
  5. Discussion
  6. Acknowledgments and conflict of interest disclosure
  7. References
Thumbnail image of graphical abstract

PC12 cells are used to study the signaling mechanisms underlying the neurotrophic and neuroprotective activities of pituitary adenylate cyclase-activating polypeptide (PACAP) and nerve growth factor (NGF). Previous microarray experiments indicated that serpinb1a was the most induced gene after 6 h of treatment with PACAP or NGF. This study confirmed that serpinb1a is strongly activated by PACAP and NGF in a time-dependent manner with a maximum induction (~ 50-fold over control) observed after 6 h of treatment. Co-incubation with PACAP and NGF resulted in a synergistic up-regulation of serpinb1a expression (200-fold over control), suggesting that PACAP and NGF act through complementary mechanisms. Consistently, PACAP-induced serpinb1a expression was not blocked by TrkA receptor inhibition. Nevertheless, the stimulation of serpinb1a expression by PACAP and NGF was significantly reduced in the presence of extracellular signal-regulated kinase, calcineurin, protein kinase A, p38, and PI3K inhibitors, indicating that the two trophic factors share some common pathways in the regulation of serpinb1a. Finally, functional investigations conducted with siRNA revealed that serpinb1a is not involved in the effects of PACAP and NGF on PC12 cell neuritogenesis, proliferation or body cell volume but mediates their ability to block caspases 3/7 activity and to promote PC12 cell survival.

Pituitary adenylate cyclase-activating polypeptide (PACAP) and nerve growth factor (NGF) induce a strong increase in serpinb1 a expression in PC12 cells. Functional investigations revealed that this increase in serpinb1a does not affect cell proliferation or differentiation but inhibits caspase 3 activity and promotes cell survival.

Abbreviations used
Akt

v-akt murine thymoma viral oncogene homolog

AP-1

activator protein 1

cAMP

cyclic adenosine monophosphate

caspases

cysteine-dependent aspartate-directed proteases

dbcAMP

N6, 2′-O-dibutyryladenosine 3′-5′-cyclic monophosphate

DCC

deleted in colorectal cancer

DISC-1

disrupted-in-schizophrenia 1

DMEM

Dulbecco's modified Eagle's medium

Egr1

early growth response 1

ERK

extracellular signal-regulated kinase

FEZ-1

fasciculation and elongation protein zeta-1

GAPDH

glyceraldehyde-3-phosphate dehydrogenase

JNK

c-Jun NH2-terminal kinase

MAPK

mitogen-activated protein kinase

NF-κB

nuclear factor-kappa B

NFATs

nuclear factor of activated T cells

NGF

nerve growth factor

p75-NTR

p75 neurotrophin receptor

PAC1

specific PACAP receptor

PACAP

pituitary adenylate cyclase-activating polypeptide

PBS

phosphate-buffered saline

PC12

rat adrenal pheochromocytoma cell

PEDF

pigment epithelium-derived factor

PI3K

phosphatidylinositol 3-kinase

PKA

protein kinase A

PKC

protein kinase C

PLC

phospholipase C

PMA

phorbol-12-myristate-13-acetate

PN-1

protease nexin-1

serpin

serine (or cysteine) peptidase inhibitor

siRNA

small interfering RNA

TrkA

tyrosine kinase member A receptor

VIP

vasoactice intestinal polypeptide

VPAC1

VIP and PACAP receptor 1

VPAC2

VIP and PACAP receptor 2

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a 38-amino acid peptide that was first isolated from ovine hypothalamic extracts for its ability to stimulate adenylyl cyclase in rat anterior pituitary cells (Miyata et al. 1989). PACAP belongs to the secretin–glucagon–vasoactive intestinal polypeptide (VIP) superfamily and its sequence has been remarkably well conserved during evolution (Vaudry et al. 2009), suggesting that it must regulate important biological functions. Three PACAP receptors have been cloned: the PACAP-selective receptor PAC1 and the VIP/PACAP mutual receptors VPAC1 and VPAC2 (Harmar et al. 2012). All PACAP receptors belong to the seven-transmembrane domain G-protein-coupled receptor superfamily and activate several signaling pathways including the cAMP/protein kinase A (PKA) (Spengler et al. 1993), phospholipase C/protein kinase C (PKC) (Spengler et al. 1993), mitogen-activated protein kinase cascades (Moroo et al. 1998), and calcium fluxes (Chatterjee et al. 1996). Among its numerous biological actions, PACAP induces neurite outgrowth in cerebellar granule neurons (Gonzalez et al. 1997), inhibits cell proliferation in the developing cerebral cortex (Suh et al. 2001), and reduces apoptosis in chick neuroblasts (Erhardt and Sherwood 2004).

Nerve growth factor (NGF), a member of the neurotrophin family (Levi-Montalcini 1987), binds and activates both the tyrosine kinase member A receptor (TrkA) (Klein et al. 1991) and the p75 neurotrophin receptor (p75-NTR) (Chao 1994). TrkA is a transmembrane protein with an extracellular immunoglobulin G portion for ligand binding and a cytoplasmic tyrosine kinase domain coupled to numerous transduction pathways (Chao 1994; Patapoutian and Reichardt 2001). Binding of NGF to TrkA leads to the formation of a protein complex that activates, for example, phospholipase C, phosphatidylinositol 3-kinase (PI3K), extracellular signal-regulated kinase (ERK), Rap1 or Raf-1 (Kaplan and Stephens 1994; Soderholm et al. 2001; Wu et al. 2001). Among its various biological activities, NGF induces neurite outgrowth in hippocampal neurons from newborn rats (Shao et al. 1993) and rescues sympathetic neurons from programmed cell death (Edwards et al. 1991).

The neurotrophic effects of PACAP and NGF have been intensively investigated by using the well-characterized rat adrenal pheochromocytoma PC12 cell line in which they promote neurite outgrowth (Greene and Tischler 1976; Deutsch and Sun 1992), inhibit cell proliferation (Greene and Tischler 1976; Vaudry et al. 2002b), and reduce apoptosis (Batistatou and Greene 1993; Tanaka et al. 1997). Some of the transduction pathways involved in these effects are now well characterized (Vaudry et al. 2002a). In particular, neurite outgrowth is induced through phosphorylation of the ERK MAP kinase but, while NGF acts through both a Ras- and Rap1-dependent B-Raf activation to stimulate neurite outgrowth (York et al. 1998; Wu et al. 2001), PACAP signaling is independent of Ras (Lazarovici et al. 1998), indicating that the transduction pathways activated upstream of ERK are different. The inhibition of PC12 cells apoptosis by PACAP seems to involve, at least in part, the PKA pathway (Reglodi et al. 2004) while NGF would prevent apoptosis through the PI3K (Shimoke and Chiba 2001; Koh et al. 2003; Salinas et al. 2003) and Akt (Wu and Wong 2005) cascades. Several genes, such as fasciculation and elongation protein zeta-1 (Kuroda et al. 1999), disrupted-in-schizophrenia 1 (Miyoshi et al. 2003), deleted in colorectal cancer (Lawlor and Narayanan 1992) or early growth response 1 (Ravni et al. 2008), have already been shown to be involved in PC12 cells differentiation. But, to get a more comprehensive view of the molecular events occurring after PACAP or NGF treatment, transcriptional investigations have been conducted (Angelastro et al. 2000; Vaudry et al. 2002b; Grumolato et al. 2003a; Ishido and Masuo 2004; Lee et al. 2005; Ravni et al. 2008) and serine (or cysteine) proteinase inhibitor, clade B, member 1a (serpinb1a) was found to be the gene that exhibits the highest level of induction after 6 h of treatment with either PACAP or NGF (Ravni et al. 2008).

So far, more than 500 serpins have been identified in the three major phyla (Bacteria, Archæa and Eukarya) as well as in several eukaryotic viruses (van Gent et al. 2003). Malfunction of serpins results in a number of diseases including emphysema, thrombosis, cirrhosis, dementia, tissue self-destruction, and hypersensitivity of the immune system (Irving et al. 2000; van Gent et al. 2003). Serpins are classified into 16 clades (A-P) based on their phylogenetic relationships (Silverman and Lomas 2004). The ov-serpins or clade B serpins, identified according to their amino acid sequence similarities with the chicken ovalbumin, constitute the largest group of serpins. In contrast to most other serpins that are secreted in the blood circulation to control proteolytic cascades, the ov-serpins lack a classical secretory signal peptide and reside primarily within cells with a cytoplasmic or nucleocytoplasmic distribution (Silverman et al. 2004). The majority of ov-serpins inhibits serine and/or papain-like cysteine proteinases and protects cells from exogenous and endogenous proteinase-mediated injury (Silverman et al. 2004). In mouse, four homologs of human serpinb1 have been identified and named serpinb1a, serpinb1b, serpinb1c, and serpinb1-ps1 (Benarafa et al. 2002). Both homologous human serpinb1 and murine serpinb1a are expressed in a wide range of tissues with the highest levels of expression in bone marrow, brain, spleen, and pancreas (Benarafa et al. 2002). Serpins participate in many biological activities (e.g. phagocytosis, blood coagulation, complement activation, fibrinolysis, programmed cell death, and inflammation) and some of them have been reported to act as neurotrophic factors. For instance, serpinf1, also known as pigment epithelium-derived factor (PEDF), promotes survival and differentiation of retinal photoreceptors (Cayouette et al. 1999; Jablonski et al. 2000), cerebellar granule neurons (Yabe et al. 2005), and spinal motor neurons (Houenou et al. 1999). SerpinI1, or neuroserpin, is involved in neurite outgrowth (Hill et al. 2000; Parmar et al. 2002) and provides neuronal protection in pathologies such as cerebral ischemia (Yepes et al. 2000) or epilepsy (Miranda and Lomas 2006). Serpin2, alternatively named protease nexin-1 (PN-1), plays a role in regulation of neurite outgrowth (Cunningham and Gurwitz 1989). On the basis of these data, we hypothesized that serpinb1a could be implicated in some of the neurotrophic effects induced by PACAP and NGF. This study investigated the signaling pathways involved in PACAP and NGF regulation of serpinb1a expression in PC12 cells and revealed that this protein acts as an anti-apoptotic factor through inhibition of caspases 3/7.

Materials and methods

  1. Top of page
  2. Abstract
  3. Materials and methods
  4. Results
  5. Discussion
  6. Acknowledgments and conflict of interest disclosure
  7. References

Reagents

The 38-amino acid isoform of PACAP and VIP was synthesized by solid phase methodology as described previously (Bourgault et al. 2008). U0126 was purchased from Promega (Charbonnieres, France). Ascomycin, cycloheximide, D600, dibutyryl-cyclic AMP (dbcAMP), forskolin, Gö6983, H7, H89, JNK inhibitor 1, K-252a, LY294002, PD98059, phorbol-12-myristate-13-acetate (PMA), and SB203580 were obtained from Calbiochem (San Diego, CA, USA). Actinomycin D, cyclosporin A, NGF (7S from murine submaxillary gland), and rapamycin were from Sigma (St. Louis, MO, USA).

Cell culture and treatment

PC12-G rat pheochromocytoma cells (Rausch et al. 1988) were grown in Dulbecco's modified Eagle's medium (Invitrogen, Carlsbad, CA, USA) supplemented with 7% horse serum (Cambrex, Emerainville, France), 7% heat-inactivated fetal bovine serum (Sigma), 2.5% HEPES buffer (Invitrogen), 1% glutamine (Invitrogen), 100 units/mL penicillin, and 100 μg/mL streptomycin antibiotic (Invitrogen). Two days before the experiments, PC12 cells were plated at a density of 100 000 cells/mL on polystyrene cell culture 10 cm2 Petri dishes or 12-well plates (Costar, Corning Life Sciences, Acton, MA, USA) pre-coated with poly-l-lysine (Sigma). After transfection, cells were plated at a density of 150 000 cells/mL. Cells were then cultured at 37°C in a 10% CO2 atmosphere. Inhibitors were added 30 min prior treatment with control medium, PACAP (10−7 M) or NGF (100 ng/mL).

Measurement of mRNA expression

Total RNA was extracted by the guanidium thiocyanate–phenol–chloroform method with Tri® Reagent (Sigma) and further purified using the RNeasy® Mini Kit (Qiagen, Courtaboeuf, France). Contaminating genomic DNA was removed by treatment with DNAse I (Qiagen) and cDNA were synthesized from 5 μg of RNA using the ImProm II Reverse Transcription System (Promega). Quantitative PCR was performed on cDNA in the presence of a Mastermix (Applied Biosystems, Courtaboeuf, France) containing dNTPs, MgCl2, and the SYBR® Green reporter dye along with specific primers, using the ABI Prism® 7000 sequence Detection System (Applied Biosystems). The cDNA-generated signals in samples were internally corrected for variations in amounts of input mRNA with the glyceraldehyde-3-phosphate dehydrogenase (GAPDH) cDNA signal. The following primers, designed with the Primer Express® software (Applied Biosystems), were used: GAPDH-forward 5′-CAGCCTCGTCTCATAGACAAGATG-3′, GAPDH-reverse 5′-CAATGTCCACTTTGTCACAAGAGAA-3′ (NM_017008; Rn.54911), serpinb1a-forward 5′-TCACACTCTGAAACTTGCCAACA-3′, serpinb1a-reverse 5′-CCAAGTCAGCACCATACATCTTCT-3′ (NM_001031642; Rn.103402), serpin b5-forward 5′-AACAGCTCCATTAAGGAACTCACA-3′, serpin b5-reverse 5′-GTAGGCAGCATTAACCACAAGGAT-3′ (NM_057108; Rn.25752). GAPDH and serpin b5 primers were used at a concentration of 300 nM while serpinb1a primers were used at a concentration of 500 nM.

Inhibition of serpinb1a protein expression

Transfection of small interfering RNA (siRNA) into PC12 cells was performed with the Amaxa Nucleofector Kit V (Amaxa, Koeln, Germany) according to the instructions of the manufacturer as described previously (Ravni et al. 2008; Dejda et al. 2010). Briefly, 2 × 106 cells were resuspended into 120 μL of Nucleofector solution containing 20 μg of siRNA. Immediately after electroporation in a Nucleofector II apparatus, Dulbecco's modified Eagle's medium was added and cells were cultured at 37°C. Each siRNA was designed to target serpinb1a mRNA sequence (Hp flexible siRNA, Qiagen) and the level of this inhibition was assessed by quantitative PCR. Sequences of the siRNA used in this study were GAG GAG AAA TTC ATG AAA CAA (Control siRNA, which did not affect serpinb1a expression), TGG CTA CAT TTC GGA TCT GAA, and ATC TGT GAA GAT GAT GTA TCA (serpinb1a siRNA and supplemental serpinb1a siRNA, respectively, which both significantly reduced serpinb1a expression by approximately 90%).

Assessment of cell proliferation and cell size

Two days after treatment, PC12 cells were washed with phosphate-buffered saline and detached by incubation with Esgro Complete Accutase® (Chemicon International, Temecula, CA, USA) at 37°C for 15 min. Cell size and number were measured with a Z2 Beckman Coulter counter (Beckman, Miami, FL, USA) with lower and upper limits set to 10 and 17 μm, respectively, as described previously (Ravni et al. 2008).

Assessment of neuronal differentiation

Two days after treatment, images of PC12 cells were randomly acquired on a Leica DM IRB/E inverted microscope (Leica, Heidelberg, Germany) and analyzed with the Metamorph® software, version 6.3 (Molecular Devices, Sunnyvale, CA, USA). Neuronal differentiation was evaluated by counting the percentage of cells bearing neurites, the number of neurites per cell, and total neurite outgrowth for each cell as described previously (Ravni et al. 2008).

Assessment of apoptosis

After two days of culture in the presence of serum, PC12 cells were washed twice and cultured in serum-free medium for 12 h before treatment with PACAP or NGF. Six hours after treatment, cells were washed and resuspended in serum-free medium at 4°C and treated with the fluorometric Apo-ONE® Homogeneous Caspases-3/7 Assay kit (Promega). Fluorescence was then measured over a 6 h period on a microplate reader FlexStation II (Molecular Devices) as reported previously (Dejda et al. 2010).

Assessment of cell survival

After two days of culture in the presence of serum, PC12 cells were washed twice and cultured in serum-free medium for 12 h before treatment with PACAP or NGF. Two days after treatment, cells were washed with phosphate-buffered saline and detached by incubation with Esgro Complete Accutase® (Chemicon International) at 37°C for 15 min. The number of surviving cells was counted with a Z2 Beckman Coulter counter. In control conditions, 37% of cells survived while in the presence of PACAP or NGF between 60% and 70% of the cells survived after 2.5 days in serum-free medium. Qualitative visualization of cell survival 48 h after PACAP or NGF treatment was conducted by using LIVE/DEAD® Viability/Cytotoxicity Kit for mammalian cells (Invitrogen) according to the manufacturer protocol, as reported previously (Vaudry et al. 2002a). Briefly, cells were incubated for 20 min with a solution of calcein (producing green fluorescence in living cells) and ethidium homodimer-1 (producing red fluorescence in dead cells). Images of PC12 cells were randomly acquired on a Leica DM IRB/E inverted microscope (Leica).

Statistical analysis

Data are presented as the mean ± SEM from at least three independent experiments in which each experimental condition was performed in triplicate. Statistical analyses were conducted using a Kruskal–Wallis test, followed by Dunn's post hoc test or by Mann–Whitney test using the PRISM software (GraphPad Software, San Diego, CA, USA).

Results

  1. Top of page
  2. Abstract
  3. Materials and methods
  4. Results
  5. Discussion
  6. Acknowledgments and conflict of interest disclosure
  7. References

Effect of PACAP and NGF on serpinb1a mRNA expression in PC12 cells

Incubation of PC12 cells with 10−7 M PACAP caused a significant increase in serpinb1a mRNA expression after only 1 h of treatment (Fig. 1a). The maximum effect (~40-fold over control) was observed after 6 h of treatment; then serpinb1a mRNA gradually declined, and returned to basal level within 48 h. The time-course effect of NGF on serpinb1a mRNA expression was very similar to that of PACAP (Fig. 1a). Since a 6-h exposure to PACAP or NGF provoked the strongest increase in serpinb1a mRNA expression, gene induction was measured after 6 h of treatment in all subsequent experiments. To look at the specificity of this PACAP-induced transcriptional effect on serpinb1a, we also measured mRNA expression of serpinb5, another member of serpin family also known as maspin. Contrary to its stimulatory effect on serpinb1a expression, PACAP (10−7 M) did not affect the mRNA expression level of serpinb5 (data not shown). Incubation of PC12 cells with graded concentrations of PACAP (from 10−12 to 10−7 M) and NGF (from 10−12 to 10−7 g/mL) for 6 h resulted in a dose-dependent stimulation of serpinb1a gene expression (Fig. 1b). A significant increase in serpinb1a mRNA level was observed in the presence of 10−9 M PACAP and 10−8 g/mL NGF. The strongest increase in serpinb1a expression was obtained with 10−7 M PACAP and 100 ng/mL NGF and these two concentrations were used for subsequent experiments. Treatment of PC12 cells with both PACAP and NGF led to a synergistic increase in serpinb1a expression that exceeded 200-fold over control (Fig. 1c).

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Figure 1. Effects of PACAP and NGF treatment on serpinb1a mRNA expression in PC12 cells. (a) Time-course effect: Cells were cultured in the presence of PACAP (10−7 M) or NGF (100 ng/mL) for durations ranging from 0 to 48 h. (b) Dose effect: Cells were cultured 6 h in the presence of PACAP (10−12 to 10−7 M) or NGF (10−12 to 10−7 g/mL). (c) Combined effect: Cells were cultured 6 h in the presence of PACAP (10−7 M) and/or NGF (100 ng/mL). Quantitative PCR results were corrected using the glyceraldehyde-3-phosphate dehydrogenase (GAPDH) signal as internal control and expressed as the mean fold induction (± SEM) of serpinb1a mRNA level compared to the control. *< 0.05 and **< 0.01 versus control.

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Transduction pathways involved in the regulation of serpinb1a expression by PACAP and NGF

The ability of PACAP and NGF to stimulate serpinb1a transcription was abolished when PC12 cells were pre-treated with the RNA polymerase blocker actinomycin D, but was not modified when they were exposed to the protein synthesis inhibitor cycloheximide (Table 1).

Table 1. Effect of transcription or translation inhibitors on PACAP- and NGF-induced serpinb1a expression in PC12 cells
 PACAPNGF
  1. **< 0.01 versus control; ##< 0.01 versus corresponding treatment without inhibitor; NS, not significantly different from corresponding treatment without inhibitor.

  2. Cells were pre-incubated for 30 min with the RNA polymerase blocker actinomycin D (10 μM) or the protein synthesis inhibitor cycloheximide (0.5 μg/mL) and then cultured 6 h in the presence of PACAP (10−7 M) or NGF (100 ng/mL). Quantitative PCR results were corrected using the GAPDH signal as internal control and expressed as the mean fold induction (± SEM) of serpinb1a mRNA expression compared to the control without any treatment (neither PACAP or NGF nor inhibitors).

No inhibitor39.8 ± 2.3**43.3 ± 2.6**
Actinomycin D1.4 ± 0.1##1.6 ± 0.2##
Cycloheximide38.4 ± 3.4NS37.7 ± 2.5NS

Exposure of PC12 cells to the adenylyl cyclase activator forskolin or to the cell permeant cAMP analog dbcAMP only induced a ~ 3.5-fold increase in serpinb1a mRNA expression, i.e., 10–12 times weaker than PACAP or NGF (Fig. 2). Activation of PKC with PMA did not affect serpinb1a mRNA expression level but concomitant incubation with both forskolin and PMA potentialized the response to forskolin, leading to an increase in serpinb1a gene transcription that was similar to the one observed after PACAP or NGF treatment (Fig. 2). On the other hand, the serpinb1a mRNA expression was not modulated in PC12 cells in response to a treatment with VIP, which is consistent with previous observations showing that PC12 cells express the PACAP-selective receptor PAC1 but not the VIP/PACAP mutual receptors VPAC1 and VPAC2 (Ravni et al. 2006b).

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Figure 2. Effects of cAMP and protein kinase C (PKC) activation as well as VIP treatment on serpinb1a mRNA expression in PC12 cells. Cells were cultured 6 h in the presence of PACAP (10−7 M), the cAMP activator forskolin (25 μM), the cAMP analog dbcAMP (10−3 M), the PKC activator phorbol-12-myristate-13-acetate (PMA) (10−7 M), a combination of PMA plus forskolin, or VIP (10−7 M). Quantitative PCR results were corrected using the glyceraldehyde-3-phosphate dehydrogenase (GAPDH) signal as internal control and expressed as the mean fold induction (± SEM) of serpinb1a mRNA level compared to the control without activator. *< 0.05 and **< 0.01 versus control.

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Considering the various transduction pathways activated by PACAP and NGF, a set of inhibitors were used to decipher the signaling mechanisms involved in serpinb1a regulation. Pre-incubation of PC12 cells with the selective PKA inhibitor H89 significantly reduced the effect of both PACAP and NGF on serpinb1a gene expression while the PKC inhibitor Gö6983 only attenuated the response to PACAP (Fig. 3a). In the presence of the PKA/PKC inhibitor H7, the effect of PACAP and NGF on serpinb1a gene expression was reduced by 6- and 2-fold, respectively (Fig. 3a). The reduction of the effect of NGF on serpinb1a expression by H7 was similar to that observed with H89, whereas H7 almost completely blocked the ability of PACAP to stimulate serpinb1a expression. The trkA inhibitor K252a increased by 1.7-fold the PACAP-evoked stimulation of serpinb1a mRNA level but diminished by more than six times the ability of NGF to increase serpinb1a expression (Fig. 3b). The PI3K inhibitor LY294003 induced a reduction of the effect of both PACAP (2.9-fold) and NGF (2.2-fold) on serpinb1a mRNA expression (Fig. 3b). Inhibition of ERK1/2 with PD98059 or U0126 and blockage of the p38 mitogen-activated protein kinase with SB203580 provoked a 2- to 15-fold decrease in the induction of serpinb1a mRNA expression by PACAP or NGF (Fig. 3c). In contrast, the JNK inhibitor had no effect on PACAP- or NGF-induced serpinb1a gene transcription (Fig. 3c). The calcineurin inhibitors ascomycin and cyclosporin A induced a 7- and 20-fold reduction, respectively, of the effects of PACAP and NGF on serpinb1a mRNA expression (Fig. 3d), whereas specific FKBP12 inhibitor rapamycin had no effect on PACAP-induced serpinb1a expression but significantly increased NGF induction by 1.5-fold (Fig. 3d). However, blockade of voltage-sensitive calcium channels with D600 reduced by 3.5- and 8-fold the effects of PACAP and NGF, respectively, on serpinb1a mRNA levels (Fig. 3d). Based on these results, it is possible to propose a schematic representation of the signaling pathways activated by PACAP and NGF that lead to the increase in serpinb1a expression (Fig. 3e).

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Figure 3. Investigation of the transduction pathways involved in the regulation of serpinb1a expression by PACAP and NGF in PC12 cells. Cells were pre-incubated for 30 min with inhibitors and then cultured for 6 h in the presence of PACAP (10−7 M) or NGF (100 ng/mL). (a) Effect of protein kinase inhibitors on PACAP- and NGF-induced serpinb1a expression. Cells were pre-incubated with the protein kinase A (PKA) inhibitor H89 (10 μM), the protein kinase C (PKC) inhibitor Gö6983 (1 mM), or the PKA/PKC inhibitor H7 (50 μM). (b) Effect of TrkA and phosphatidylinositol 3-kinase (PI3K) inhibitors on PACAP- and NGF-induced serpinb1a expression. Cells were pre-incubated with the TrkA inhibitor K252a (1 μM) or the PI3K inhibitor LY294002 (20 μM). (c) Effect of MAP kinase inhibitors on PACAP- and NGF-induced serpinb1a expression. Cells were pre-incubated with the MEK inhibitors PD98059 (50 μM) or U0126 (25 μM), the p38 inhibitor SB203580 (10 μM), or the JNK inhibitor I (2 μM). (d) Effect of calcineurin inhibitors and calcium channel blockers on PACAP- and NGF-induced serpinb1a expression. Cells were pre-incubated with the calcineurin inhibitors ascomycin (100 nM) or cyclosporin A (100 nM), the FKBP12 inhibitor rapamycin (100 nM), or the calcium channel blocker D600 (30 μM). (e) Schematic representation of the signaling pathways activated by PACAP and NGF that lead to the increase in serpinb1a expression. Quantitative PCR results were corrected using the glyceraldehyde-3-phosphate dehydrogenase (GAPDH) signal as internal control and expressed as the mean fold induction (± SEM) of serpinb1a mRNA level compared to the control without inhibitor. **< 0.01 versus control; #< 0.05 and ##p < 0.01 versus PACAP without inhibitor; §< 0.05 and §§p < 0.01 versus NGF without inhibitor; NS, not significantly different from corresponding treatment without inhibitor.

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Involvement of serpinb1a in the neurotrophic effects of PACAP and NGF

Transfection of PC12 cells with siRNA showed that control siRNA did not alter serpinb1a mRNA increase by PACAP or NGF, whereas specific-serpinb1a siRNA reduced by approximately 90% PACAP- and NGF-induced serpinb1a mRNA expression (Fig. 4a). Blockage of serpinb1a mRNA induction by specific siRNA did not modify the effect of PACAP or NGF on neurite outgrowth (Fig. 4b), growth arrest or cell size (Fig. 4c). In serum-free medium, PACAP and NGF were still able to increase serpinb1a expression but the amplitude of the response was reduced with a 15.9- and 3.8-fold increase, respectively (Fig. 5a). In those conditions, serpinb1a siRNA blocked the inhibitory effect of PACAP and NGF on caspases 3/7 activity (Fig. 5b) and totally abolished the anti-apoptotic effects of PACAP and NGF on PC12 cells cultured in serum-free conditions (Fig. 5c and d). These effects of serpinb1a blockade on caspases 3/7 activity and cell survival were confirmed by transfecting a distinct inhibitory serpinb1a siRNA, the sequence of which is specified in the materials and methods section (data not shown). The fact that serpinb1a induction was reduced in the absence of serum may have contributed to determine its implication in the anti-apoptotic effect of PACAP and NGF and we should not exclude that serpinb1a could be involved in other cellular functions. It was also noted that serpinb1a siRNA had no impact by itself on cell survival in control conditions (Fig. 5c).

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Figure 4. Involvement of serpinb1a in the neurotrophic effects of PACAP and NGF on PC12 cells. Cells were transfected with control siRNA or siRNA directed against serpinb1a mRNA, and were treated with PACAP (10−7 M) or NGF (100 ng/mL) two days later. (a) Effect of anti-serpinb1a siRNA on cell differentiation (photomicrographs) and serpinb1a mRNA expression after treatment with PACAP or NGF for 24 h and 6 h, respectively. Quantitative PCR results were corrected using the glyceraldehyde-3-phosphate dehydrogenase (GAPDH) signal as internal control and expressed as the mean fold induction (± SEM) of serpinb1a mRNA level compared to untreated cells transfected with control siRNA. (b) Neuritogenesis assessment by measuring the percentage of cells with neurites, the number of neurites per cell, and the overall neurite outgrowth after 48 h of treatment with PACAP or NGF in the presence of control siRNA or anti-serpinb1a siRNA. (c) Quantification of cell proliferation and cell size by counting the number of cells and the percentage of cells with a diameter greater than 17 μm after 48 h of treatment with PACAP or NGF in the presence of control siRNA or anti-serpinb1a siRNA. *< 0.05 and **< 0.01 versus control in the presence of control siRNA; ##p < 0.01 versus PACAP in the presence of control siRNA; §§p < 0.01 versus NGF in the presence of control siRNA. Scale bar: 15 μm.

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Figure 5. Involvement of serpinb1a in PACAP- and nerve growth factor (NGF)-induced survival of PC12 cells. Cells were transfected with control siRNA or siRNA directed against serpinb1a mRNA and were serum-deprived two days later. After 12 h in serum-free conditions, cells were treated with PACAP (10−7 M) or NGF (100 ng/mL). (a) Effects of a 6 h PACAP and NGF treatment on serpinb1a mRNA expression in serum-deprived PC12 cells. (b) Quantification of caspases 3/7 activity after 6 h of treatment with PACAP or NGF. (c) Typical microphotographs illustrating the effect of anti-serpinb1a siRNA on cell survival. Two days after PACAP or NGF treatment, living cells were labeled with calcein (green fluorescence) and dead cells were labeled with ethidium homodimer-1 (red fluorescence). (d) Quantification of cell survival by counting the number of cells after 48 h of treatment with PACAP or NGF in serum-deprived conditions. *< 0.05 versus control in the presence of control siRNA; #p < 0.05 versus PACAP in the presence of control siRNA; §p < 0.05 versus NGF in the presence of control siRNA. Scale bar: 100 μm.

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Discussion

  1. Top of page
  2. Abstract
  3. Materials and methods
  4. Results
  5. Discussion
  6. Acknowledgments and conflict of interest disclosure
  7. References

In previous studies, the PC12 cell line has been used to investigate the various effects of PACAP and NGF on neuronal differentiation including neurite outgrowth, inhibition of cell division, and neuroprotection (Vaudry et al. 2002a; Grumolato et al. 2003b; Sakai et al. 2004; Ravni et al. 2008). Like NGF, which is the classical inducer of neuronal differentiation (Greene and Tischler 1976), PACAP promotes growth arrest and causes neuritogenesis in PC12 cells (Deutsch and Sun 1992). To identify genes involved in PC12 cell differentiation, we previously investigated the transcriptome of PC12 cells exposed to PACAP or NGF for 6 h (Ravni et al. 2008). Among the 99 transcripts induced by PACAP and the 114 induced by NGF, 19 were shared by the two neurotrophic factors and serpinb1a represented one of the most induced genes by both PACAP and NGF. The aim of this study was thus to elucidate the signal transduction pathway(s) leading to PACAP- and NGF-evoked serpinb1a transcription and to determine the functional role of this gene.

The obtained results confirmed that both PACAP and NGF markedly increase serpinb1a expression. In PC12 cells, PACAP acts through the seven-transmembrane G-protein-coupled receptor PAC1 (Ravni et al. 2006a), whereas NGF activates the TrkA receptor tyrosine kinase and p75-NTR (Berg et al. 1991; Cordon-Cardo et al. 1991). The fact that the two trophic factors exerted similar effects on serpinb1a kinetics and on some signaling pathway components suggested an indirect regulation by PACAP through activation of the neurotrophin receptor, as reported previously (Lazarovici and Fink 1999; Rajagopal et al. 2004). However, this hypothesis was invalidated by the fact that PACAP-induced serpinb1a mRNA expression was not blocked by K252a, a TrkA tyrosine kinase inhibitor (Lee et al. 2002). On the other hand, K252a actually blocked the NGF-induced increase in serpinb1a expression, confirming the involvement of TrkA in the anti-apoptotic effects of NGF. Thus, although some regulatory elements are different, PACAP and NGF transduction pathways proceed through several common actors including ERK and PKA. Accordingly, several studies have already pointed out the ability of PACAP and NGF to activate transduction pathways that converge to activate the same target genes (Hashimoto et al. 2000; Vaudry et al. 2002c). Nevertheless, the synergistic effect of PACAP and NGF on PACAP (Hashimoto et al. 2000) and serpinb1a gene expression suggests that besides the common pathways activated by both trophic factors, complementary mechanisms are also presumably induced. In fact, the synergistic stimulation of serpinb1a cotreatment by PACAP and NGF illustrates the possible cooperation between a G-protein-coupled receptor and a tyrosine kinase receptor as already reported for cell differentiation (Sakai et al. 2004). As previously shown by microarray, PACAP and NGF activate common and distinct mechanisms (Ravni et al. 2008), suggesting that they both exert neurotrophic activities even if their functions are not exactly the same. In fact, PACAP is acting rapidly on the cells but its action may be transient while NGF initiates slower response but induces a greater final effect. In the presence of both factors, the cell generates faster response (Sakai et al. 2004) which could contribute to an adaptation of the cellular response to some physiological situations.

PACAP simultaneously activates several signaling pathways that mediate its various effects (Vaudry et al. 2000b; Ravni et al. 2008). For instance, neuritogenesis is activated through a cAMP- and ERK-dependent, PKA/PKC-independent pathway, while cell size increase involves ERK and PKA, but not PKC (Ravni et al. 2008; Emery and Eiden 2012). The fact that multiple transduction pathways promote serpinb1a expression suggests that this gene could be involved in several cellular functions. Nevertheless, investigations achieved by using siRNA revealed that in contrast to what has recently been reported by Watanabe and collaborators (Watanabe et al. 2012), in our hands serpinb1a inhibition with two different siRNA does not affect the ability of PACAP or NGF to promote neuritogenesis, induce growth arrest, or increase cell size. In contrast, it appeared that blockade of serpinb1a expression suppressed the ability of PACAP and NGF to inhibit caspases 3/7 activity. Moreover, siRNA targeting serpinb1a abrogated the protective effect of PACAP and NGF on cell death induced by serum deprivation. Serpinb1a might exert its antiapoptotic effect by directly inhibiting the activity of the cysteine protease caspases 3/7, as already reported for other serpins (Antoku et al. 1997). For instance, the viral CrmA serpin, a caspase inhibitor whose physiological target is likely to be the apical caspase in the cascade (Zhou et al. 1997), uses such mechanisms to block programmed cell death induced by Fas (Tewari and Dixit 1995). Serpin1 of Arabidopsis thaliana was also reported to act as a suicide inhibitor for metacaspase 9 (Vercammen et al. 2006). Nevertheless, serpinb1a might also exert its anti-apoptotic effect indirectly, by changing the stability of bcl2 and Bax proteins as previously reported for serpinb5 in tumor cells (Zhang et al. 2005). Interestingly among the different pathways involved in the regulation of serpinb1a, the cAMP/PKA has been reported in several cellular models to be involved in neuroprotection (Vaudry et al. 2003; Racz et al. 2007; Baxter et al. 2011). Furthermore, inhibition of both PKA and PKC pathways, which induce a very strong inhibition of serpinb1a have also been previously shown to be required for the inhibition of caspases 3/7 by PACAP (Vaudry et al. 2000a). The PKA and PKC pathways seem to play a key role in the regulation of serpinb1a by PACAP if we consider that incubation with the PKA stimulator forskolin, together with the PKA stimulator PMA, mimics the effect of PACAP on serpinb1a expression. But, as shown with the Gö6983 compound, the PKC pathway does not seem to mediate the activation by NGF, which is consistent with the idea that PKC is not involved in the effects of NGF (Sigmund et al. 1990). This observation suggests that besides PKA, NGF must activate another pathway that remains to be identified and which would explain the synergistic action of both PACAP and NGF.

This study established that the anti-apoptotic effect of PACAP and NGF in PC12 cells involves serpinb1a expression. Thus, serpinb1a should be considered as another serpin family member acting as a neurotrophic factor. Taking into account that PACAP and NGF block programmed cell death in many neuronal cell types (Seaborn et al. 2011), it will now be of great interest to investigate the possible anti-apoptotic action of serpinb1a over-expression in some neurodegenerative pathologies such as Alzheimer's disease.

Acknowledgments and conflict of interest disclosure

  1. Top of page
  2. Abstract
  3. Materials and methods
  4. Results
  5. Discussion
  6. Acknowledgments and conflict of interest disclosure
  7. References

The authors wish to thank Mr Colas Calbrix and Mrs Milène Tetsi for skillful technical assistance. A. R. was the recipient of a doctoral fellowship from the Ministry of Education. T. S. was the recipient of an INSERM postdoctoral fellowship. D. V. and H. V. are Affiliated Professors at the INRS − Institut Armand-Frappier. This study was supported by INSERM (U982), the National Institute of Mental Health (NIMH) Intramural Research Program, an INSERM-FRSQ program (to A. F. and D. V.), the Interreg TC2N project, and the Région Haute-Normandie. Authors declare that they have no conflict of interest. Part of this work was presented on a poster during the 11th International Symposium on VIP, PACAP and related Peptides that took place in Pécs, Hungary from August 23 to 31, 2013.

All experiments were conducted in compliance with the ARRIVE guidelines.

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  6. Acknowledgments and conflict of interest disclosure
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