A role for protein kinase C in the growth of human erythroid progenitor cells

Effect of inhibitors of protein phosphatase 1 and 2A on erythroid colony formation: an investigation of the specificities of inhibitors

To investigate the possible involvement of protein phosphatase (PP)1 and PP2A in the process of erythropoiesis, we assessed the effect of PP1 and PP2A inhibitors on erythroid colony formation using an in vitro colony formation assay. Okadaic acid (OKA), calyculin A (Cal-A) and tautomycin suppressed colony formation but 1-nor-okadaone did not. These results suggest that PP1 and PP2A both play an important role in erythropoiesis. Furthermore, higher concentrations of tautomycin were needed to suppress colony formation compared to concentrations of OKA and Cal-A. The target enzyme of inhibitors in erythropoiesis may be PP2A.

Possible involvement of protein kinase C in the aberrant regulation of erythropoiesis in polycythemia vera

To examine the possible involvement of protein kinase C (PKC) in the regulation of aberrant erythropoiesis of polycythemia vera (PV), we investigated the effects of PKC inhibitors on in vitro burst-forming unit of erythroid (BFU-E)-derived colony formation by bone marrow (BM) and peripheral blood (PB) cells obtained from five PV patients. 1-(Isoquinoline-sulfonyl)-2-methylpiperazine dihydrochloride (H-7), an inhibitor of PKC, suppressed the colony formation by BM and PB cells of PV patients in a dose-dependent manner, similar to those in the normal individuals. However, the 50% inhibitory concentrations (IC50) of H-7 in PV BM and PB cells were significantly higher than those in normal BM and PB cells, respectively. The BFU-E-derived colony formation by PV BM and PB cells was also less affected by Staurosporine, another PKC inhibitor, than those in a normal subject. Furthermore, in the study of PV, the IC50 of endogenous colonies formed in the absence of erythropoietin was much higher than that of colonies formed by the stimulation of erythropoietin. By contrast, N-(2-guanidinoethyl)-5-isoquinolinesulfonamide dihydrochloride (HA1004), a cyclic AMP-dependent kinase inhibitor, did not have such inhibitory effects. These findings suggest that PKC, as a second messenger, is involved in the regulation of aberrant erythropoiesis of PV.

Role of protein phosphorylation in EPO-mediated early signal transduction: analysis in the EPO-reactive cell line ELM-I-1 transfected with a c-fos-enhancer/promoter-luciferase reporter gene

To investigate the role of protein phosphorylation in the early phase of EPO-mediated signal transduction, we EPO-stimulated a murine erythroid cell line ELM-I-1 transformed by plasmids comprised of the c-fos enhancer/promoter linked to the luciferase gene. Using this reporter gene system, we previously showed that EPO-induced activation of the c-fos promoter can be detected rapidly and sensitively as an elevation of cellular luciferase activity. In this study, we first examined the role of protein tyrosine phosphorylation. The tyrosine phosphatase inhibitor orthovanadate not only induced luciferase activity by itself but enhanced the action of EPO. On the other hand, the tyrosine kinase inhibitors erbstatin and herbimycin suppressed the effect of EPO. Next, the role of protein kinase C (PKC) in the EPO response was assessed. The PKC activator phorbol myristate acetate (PMA) not only induced luciferase activity by itself but enhanced the action of Epo. On the other hand, the PKC inhibitor 1-(5-isoquinolynyl-sulfonyl)-2-methylpiperazine (H7) suppressed the effect of Epo and PMA, whereas a nonspecific protein kinase inhibitor, N-(2-Guanidinoethyl)-5-Isoquinolinesulfornamine (HA1004) inhibited the action of neither Epo nor PMA. Another known PKC inhibitor staurosporine (STSP) did not inhibit but rather enhanced the effect of Epo. This action of STSP was blocked by H7 but not by HA1004. These results suggest that the EPO-mediated early signal transduction pathway leading to c-fos expression involves protein-tyrosine phosphorylation, is modulated by tyrosine phosphatase activity and is positively regulated by PKC.

The expression and possible roles of protein kinase C in haematopoietic cells

Protein kinase C (PKC) activation and/or modulation of its isoenzyme expression play key roles in regulating the response of haematopoietic cells to both growth factors and non-physiological inducers of cell growth and differentiation. The level of PKC activities for both cytosol and particulate fractions of ALL and CLL cells are lower than those of AML type. Atypical AML blasts expressing T-cell associated CD2 and CD7 determinants have significantly lower PKC activities compared to typical AML blasts. Analyses of PKC isoforms (-alpha, -beta, and -gamma) show considerable variation with respect to leukaemic cell distributions and subcellular localisations. PKC-alpha and -beta are usually the major species in cytosolic fractions, whereas PKC-gamma is the predominant type in particulate fractions. All lymphoid cells express PKC-gamma in the cytosol, albeit as a minor component, while the occurrence of cytosol PKC-gamma in AML cells appears to be associated in particular with a typical lymphoid antigen expression.