Functional role for protein kinase C beta as a regulator of stress-activated protein kinase activation and monocytic differentiation of myeloid leukemia cells

Human myeloid leukemia cells respond to 12-O-tetradecanoylphorbol-13-acetat e (TPA) and other activators of protein kinase C (PRC) with induction of mo nocytic differentiation. The present studies demonstrated that treatment of U-937 and HL-60 myeloid leukemia cells,vith TPA, phorbol-12,13-dibutyrate, or bryostatin 1 was associated with the induction of stress-activated prot ein kinase (SAPK). In contrast, TPA-resistant TUR and HL-525 cell variants deficient in PKC beta failed to respond to activators of PKC with the induc tion of SAPK. A direct role for PKC beta in TPA-induced SAPK activity in TU R and HL-525 tells that stably express PKC beta was confirmed. We showed th at TPA induced the association of PKC beta with MEK kinase 1 (MEKK-1), an u pstream effector of the SAPK/ERK kinase 1 (SEK1)-->SAPK cascade. The result s also demonstrated that PKC beta phosphorylated and activated MEKK-1 in vi tro. The functional role of MEKK-1 in TPA-induced SAPK activity was further supported by the demonstration that the expression of a dominant negative MEKK-1 mutant abrogated this response. These findings indicate that PKC bet a activation is necessary for activation of the MEK-1-->SEK1-->SAPK cascade in the TPA response of myeloid leukemia cells.