Is PKC activation required for leukemia cell differentiation?

Role of calcium-dependent protein kinases in chronic myeloid leukemia: combined effects of PKC and BCR-ABL signaling on cellular alterations during leukemia development

Calcium-dependent protein kinases (PKCs) function in a myriad of cellular processes, including cell-cycle regulation, proliferation, hematopoietic stem cell differentiation, apoptosis, and malignant transformation. PKC inhibitors, when targeted to these pathways, have demonstrated efficacy against several types of solid tumors as well as leukemia. Chronic myeloid leukemia (CML) represents 20% of all adult leukemia. The aberrant Philadelphia chromosome has been reported as the main cause of CML development in hematopoietic stem cells, due to the formation of the BCR-ABL oncogene. PKCs and BCR-ABL coordinate several signaling pathways that are crucial to cellular malignant transformation. Experimental and clinical evidence suggests that pharmacological approaches using PKC inhibitors may be effective in the treatment of CML. This mini review summarizes articles from the National Center for Biotechnology Information website that have shown evidence of the involvement of PKC in CML.

Differential redistribution of protein kinase C isoforms by cyclic AMP in HL60 cells

In this study we have analyzed the distribution of protein kinase C isoforms in cytosol, membrane, and nucleus in HL60 cells. Furthermore, we have studied the redistribution of these isoforms after cyclic AMP treatment. Protein kinase C localization and cyclic AMP-induced translocation was demonstrated by Western blot analysis. Cytosol, membrane and nucleus in HL60 cells expressed the abundance of protein kinase C alpha, betaI, betaII, delta, lambda, and zeta isoforms. After cyclic AMP treatment, the amount of protein kinase C betaI and zeta increased only in the nucleus, while protein kinase C delta increased in the three fractions tested. These effects were dependent on the cyclic AMP concentration and duration of action. Our results suggest the existence of cross-talk between the cyclic AMP system and protein kinase C in HL60 cells. Taking into account the processes regulated by protein kinase C, these findings also suggest that cyclic AMP plays a regulatory role in various cellular responses in HL60 cells, such as differentiation and gene expression. The increase observed in PKC delta was due to cyclic AMP-dependent protein kinase C activation, and the synthesis of enzyme was probably activated by the nucleotide.

Evidence of the role of protein kinase C during aclacinomycin induction of erythroid differentiation in K562 cells

At subtoxic concentrations, aclacinomycin is effective in controlling erythroid differentiation of K562, a human erythroleukemic cell line. To better understand early events implicated in this process, we have used bisindolylmaleimide (GF109203X), an inhibitor with a high selectivity for protein kinase C (PKC). Our data show that GF109203X inhibits aclacinomycin effects on K562, evidenced by a strong reduction of hemoglobinized cells and a marked decrease of mRNA rates of erythroid genes. To establish firmly PKC involvement, we also verified that aclacinomycin stimulates its rapid translocation, from the cytosolic to the membrane compartment. By Western blot analysis, we also show that after short induction times, PKCalpha was the most implicated.

Erythropoietin– and Stem Cell Factor–Induced DNA Synthesis in Normal Human Erythroid Progenitor Cells Requires Activation of Protein Kinase C and Is Strongly Inhibited by Thrombin

Proliferation, differentiation, and survival of erythroid progenitor cells are mainly regulated by stem cell factor (SCF) and erythropoietin (Epo). Using normal human progenitors, we analyzed the role of Ca2+-sensitive protein kinase C (PKC) subtypes and of G-protein–coupled receptor ligands on growth factor–dependent DNA synthesis. We show that stimulation of DNA synthesis by the two growth factors requires activation of PKC. Inhibitors of Ca2+-activated PKC subtypes blocked the growth factor–induced 3H-thymidine incorporation. SCF and Epo caused no significant translocation of PKC into the membrane, but treatment of intact cells with either of the two cytokines resulted in enhanced activity of immunoprecipitated cytosolic PKC. Stimulation of PKC with the phorbol ester PMA mimicked the cytokine effect on DNA synthesis. Epo-, SCF-, and PMA-induced thymidine incorporation was potently inhibited by thrombin (half-maximal inhibition with 0.1 U/mL). This effect was mediated via the G-protein-coupled thrombin receptor and the Rho guanosine triphosphatase. Adenosine diphosphate caused a modest Ca2+-dependent stimulation of DNA synthesis in the absence of cytokines and specifically enhanced the effect of SCF. Cyclic 3′,5′-adenosine monophosphate exerted a selective inhibitory effect on Epo-stimulated thymidine incorporation. Our results define PKC as major intermediate effector of cytokine signaling and suggest a role for thrombin in controlling erythroid progenitor proliferation.