Changes of nuclear protein kinase C activity and isotype composition in PC12 cell proliferation and differentiation

To establish whether protein kinase C was involved in the nuclear events underlying cell differentiation and proliferation, rat pheochromocytoma PC12 cells, serum-starved for 24 h, were treated with either differentiating doses of nerve growth factor or high serum concentrations, which represented a powerful mitogenic stimulus. Western blot analysis with isoform-specific antibodies, performed on whole cell homogenates, cytoplasms, and purified nuclei, showed that PKC isotypes alpha, beta I, beta II, delta, epsilon, eta, and zeta were expressed in PC12 cells and that all of them, except for beta I, were found at the nuclear level, variably modulated depending on the cell treatment. Compared to serum-stimulated cells, in which an early (1 day) and marked rise of protein kinase C activity was followed by a plateau, nerve growth factor-treated cells showed a progressive increase of protein kinase C activity coincident with the onset and maintenance of the differentiated phenotype. Western blot analysis of nuclei isolated from fully differentiated cells demonstrated an increase of protein kinase C alpha, paralleled by enhanced phosphotransferase activity along with the nerve growth factor treatment, and complete loss of the delta isotype. In contrast, in nuclei of proliferating PC12 cells, after an early but modest increase at 1 day of mitogenic stimulation, protein kinase C activity reached a plateau. Isotype-specific analysis indicated a concomitant increase of protein kinase C beta II, delta, and zeta and the appearance of protein kinase C epsilon and eta at the nuclear level. Considering the relative intensity of the cytoplasmic and nuclear immunoreactive bands under the three conditions examined, clear-cut translocation to the nucleus occurred for PKC epsilon and eta in serum-stimulated cells. Additional nuclear accumulation of PKC by translocation from the cytoplasm was prominently induced for the zeta isoform after mitogenic stimulation and for PKC alpha during prolonged NGF treatment. Our data suggest that nuclear translocation and selective activation of distinct protein kinase C isoforms play a relevant role in the control of proliferation and differentiation of the same cell type and that nuclear protein kinase C is crucial to the induction and persistence of the differentiated neuronal phenotype of PC12 cells.