No abstract

Exposure of quiescent mouse fibroblasts to platelet-derived growth factor induces mRNA from the c-fos proto-oncogene within 10 min followed by synthesis of nuclear c-fos proteins. These are amongst the earliest described nuclear events that follow a mitogenic stimulus.

IL (interleukin)-22 is an IL-10-related cytokine; its main biological activity known thus far is the induction of acute phase reactants in liver and pancreas. IL-22 signals through a receptor that is composed of two chains from the class II cytokine receptor family: IL-22R (also called ZcytoR11/CRF2-9) and IL-10R␤ (CRF2-4), which is also involved in IL-10 signaling. In this report, we analyzed the signal transduction pathways activated in response to IL-22 in a rat hepatoma cell line, H4IIE. We found that IL-22 induces activation of JAK1 and Tyk2 but not JAK2, as well as phosphorylation of STAT1, STAT3, and STAT5 on tyrosine residues, extending the similarities between IL-22 and IL-10. However our results unraveled some differences between IL-22 and IL-10 signaling. Using antibodies specific for the phosphorylated form of MEK1/2, ERK1/2, p90RSK, JNK, and p38 kinase, we showed that IL-22 activates the three major MAPK pathways. IL-22 also induced serine phosphorylation of STAT3 on Ser 727 . This effect, which is not shared with IL-10, was only marginally affected by MEK1/2 inhibitors, indicating that other pathways might be involved. Finally, by overexpressing a STAT3 S727A mutant, we showed that serine phosphorylation is required to achieve maximum transactivation of a STAT responsive promoter upon IL-22 stimulation.

Phosphatidic acid (PA), an intriguing phospholipid that is rapidly produced during receptor-stimulated breakdown of phosphoinositides, has often been proposed to function as a Ca2+ ionophore in activated cells. The PA-ionophore hypothesis is supported by the fact that exogenously applied PA stimulates Ca2+ uptake in various cells and can evoke Ca2+-mediated physiological responses, but it is not known whether PA accumulation affects cytoplasmic free Ca2+ concentration ([Ca2+]i). Here we report that PA elicits a transient rise in [Ca2+]i in cultured cells, not by stimulating Ca2+ influx, but, surprisingly, by releasing Ca2+ from intracellular stores. We further show that PA evokes growth factor-like effects in that it raises cytoplasmic pH, induces expression of the c-fos and c-myc proto-oncogenes and stimulates DNA synthesis. Our results indicate that, unlike an ionophore, PA acts by triggering the hydrolysis of phosphoinositides, with consequent formation of second messengers such as inositol trisphosphate signalling Cai2+ release. Furthermore, our data strengthen the notion that any Ca2+-mobilizing stimulus acting through phospholipase C may ultimately function as a growth factor.

Nerve growth factor (NGF) causes the differentiation of PC12 cells to sympathetic neuron-like cells and also induces a rapid but transient expression of fos mRNA and protein. fos mRNA transcripts can be detected 5 min after the addition of NGF, are maximally abundant after 30 min, and then their levels decrease. fos protein synthesis parallels the expression of fos mRNA, and the induced fos proteins are located in the nucleus. cAMP, epidermal growth factor, the phorbol ester phorbol 12-myristate 13-acetate, and K+ depolarization also induce thefos gene. Growth of PC12 cells in the presence of dexamethasone, which induces differentiation into chromaffim-like cells, is not accompanied byfos expression. We propose that while fos gene induction is associated with the differentiation of PC12 cells to sympathetic nerve, its enhanced expression is primarily involved in the anabolic responses induced by NGF and many growth factors.It is the general consensus that proto-oncogenes are involved in the regulation of cell growth and differentiation (1-3). They are conserved during evolution and expressed in a tissue-and stage-specific manner during development (3-5). The expression of some proto-oncogenes is induced when resting cells proliferate in response to mitogens (6)(7)(8)(9). A direct link between proto-oncogenes and cell-growth regulation was established when it was discovered that the proto-oncogne sis encodes the B chain ofplatelet-derived growth factor (PDGF) and erbB gene product is a truncated form of epidermal growth factor (EGF) receptor (10-13). We have been studying the proto-oncogenefos, whose viral cognate, v-fos, is the transforming gene of two murine retroviruses that induce osteosarcomas in vivo and transform fibroblasts in vitro (14)(15)(16)(17). The viral and cellular fos proteins differ at their COOH termini, but both are found in the nucleus and both induce transformation (18)(19)(20). Finally, the expression offos is inducible in response to mitogens and certain differentiation-specific agents (21-23).To investigate further the role of fos gene in growth and differentiation, we have studied its induction by nerve growth factor (NGF) in the clonal rat pheochromocytoma cell line, PC12. In the presence of NGF, PC12 cells acquire properties of sympathetic neurons, including neurite outgrowth (24), increased electric excitability (25), and changes in neurotransmitter synthesis (26). NGF may also act as a weak mitogen in these cells (27). In contrast to the effect of NGF, when PC12 cells are grown in the presence of dexamethasone, they acquire the characteristics of chromaffin cells (28). We report here that the fos gene is rapidly and transiently expressed when NGF is added to PC12 cells, but no expression ofthefos gene is detected in PC12 cells upon the addition of dexamethasone. MATERIALS AND METHODSPC12 cells were grown in Dulbecco's modified Eagle's medium containing 10% fetal calf serum and 5% horse serum as described (28). For induction, the culture medium was aspirated and replaced by N2...