Potential conflict of interest: Nothing to report.
Supported by MSTP grant (to Z.G.N.), DK56621, DK47340, and P20AA017067 (to S.L.F.), Summer Student support from MSSM.
Kruppel-like factor 6 (KLF6), a zinc finger transcription factor and tumor suppressor, is induced as an immediate-early gene during hepatic stellate cell (HSC) activation. The paradoxical induction of a tumor suppressor in HSCs during proliferation led us to explore the biology of wildtype KLF6 (KLF6WT) and its antagonistic, alternatively spliced isoform KLF6SV1 in cultured HSCs and animal models. The animal models generated include a global heterozygous KLF6 mouse (Klf6+/−), and transgenic mice expressing either hKLF6WT or hKLF6SV1 under the control of the Collagen α2 (I) promoter to drive HSC-specific gene expression following injury. The rat Klf6 transcript has multiple splice forms that are homologous to those of the human KLF6 gene. Following a transient increase, all rat Klf6 isoforms decreased in response to acute carbon tetrachloride (CCl4) liver injury and culture-induced activation. After acute CCl4, Klf6+/− mice developed significantly increased fibrosis and enhanced fibrogenic messenger RNA (mRNA) and protein expression. In contrast, HSC-specific transgenic mice overexpressing KLF6WT or KLF6SV1 developed significantly diminished fibrosis with reduced expression of fibrogenic genes. Chromatin IP and quantitative reverse-transcription polymerase chain reaction in mouse HSCs overexpressing KLF6WT demonstrated KLF6WT binding to GC boxes in promoters of Colα1 (I), Colα2 (I), and beta-platelet-derived growth factor receptor (β-Pdgfr) with reduced gene expression, consistent with transcriptional repression by KLF6. Stellate cells overexpressing either KLF6WT or KLF6SV1 were more susceptible to apoptotic stress based on poly (ADP-ribose) polymerase (PARP) cleavage. Conclusion: KLF6 reduces fibrogenic activity of HSCs by way of two distinct mechanisms, direct transcriptional repression of target fibrogenic genes and increased apoptosis of activated HSCs. These results suggest that following its initial induction, sustained down-regulation of KLF6 in liver injury may allow de-repression of fibrogenic genes and decreased stellate cell clearance by inhibiting apoptosis. (HEPATOLOGY 2013)
Our previous efforts to understand the molecular basis of stellate cell activation used subtraction hybridization to clone a novel zinc finger transcription factor, Klf6 (previously called “Zf9”), which is induced as an immediate-early gene in hepatic stellate cells (HSCs) during liver injury in vivo.1 Subsequent studies have broadened KLF6′s roles in injury to include growth responses of vascular endothelial cells and hepatocytes, among others.2, 3 KLF6 is a member of a large family of zinc finger transcription factors that have a conserved C-terminal C2H2 DNA binding domain recognizing “GC box” motifs in responsive promoters. Studies originally performed in liver ultimately led to the identification of KLF6 as a growth-inhibitory tumor suppressor gene that is inactivated in several human cancers.4, 5
The discovery that Klf6, a growth suppressive gene, is rapidly induced when stellate cells undergo a proliferative burst presented a paradox that the current study sought to reconcile. In addition to full-length KLF6, human KLF6 has several shorter antagonistic splice isoforms, KLF6SV1-3; among these, KLF6SV1 is the best studied.6, 7 Based on this information we hypothesized that the cloning of Klf6 in rat stellate cells (1) actually represented a growth-promoting splice isoform that contributes to stellate cell activation, yet rodent splice isoforms were not described.
In our studies exploring the role of KLF6 during stellate cell activation we employed Klf6+/− mice, lacking one allele of Klf6 in all cells; these mice are viable, fertile, and phenotypically normal, whereas Klf6 null mice are embryonic lethal.8 We used these animals, in addition to novel stellate cell-specific transgenic mice, to explore the role of KLF6 in HSC activation and fibrogenic gene induction.
Semiquantitative Polymerase Chain Reaction (PCR) and Cloning of rat Klf6 Splice Variants.
PCR was performed using primers to rat Klf6 messenger RNA (mRNA) untranslated regions (UTRs): 5′UTR: AACTTTCACCTGCGCTCCCG, 3′UTR: TGGCTG GTACAGGTATCCCTC with rat primary stellate cell complementary DNA (cDNA) as a template. The products of the reaction were separated using gel electrophoresis. Bands were excised and extracted using the QIAquick Gel Extraction Kit (Qiagen, Chatsworth, CA) according to the manufacturer's protocol. The DNA fragments were then cloned into the pCR8/GW/TOPO TA vector (Invitrogen, Carlsbad, CA) for sequencing and subsequently into the pcDNA3.1/V5-His TOPO TA Expression Kit (Invitrogen) according to the manufacturer's protocol.
All animals received humane care according to the criteria outlined in the “Guide for the Care and Use of Laboratory Animals” prepared by the National Academy of Sciences and published by the National Institutes of Health (NIH publication 86-23 revised 1985).
Murine and Rat Stellate Cell Isolation.
Mice were injected intraperitoneally with carbon tetrachloride (CCl4) (5 μL/g mouse of 10% CCl4 in corn oil) (Sigma) three times (alternating days), and mouse stellate cells were isolated 2 days after the last dose of CCl4. mHSCs were isolated from C57/Bl6 wildtype, Klf6F/F, Cola2(I)-KLF6WT, and Cola2(I)-KLF6SV1 mice by enzymatic digestion and Percoll density gradient centrifugation,9 with modifications. For the transgenic mouse HSC studies, stellate cells were isolated and mRNA was harvested without culturing the cells. For the in vitro KLF6 deletion studies, Klf6F/F HSCs were cultured for 10 days and then infected with either Ad-lacZ or Ad-Cre at a multiplicity of infection (MOI) of 10. Viral transduction efficiency was 85% as assessed using Ad-EGFP and fluorescence microscopy. Protein lysates were collected for western blot.
Rat Klf6 Isoform Quantitation.
For liver injury experiments, Sprague-Dawley rats were injected with CCl4 (2 μL/g of a 50% CCl4 solution in corn oil) or corn oil, followed by isolation of HSCs (see below) at 24 hours or 48 hours after injection (n = 3 for each condition). Messenger RNA was isolated and quantitative RT-PCR (see next section) was performed using isoform-specific primers to identify any rat Klf6 isoforms (see Table 1 for list of primers). For culture activation experiments, HSCs were isolated from untreated rats and activated by growth in primary culture on noncoated plastic dishes for up to 1 week. Messenger RNA was harvested from the cells at days 0, 2, 4, and 7, in three separate cell isolates.
Table 1. List of Primers
rKlf6 Bot F
rKlf6 Bot R
rKlf6 Mid F
rKlf6 Mid R
rKlf6 Top F
rKlf6 Top R
rCOLa2 (I) F
rCOLa2 (I) R
rCola1 (I) F
rCola1 (I) R
mCOLa1 (I) F
mCOLa1 (I) R
mCOLa2 (I) F
mCOLa2 (I) F
Reverse-Transcription and Real-Time Quantitative PCR.
RNA was extracted from cells/liver tissue and reverse-transcribed into cDNA using an RNeasy kit (Qiagen, Valencia, CA) and using Sprint RT Complete-Double PrePrimed tubes (ClonTech, Mountain View, CA), respectively, and analyzed by quantitative PCR using SYBR green qPCR Master Mix (Roche) on the lightCycler480 System (Roche). Data are represented as the relative expression of fibrogenic genes after normalizing to glyceraldehyde 3-phosphate dehydrogenase; (GAPDH). (Please refer to Table 1 for a complete list of primers.)
Western blots of cell extracts were prepared by centrifuging the cells with lysis buffer complemented with protease inhibitor (Complete Lysis-M kit, Roche Diagnostics, Indianapolis, IN) Protein concentration was determined with a Bio-Rad DC kit. Antibodies used were as follows: rabbit anti-Collagen I (1:1,000) (Rockland), rabbit anti-β-PDGFR (beta-platelet-derived growth factor receptor) (1:500) (Santa Cruz), mouse anti-alpha-smooth muscle actin (ASMA) (1:500) (Millipore).
Rat KLF6 Degradation Analysis.
293FT cells were plated in six-well plates and serum-starved for 4 hours prior to transfection. Cells were transiently transfected using the Lipofectamine 2000 Reagent (Invitrogen) according to the manufacturer's protocol with pcDNA3.1/rKlf6top-V5, pcDNA3.1/rKlf6mid-V5, pcDN A3.1/rKlf6bot-V5, or pcDNA3.1/V5. Twenty-four hours later cells were either treated with proteasomal inhibitor MG132 (Sigma) at a final concentration of 10 μM or an equal volume of dimethyl sulfoxide (DMSO). Protein was harvested 5 hours after addition of MG132.
Rat KLF6 Cellular Localization.
293FT cells were plated in six-well plates and serum-starved for 4 hours prior to transfection. Cells were transiently transfected using the Lipofectamine 2000 Reagent (Invitrogen) according to the manufacturer's protocol with pcDN A3.1/rKlf6top-V5, pcDNA3.1/rKlf6mid-V5, pcDNA3. 1/rKlf6bot-V5, or pcDNA3.1/V5. Cells were fixed with ice-cold methanol and stained first with monoclonal mouse anti-V5 antibody (1:50) (Abcam), followed by FITC-conjugated goat antimouse antibody (1:250) (Invitrogen). Cells were mounted using Vectashield containing DAPI (Invitrogen).
Generation of HSC-Specific KLF6 Transgenic Mice.
Colα2(I)-KLF6WT and Colα2(I)-KLF6SV1 constructs were generated by cloning KLF6WT and KLF6SV1 downstream of the Colα2(I) enhancer promoter using a promoter provided by Dr. Yutaka Inagaki.10 These constructs were then used to generate transgenic mice on a C57/Bl6 background. (Mouse Genetics Shared Resource Facility at Mt. Sinai) The same transgene-specific primers were used to identify both KLF6WT and KLF6SV1 transgene positive founder mice. Geno forward: GGAACGGTCCACGATTG CCAAGTCT, Geno reverse: TAACGTTCCAGCT CTAGGCAGGTCTG. Transgenic lines were selected based on good transgene induction following a single dose of CCl4.
Chronic CCl4 Injury.
For chronic CCl4 experiments, mice were injected intraperitoneally twice per week for 8 weeks with a dose of 5 μL/g mouse of 10% CCl4 in corn oil. Mice were sacrificed 48 hours after the last dose of CCl4 and liver tissue was collected for mRNA/protein analysis, hematoxylin and eosin (H&E) staining, Sirius Red staining, and ASMA immunohistochemistry for the Klf6+/− mice (n = 7 per group) and for the HSC-specific transgenic animals (n = 6 per group).
Quantitative Fibrosis Morphometry in Liver Sections.
Formalin-fixed, paraffin-embedded liver tissue was stained with Sirius Red. For each animal 40 images were taken at low magnification (10× objective lens) and subsequently used for quantifying percentage of fibrotic tissue using BioQuant Software.
Chromatin Immunoprecipitation Assays (ChIP).
Chromatin immunoprecipitation assays (ChIP) were performed using a commercial kit according to the manufacturer's instructions (Active Motif-ChIP-IT Express). Proteins crosslinked to DNA were immunoprecipitated with 10 μg of anti-KLF6 antibody (0.2 μg/μL) (R-173) (Santa Cruz Biotechnology), 10 μg anti-histone H3 rabbit antiserum (0.2 μg/μL) (07-690) (Upstate Biotechnology), 10 μg anti-KLF6SV1 antibody (0.5 μg/μL) (Invitrogen) or 10 μg control IgG (0.4 μg/μL) (sc-2027) (Santa Cruz Biotechnology) and protein-G-magnetic beads. Genomic sequence primers encompassing “GC-boxes” in promoter regions (within 2 kbp) upstream of transcriptional start site were used to amplify immunoprecipitated DNA. Colα1(I) Fwd: 5′-C CCTTCCTTTCCCTCCTCC-3′, Colα1(I) Rev: 5′-TG GCCTGGGCCCCTTTTAT-3′; Colα2(I) Fwd: 5′-CCA GGTCCCTCAACACCTG-3′, Colα2(I) Rev: 5′-AGTG CTGGGAGGAGACTGC-3′; β-PDGFR Fwd: 5′-GC GAAAGTGGAAGAGAACCAG-3′, β-PDGFR Rev: 5′-GGACTAAGTTGTCTGGAACCAC-3′.
We employed a human HSC line (LX2) stably transfected with adenovirus containing pBabe, pBabe-KLF6WT, or pBabe-KLF6SV1. Briefly, LX2 cells were infected with adenovirus containing the above sequences and cell selection was performed using puromycin at a concentration of 0.45 μg/mL. Stable colonies were selected at 1 week, amplified, and used in further experiments.
Rat Klf6 Is Alternatively Spliced, with All Splice Isoforms Decreased During Stellate Cell Activation.
Klf6 was first cloned from primary rat stellate cells1 using subtractive hybridization, which yielded a cDNA fragment that was used to screen a full-length, sized cDNA library from stellate cells; this approach led to the isolation of the Klf6 full-length rat cDNA. Using this cloning strategy, we were not able to recognize that the rat KLF6 transcript might be alternatively spliced. Subsequently, three alternative splice isoforms of human KLF6 (hKLF6) were discovered (KLF6SV1-3) in the context of carcinogenesis.11 Among these, KLF6SV1, is the best studied and has been further identified as a growth-promoting isoform that antagonizes the function of full-length KLF6 (“KLF6WT”).6, 7, 12 Based on this finding, we then surmised that KLF6 might also be alternatively spliced during HSC activation in response to tissue injury, leading to increased generation of an antagonistic splice form that promotes stellate cell activation.
To test this hypothesis, we explored whether Klf6 was alternatively spliced in rat stellate cells by using RT-PCR based on flanking primer sequences in the rat 5′ and 3′ KLF6 UTRs. Using cDNA primary rat stellate cells from untreated animals we consistently detected three distinct bands, which were cloned, sequenced, and aligned against the rat genome. This approach yielded three novel rKlf6 splice forms. The rKlf6 splice forms were labeled “Top,” “Middle,” and “Bottom” based on their size, shown in Fig. 1A.
Even though the splicing pattern for rKlf6 is different from hKLF6, there are several key similarities. The rKlf6top (283 amino acids [aa]) corresponds to hKLF6WT, with a homology of 93% at the amino acid level. A 243-basepair (bp) stretch of nucleotides of exon 2 (bp 201-443) is spliced out in rKlf6mid (72 aa) and this splicing pattern leads to a frameshift with a premature stop codon (asterisk in Fig. 1A). For rKlf6bot (55 aa), there is a complete skipping of exon 2, also leading to a frameshift and premature stop codon. Interestingly, rKlf6bot is very similar to hKLF6SV1 (the shortest variant for human KLF6). The hKLF6SV1 isoform has a unique 21 amino acid stretch at its C-terminus that is thought to be critical for its function; rKlf6bot shares 19 of these 21 amino acids.
To determine whether the shorter rKlf6mid and rKlf6bot isoforms are translated into proteins we transfected 293FT cells with V5-tagged expression constructs encoding each of the three rKlf6 cDNA isoforms. Cells were grown in the presence or absence of proteasomal inhibitor MG132 to account for potential rapid degradation by the proteasome, because this pathway is important in the degradation of hKLF6.13 All three rat Klf6 isoforms were translated into protein and were stabilized in the presence of the proteasomal inhibitor, indicating their degradation by the proteasome (Supporting Fig. 1A). Moreover, the putative nuclear localization signal for hKLF6 lies in the C-terminus region, which is absent in rKlf6mid and rKlf6bot, indicating that the isoforms should be cytoplasmic. To verify the localization of these rat KLF6 isoforms, confocal microscopy was performed on 293FT cells transfected with V5-tagged rKlf6 expression constructs that were immunostained for the V5 epitope tag. As predicted, rKLFtop was present in both the nucleus and cytoplasm, while the two lower molecular weight isoforms, rKLF6mid and rKLF6bot, were unable to enter the nucleus where they displayed a cytoplasmic punctate pattern (Supporting Fig. 1B).
By exploiting the unique splice junctions within the rKlf6 isoforms, real-time PCR primers were designed to perform isoform-specific real-time PCR (see Table 1 for list of primers). With these tools we next assessed rKlf6 isoform expression during stellate cell activation following liver injury and during activation induced by primary culture on plastic.14 Rat stellate cells were isolated 24 and 48 hours after a single dose of oil (control) or CCl4. As shown in Fig. 1B, there was a dramatic decrease in levels of all rKlf6 isoforms at 24 and 48 hours postinjection of CCl4 and following activation in primary culture (Fig. 1C). To confirm that the cells were activating in response to culture, we assessed collagen (I) expression levels, which increased progressively during the culture activation, as expected (Fig. 1D). To establish the relevance of these findings in rats to human stellate cells, expression of hKLF6WT and hKLF6SV1 mRNAs was assessed in primary human stellate cells during culture activation. As for rKlf6, both human KLF6 isoforms decreased significantly with progressive activation in culture (Fig. 1E).
Loss of Klf6 in HSCs Leads to More Fibrogenic Activity and Increased Liver Fibrosis Following Liver Injury.
To further define the contribution of KLF6 to stellate cell activation in vivo, we next assessed the response of Klf6+/− mice to chronic CCl4, reasoning that loss of a single allele of Klf6 in stellate cells might modify their fibrogenic behavior in this model. Remarkably, there was a marked increase in inflammation along the fibrotic tracts in the Klf6+/− animals (Fig. 2A). Likewise, Klf6+/− mice displayed a marked increase in fibrotic area compared to wildtype mice, as assessed by Sirius Red staining and morphometry (3.6% versus 2.5%, P < 0.01) with comparable levels of aspartate aminotransferase (AST) and alanine aminotransferase (ALT) between the two groups (Fig. 2B,C). Consistent with the increased fibrosis accumulation, stellate cell-derived fibrogenic mRNAs were significantly increased in the Klf6+/− mice, including Asma, Colα1 (I), Timp1, Timp2, β-Pdgfr, and Tgf-β mRNA transcripts, along with a decrease in levels of Klf6 mRNA, as expected (Fig. 2D). Western blot analysis of whole liver protein lysates confirmed an increased expression of the fibrogenic proteins β-PDGFR, COL(I), and ASMA in Klf6+/− animals (Fig. 2E).
Because Klf6 depletion in Klf6+/− mice is not restricted to stellate cells, we sought to exclude the possibility that allelic loss of Klf6 in hepatocytes might contribute to the observed phenotype, even though the extent of injury based on serum AST and ALT levels was not affected by Klf6 heterozygosity. To do so, we examined the impact of chronic CCl4 on mice with hepatocyte-specific deletion of Klf6 generated by crossing mice with a floxed allele of (Klf6F/+)15 to animals expressing Cre-recombinase driven by the albumin promoter. In these mice, chronic CCl4 did not alter fibrogenic gene expression or the extent of fibrosis as observed in the Klf6+/− animals (data not shown). These findings implicate loss of KLF6 in a nonparenchymal cell(s) of the liver as the reason for the observed increase in fibrosis.
To more specifically characterize the impact of KLF6 depletion on stellate cells, mouse stellate cells were isolated from Klf6F/F mice and activated by primary culture on plastic for 7 days. The stellate cells were then infected with adenoviruses expressing either LacZ or Cre-recombinase and the expression of COL(I) protein was assessed by western blot. As predicted, Cre-mediated deletion of Klf6 in culture-activated stellate cells led to increased collagen expression (Fig. 2F).
Transgenic Mice with Overexpression of KLF6WT or KLF6SV1 in Stellate Cells Have Reduced Fibrosis After Injury.
Despite numerous attempts to clone and characterize Klf6 splice forms in the mouse, we repeatedly detected only one Klf6 isoform by 5′/3′ flanking PCR. Therefore, we instead established mice expressing human isoforms as a relevant model by first establishing that the biologic activity of human splice variants in mouse stellate cells is identical to their known activity in human stellate cells. In doing so, we thereby validated the use of transgenes in which the human isoforms were expressed. In fact, the absence of multiple mouse Klf6 isoforms facilitated our ability to attribute biology directly to human isoforms rather than through their potential interactions with mouse isoforms.
Based on our initial hypothesis that injury might be stimulating a change in the splicing pattern of KLF6, we examined the impact of overexpressing of KLF6WT or KLF6SV1 on the fibrogenic activity of stellate cells. We employed the Colα2(I) enhancer promoter, which is active as early as 12 hours after injury.16 The KLF6WT or KLF6SV1 isoforms were cloned downstream of this promoter. After validating expression of the expression constructs in a mouse stellate cell line (JS1)17 (Supporting Fig. 2A), stellate cell-specific transgenic mice were generated. At least two transgenic mouse lines were selected for efficient expression of the transgene by qRT-PCR (Supporting Fig. 2B), and all experiments were validated in these two lines for each genotype. Because the experimental findings were consistent between both mouse lines only results from a single line of Colα2(I)-KLF6WT and Colα2(I)-KLF6SV1 mice are included.
The loss-of-function studies in Klf6+/− mice suggested that Klf6 is antifibrotic in stellate cells, and its depletion augments their fibrogenic activity. Therefore, we next examined whether overexpression of KLF6WT in stellate cells would yield the opposite result, decreased fibrosis. Based on our characterization of the endogenous rat isoforms, as well as results from the chronic CCl4 experiments in the Klf6+/− mice, we anticipated decreased fibrosis in the Colα2(I)-KLF6WT mice. As predicted, increased KLF6WT expression in stellate cells reduced the amount of fibrosis in mouse livers following chronic CCl4 injury (Fig. 3A,C). Moreover, we anticipated the opposite phenotype in Colα2(I)-KLF6SV1 transgenic mice, because in human cancer KLF6SV1 antagonizes the activity of KLF6WT. Surprisingly, these Colα2(I)-KLF6SV1 mice also had dramatically decreased fibrosis, similar to Colα2(I)-KLF6WT transgenic mice (Fig. 3A,C). Injury levels assessed by AST and ALT were similar between the three groups (Fig. 3B). The reduced fibrosis based on morphometry was correlated with decreased expression of fibrogenic mRNAs and proteins in whole liver by real-time PCR (Asma, Colα1 (I), Timp2, β-Pdgfr, and Tgf-β) (Fig. 3D) and western blot (Fig. 3E).
The reduced fibrosis in mice overexpressing KLF6WT or KLF6SV1 in stellate cells could result from a decrease in total number of stellate cells following injury, a decrease in fibrogenic potential of each cell, or a combination of the two. In order to address these possibilities, we examined the fibrogenic activity of stellate cells isolated from both Colα2(I)-KLF6WT and Colα2(I)-KLF6SV1 mice following acute CCl4 administration. Whereas stellate cells isolated from the Colα2(I)-KLF6WT mice expressed significantly less Colα1 (I), Colα2 (I), and β-Pdgfr, fibrogenic gene expression was not altered in cells from the Colα2(I)-KLF6SV1 animals (Fig. 4A). In order to further clarify this issue, we performed ASMA immunohistochemistry on liver sections of transgenic and wildtype mice following chronic CCl4 administration. Interestingly, there was a decrease in the number of ASMA-positive cells in both the Colα2(I)-KLF6WT and Colα2(I)-KLF6SV1 livers (Supporting Fig. 3A,B), consistent with the possibility that the decreased fibrosis might reflect decreased numbers of activated cells in the presence of either KLF6WTorKLF6SV1.
These findings suggested that transgenic stellate cells expressing KLF6WT or KLF6SV1 might be more prone to apoptosis. We therefore assessed the impact of KLF6WT or KLF6SV1 on apoptosis in a human HSC line (LX2) that was stably transfected with pBabe, pBabe-KLF6WT, or pBabe-KLF6SV1. Indeed, apoptosis was increased in stellate cells expressing pBabe-KLF6WT and pBabe-KLF6SV1, as indicated by increased PARP cleavage (Fig. 4B).
In order to confirm that KLF6WT was acting as a transcription factor that binds to GC boxes in the promoter regions of Colα1 (I), Colα2 (I), and β-PDGFR, we performed ChIP. For this experiment, the JS1 mouse stellate cell line (JS1) was transfected with either Colα2(I)-KLF6WT, Colα2(I)-KLF6SV1, or Colα2(I)-GFP constructs. Based on this analysis, KLF6WT but not KLF6SV1 bound to the promoters of Colα1 (I), Colα2 (I), and β-Pdgfr (Fig. 4C). The lack of chromatin binding by KLF6SV1 is consistent with its lack of a DNA-binding domain. To reinforce the conclusion that KLF6WT can repress endogenous promoters associated with stellate cell activation, JS1 cells were transfected with pBabe-KLF6WT or the pBabe control plasmid. Similar repression was seen in the levels of Colα1 (I), Colα2 (I), and β-Pdgfr but not Asma transcripts (Fig. 4D).
Our data implicate KLF6WT as a transcription factor that preserves the quiescent phenotype of HSCs by inhibiting expression of fibrogenic genes, including Colα1 (I), Colα2 (I), and β-Pdgfr, as well as by inducing apoptosis of stellate cells. KLF6SV1 normally acts as a splice form that antagonizes KLF6WT. Interestingly, however, unlike transformed cancer cells where KLF6SV1 promotes survival,6, 18 in stellate cells KLF6SV1 promotes apoptosis. Moreover, because endogenous levels of KLF6 decrease with activation, there is diminishing KLF6WT for KLF6SV1 to antagonize.
Our initial hypothesis that KLF6 alternative splicing yields increased antagonistic splice forms that promote stellate cell activation is refuted by these data. In fact, there are parallel decreases in full-length KLF6 (rKlf6top), and the shorter KLF6 isoforms (rKlf6mid and rKlf6bot) with progressive activation. The parallel decreases in expression of all isoforms suggests that the pre-mRNA for Klf6 is reduced, resulting in similar changes for all splice isoforms, rather than due to a change in the splicing pattern.
Reduced Klf6 mRNA levels during activation at first seem to contradict our earlier finding of transient induction during early stellate cell activation (i.e., 6-12 hours after CCl4).1 KLF6 may have a unique role and distinct transcriptional targets early in stellate cell activation, which are different from its activities later during the activation. However, the transgenic Colα2(I)-promoter is only expressed at least 12 hours after injury, so that the contribution of Klf6 to very early stellate cell activation cannot be assessed. Indeed, the findings suggest that Klf6 may be solely antifibrogenic. The early, transient peak of Klf6 mRNA immediately following injury could serve to preserve cellular quiescence in order to prevent unwanted, premature activation of stellate cells. Thereafter, decreasing expression of Klf6 mRNA could enable cells to activate. This role of KLF6 as a potential “quiescence-preserving” gene is consistent with findings for another transcription factor, Lhx2, whose absence in development leads to spontaneous neonatal liver fibrosis.19
The findings of reduced fibrosis in Klf6+/− mice suggest that allelic loss of Klf6 in stellate cells might contribute to increased fibrosis. However, because heterozygosity is present in all cell types, the observed phenotype cannot be definitively ascribed directly to KLF6 loss in stellate cells. Moreover, Klf6+/− mice displayed an increased inflammatory infiltrate in liver after CCl4, compared to wildtype animals, raising the prospect that loss of KLF6 in the inflammatory cells might alter their response to injury even though AST and ALT levels after CCl4 were unaffected by Klf6 heterozygosity. A more specific way to establish that stellate cell-specific loss of KLF6 leads to increased fibrosis would be to cross Klf6F/F mice with GFAP-Cre mice. However, this attempt resulted in a profound neurologic phenotype in mice, which precluded analysis of liver injury (data not shown). Studies using isolated stellate cells from Klf6F/F mice support the idea that loss of KLF6 in stellate cells is an important contributor to enhanced fibrosis.
Data from mice with transgenic expression of KLF6WT or KLF6SV1 is complementary and supports an antifibrotic role of this gene. Reduced fibrosis in mice overexpressing KLF6WT or KLF6SV1 in stellate cells reflects either a decrease in stellate cell numbers following injury, a decrease in fibrogenic potential of each cell, or a combination of the two. The experiments utilizing stable overexpression of KLF6WT or KLF6SV1 in HSCs demonstrate an increase in apoptosis in both cases. Furthermore, although stellate cells isolated from the Colα2(I)-KLF6WT mice expressed significantly less Colα1 (I), Colα2 (I), and β-Pdgfr, fibrogenic gene expression was not altered in cells from the Colα2(I)-KLF6SV1 animals. KLF6WT but not KLF6SV1 interacts directly with “GC” boxes in the promoter regions of Colα1 (I), Colα2 (I), and β-Pdgfr. These data, combined with findings following the overexpression of KLF6WT in JS1 cells demonstrating transcriptional inhibition, suggest that KLF6WT directly represses these promoters. In short, the phenotype in the KLF6WT transgenic mice is due to both a decrease in fibrogenic potential, as well as decreasing stellate cell number, which is likely a result of increased apoptosis.
KLF6 is a well-studied transcription factor that can repress or activate target genes by interacting with coregulator proteins such as histone deacetylase (HDAC).20 A recent article implicates ligand-dependent corepressor (LCoR) in the regulation of KLF6 target genes,21 notably p21 and E-cadherin. Promoter bound KLF6 inhibits downstream genes by tethering a transcriptional corepressor complex containing LCoR, with specific contributions by C-terminal binding protein (CtBP1) and HDAC. It would be informative to determine which coregulator proteins interact with KLF6 during the inhibition of fibrogenic genes in quiescent stellate cells.
In summary, our findings clarify the role of KLF6 and its isoforms during HSC activation. Moreover, the parallel activities of both KLF6WT and its splice isoforms contrasts entirely with their behavior in human cancer,6, 7, 12, 18 reinforcing the conclusion that activities of splice isoforms may be highly context-dependent not only for KLF6, but possibly other genes that are alternatively spliced.