In eukaryotic cells, enzymatic isoforms of MAPK, such as Jun kinase-1 (JNK1) and p38 kinase, which can mediate anti-proliferative stimuli and apoptosis, have been identified. They have large sequence homology, but are functionally different from proliferative pathway-associated erk1/2. In fact, JNK1 and p38 kinase are part of enzymatic cascades activated by anti-proliferative agents such as ionizing and ultraviolet rays and cytokines. Recently, it has been demonstrated a role of JNK1 and p38 kinase in the onset of apoptosis in several cell models. In this regard, in addition to the STAT pathway, type I IFNs activate members of the MAPK family, including erk (David et al., 1995) and the p38 MAPK (that belongs to the stress-activated kinases) (Goh et al., 1999; Uddin et al., 1999, 2000). It was recently shown that activation of p38 is required for transcriptional activation of IFN-sensitive genes (Goh et al., 1999; Uddin et al., 1999, 2000). In addition, it was demonstrated that such transcriptional regulation of IFN-sensitive genes is unrelated to effects on DNA binding of STAT complexes or serine phosphorylation of STATs (Uddin et al., 2000), apparently involving a STAT-independent nuclear mechanism. Thus, coordination of the functions of the IFN-activated STAT and p38 pathways is necessary for full transcriptional activation in response to interferons (Goh et al., 1999; Uddin et al., 1999, 2000). It was, moreover, found that p38 MAPK pathway is engaged in type I IFN signaling in primary human hematopoietic progenitors and its function is required for the generation of the suppressive effects of interferons on normal hematopoiesis. In details, p38 and its downstream effector, mitogen-activated protein kinase activated protein kinase 2 (MapKapK-2), are rapidly activated by IFNα treatment of enriched primary human progenitor cells and pharmacological inhibition of p38 MAPK activation reverses the type I IFN-dependent inhibition of hematopoietic progenitor colony formation (Verma et al., 2002). Moreover, p38 MAPK, is involved in the generation of the antileukemic effects of IFNα in break cluster region (BCR)-ABL-expressing cells of acute myeloid leukemia (Mayer et al., 2001). We have reported that IFNα increases the expression of the epidermal growth factor receptor (EGF-R) at the surface of human epidermoid carcinoma cells (Budillon et al., 1991). We have also found that IFNα enhances the activity of EGF on these cells. In fact, IFNα-treated KB cells (human epidermoid carcinoma) are sensitized to the growth promoting effects of EGF. Moreover, the EGF-induced tyrosine phosphorylation of total cellular proteins and of the EGF-R is increased in the IFNα-treated cells (Caraglia et al., 1995). On the bases of these findings, we have hypothesized that the increased expression and function of the EGF-R could represent a protective response of tumour cells (STRESS RESPONSE) to the antiproliferative effect of IFNα (Tagliaferri et al., 1994). In order to verify this hypothesis we have studied, in KB cells, the expression of heat shock proteins (HSP) which are molecules involved in the protective response of eukaryotic cells to stress. IFNα increases the expression of HSP27, HSP90, and HSP70 inducible forms while it does not change the levels of the constitutive form of HSP70. After EGF addition to IFNα-treated KB cells, the levels of the HSPs are resumed to the levels of untreated control cells (Caraglia et al., 1999). We have moreover found that IFNα induces apoptosis on human epidermoid cancer KB cells and that also this effect is antagonized by EGF. We have evaluated the effects of IFNα and EGF on the stress-induced pathway of MAPK isoenzymatic activity JNK1 and MAPKp38 in KB cells. We have found that IFNα induces an about fivefold increase of activity of these proteins while the addition of EGF to IFNα-treated cells causes a progressive reduction of the activity of the two enzymes which reaches almost basal levels after 6 h of exposure to EGF. However, EGF alone does not induce any change in the activity of JNK1 and MAPKp38 in untreated KB cells. We have evaluated the involvement of JNK1 in the triggering of IFNα-induced apoptosis by transfecting KB cells with a plasmid encoding for a wild type form of JNK1 (JNK1wt). Either the treatment of parental cells with IFNα or the overexpression of JNK1wt in transfected cells induce apoptosis and the exposure of JNK1wt-transfected cells to IFNα causes a potentiation of apoptosis. The addition of EGF to JNK1wt-transfected cells exposed to IFNα is again able to revert this effect. Therefore, the effects of EGF and IFNα on apoptosis are paralleled by changes of the activity of the stress-inducible JNK1 that appears responsible, at least in part, for the apoptotic effects of IFNα (Caraglia et al., 1999).
A tumour suppressor gene specifically activated after a genotoxic stress is p53. Takaoka et al. have recently shown that transcription of the p53 gene is induced by IFNα/β, accompanied by an increase in p53 protein level. IFNα/β signaling itself does not activate p53; rather, it contributes to boosting p53 responses to stress signals. In these experimental conditions p53 gene induction by IFNα/β contributes to tumour suppression, is activated in virally infected cells to evoke an apoptotic response and is critical for antiviral defence of the host (Takaoka et al., 2003). The role of NF-κB in the apoptosis induced by IFNα is controversial. In fact, it has been demonstrated that IFNα can activate NF-κB through STAT3 and via phosphatidyl-inositol 3 kinase (PI3K) and Akt activation in lymphoma cells and promotes survival of human primary B-lymphocytes via PI3K (Yang et al., 2001). Other studies demonstrate that IFNα sensitizes human hepatoma cells to TRAIL-induced apoptosis through DR5 upregulation and NF-κB inactivation or suppresses the antiapoptotic effect of NF-κB and sensitizes renal cell carcinoma cells in vitro to chemotherapeutic drugs (Steiner et al., 2001; Shigeno et al., 2003).