Protein kinase C isotypes in human erythroleukemia (K562) cell proliferation and differentiation: Evidence that β_II protein kinase C is required for proliferation

The human erythroleukemia (K562) cell line undergoes megakaryocytic differentiation and cessation of proliferation when treated with phorbol myristate acetate (PMA). To investigate the role of individual protein kinase C (PKC) isotypes in these events, we have assessed PKC isotype expression during leukemic proliferation and PMA-induced differentiation. Immunoblot analysis using isotype-specific antibodies demonstrates that proliferating K562 cells express the α, β_II, and ζ PKC isotypes. PMA-induced differentiation and cytostasis lead to a decrease in β_II PKC and increases in α and ζ PKC levels. The role of the α and β_II PKC isotypes was further assessed in cells overexpressing these isotypes. K562 cells overexpressing human α PKC grew more slowly and were more sensitive to the cytostatic effects of PMA than control cells, whereas cells overexpressing β_II PKC were less sensitive to PMA. PMA-induced cytostasis is reversed upon removal of PMA. Resumption of proliferation is accompanied by reexpression of β_II PKC to near control levels, whereas α and ζ PKC levels remain elevated for several days after removal of PMA. Proliferation of PMA-withdrawn cells can be partially inhibited by antisense β_II PKC oligodeoxyribonucleotide. Growth inhibition is dose-dependent, specific for β_II PKC-directed antisense oligonucleotide, and associated with significant inhibition of β_II PKC levels indicating that β_II PKC is essential for K562 cell proliferation. Sodium butyrate, which unlike PMA induces megakaryocytic differentiation without cytostasis, causes increases in both α and β_II PKC levels. These data demonstrate that β_II PKC is required for K562 cell proliferation, whereas α PKC is involved in megakaryocytic differentiation.