| Cells continually respond to environmental cues such as changes in nutrient availability, environmental stress, toxins, hypoxia or signals from other cells. Critical to these responses to extracellular signals are protein kinase cascades that lead to the activation of mitogen-activated protein kinases (MAPK) family members. At least three distinct MAPK pathways are known: the p38HOG MAPK, the Stress-Activated Protein Kinases (SAPKs) and the Extracellular Signal-Regulated Kinases (ERKs).; Here, we describe the characterization of a novel MAPK-like molecule from rat and its evolutionary orthologues in D. melanogaster and C. elegans. Rat NKIATRE showed similarity to both the MAPKs and cyclin-dependent kinases (CDKs). Expression of NKIATRE was restricted to testis, brain and skeletal muscle, with no expression in proliferative tissues or cultured cell lines. Two isoforms of NKIATRE were identified which differ in their carboxyl-terminal ends and subcellular localization.; I also investigated the roles of the SAPK and p38HOG MAPK pathways in cellular senescence. Specific activation of the p38HOG pathway, but not SAPK, induced permanent and irreversible cell cycle arrest at the G1 phase of the cell cycle in tumor cells. This cell cycle arrest was concomitant with features of cellular senescence, including upregulation of cyclin-dependent kinase p21Cip1, histochemical markers of senescence, and gene expression previously associated with senescence.; Finally, I describe the roles of the MAPKs in erythropoietin (EPO) signal transduction. Through the use of phosphorylation-specific antibodies, I have shown that EPO induces activation of p38, SAPK and ERK, concomitant with serine phosphorylation of STAT1, STAT3 and STAT5A/B transcription factors. Distinct MAPK pathways were required for the serine phosphorylation events: STAT1 serine phosphorylation was dependent on p38MAPK, whereas STAT3 serine phosphorylation required both p38 and ERK. No evidence was found for a role of SAPK in STAT serine phosphorylation. Serine phosphorylation of STAT1 and STAT3 required a proximal portion of the EPOR that correlates with JAK2 activation, and JAK2 overexpression was sufficient to activate p38 and ERK.; Collectively, these results suggest complex, contrasting roles of MAPKs in cell differentiation, cellular senescence and erythropoietin signal transduction. |
