Selective translocation of β(II)-protein kinase C to the nucleus of human promyelocytic (HL60) leukemia cells

The promyelocytic leukemia (HL60) cell line differentiates into monocyte-like cells after treatment with phorbol dibutyrate (PBt₂). In contrast, bryostatin 1 (bryo), a structurally distinct protein kinase C (PKC) activator, does not induce differentiation and blocks the cytostatic effect of PBt₂.The divergent responses to these agents correlate with activation of a PKC-like activity at the nucleus in response to bryo but not PBt₂ (Fields, A.P., Pettit, G.R., and May, W.S. (1988) J. Biol. Chem. 263, 8253-8260). In the present study, this nuclear PKC-like activity (termed PKC(n)) was isolated from HL60 cells and shown to phosphorylate its known nuclear substrate, lamin B. PKC(n)-mediated phosphorylation of nuclear envelope-associated lamin B in vitro is calcium-dependent and is stimulated by bryo and 1,2-dioctanoylglycerol (DiC₈), but not PBt₂. In contrast, PKC(n)-mediated phosphorylation of histone IIIS is stimulated equally by all three activators. PKC(n) mediates calcium- and phosphatidylserine-dependent phosphorylation of both histone IIIS and partially purified lamin B. PKC(n) activity can be inhibited by an anti-PKC monoclonal antibody which specifically inhibits PKC. Isotype-specific PKC antibodies identify PKC(n) as β(II)-PKC. Immunoblot analysis indicates that HL60 cells express both α- and β(II)-PKC but no β(I)- or γ-PKC. Treatment of intact cells with bryo for 30 min leads to complete translocation of both α- and β(II)-PKC from the cytosol to the membrane fractions. Approximately 8-10% of the total β(II)-PKC (and < 0.3% of the α-PKC) is found associated with the nuclear membrane of bryo-treated cells. In contrast, PBt₂ treatment leads to complete translocation of α-PKC, but only partial translocation of β(II)-PKC to the plasma membrane fraction. Neither PKC isotype is found associated with the nuclear membrane of PBt₂-treated cells. These data demonstrate that α- and β(II)-PKC differ with respect to activator responsiveness, intracellular distribution, and substrate specificity and indicate that their selective activation at distinct intracellular sites, including the nucleus, can have a dramatic effect on resulting cellular responses.