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The effect of erythropoietin (Epo) on the expression of mitogen-activated protein kinase (MAPK) target genes egr-1 and c-fos was investigated in Epo-responsive murine erythroblastic cell line ELM-I-1. Epo induced a transient rise in egr-1 mRNA without a similar effect on c-fos expression. The induction of egr-1 correlated with a rapid ERK1/2 phosphorylation and was prevented with MEK1/2 inhibitors PD 98059 and UO126. The p38 inhibitor SB 203580 enhanced ERK1/2 phosphorylation and egr-1 mRNA levels. Longer incubations of ELM-I-1 cells with Epo revealed a second later phase of increase in egr-1 expression which was also prevented by MEK1/2 inhibitors, whereas SB 203580 had a stimulatory effect. In contrast, the beta-globin mRNA production was enhanced in the presence of PD 98059 and UO126 and reduced by SB 203580. The results suggest a regulatory role of egr-1 expression in Epo signal transduction and provide pharmacological evidence for the negative modulation of differentiation-specific gene expression by the ERK1/2 pathway in murine erythroleukemia cells.

The involvement of MAPK pathways in differentiation, proliferation and survival was investigated by comparing Epo and GM-CSF signalling in human factor-dependent myeloerythroid TF-1 cells with abnormal Epo-R. GM-CSF withdrawal induced cell-cycle arrest and apoptosis accompanied by increased caspase-3 activity, DNA degradation and reduced expression of the antiapoptotic Bcl-2 and Bcl-xl proteins. Readministration of GM-CSF but not Epo reversed these processes and induced proliferation. The GM-CSF promoted cell survival and proliferation correlated with MEK-1 dependent ERK1/2, Elk-1 and CREB phosphorylation and Egr-1, c-Fos expression as well as with increased STAT-5, AP-1, c-Myb and NF-kappaB DNA-binding. In contrast, Epo failed to activate the Raf-1/ERK1/2 MAPK pathway or to induce Egr-1 and/or c-Fos expression, while it induced erythroid differentiation in GM-CSF-deprived cells. In addition, the Epo-induced haemoglobin production was inhibited in the presence of GM-CSF. These results demonstrate that the activation of MAPK cascade is not necessary for Epo-induced haemoglobin production in TF-1 cells and suggest a negative cross-talk between the signalling of GM-CSF-stimulated cell proliferation and Epo-induced erythroid differentiation.

The glycoprotein hormone, erythropoietin is the principal regulator of the production of circulating erythrocytes by controlling proliferation, differentiation and survival of its target erythroid progenitor cells. The receptor for erythropoietin is a type I cytokine receptor lacking intrinsic tyrosine kinase activity. It mediates tyrosine phosphorylation through its association with nonreceptor tyrosine kinases such as JAK2 and initiates a cascade of signalling events in response to erythropoietin. Significant progress has been made in identifying signalling pathways triggered by erythropoietin. However, the exact signalling mechanisms mediating the known physiological effects of erythropoietin in erythroid progenitor cells are poorly understood. There are many open questions including the role of Ca2+ in erythropoietin induced signal transduction. Although the results concerning the effect of erythropoietin on [Ca2+]i in various erythroid cells are conflicting, [Ca2+]i-increasing agents mimic the effect of erythropoietin on c-myb expression and activate the program of haemoglobin synthesis in murine erythroleukemia cells. An attempt is made in this review to survey recent data on the erythropoietin-induced signal transduction with respect to the different physiological effects of this hormone.