The p44/42 mitogen-activated protein kinase cascade is involved in the induction and maintenance of astrocyte stellation mediated by protein kinase C.

The mitogen-activated protein kinase (MAPK) is known to be involved in the differentiation of various types of cells. To understand the role of p44/42 MAPK (ERK1/2) in astrocyte differentiation, we investigated the effects of U0126 and PD98059, specific inhibitors of the MAPK-activating enzyme MEK, on astrocyte morphology in culture. Cultured rat cortical astrocytes exhibited flattened, polygonal morphology in the absence of stimulation, but differentiated into process-bearing stellate cells in response to the membrane-permeable cyclic AMP analog dibutyryl cyclic AMP (dBcAMP) or the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA). dBcAMP-induced astrocyte stellation was not affected by MEK inhibitors, while PMA-induced astrocyte stellation was significantly blocked by U0126 (0.1-10 microM) and PD98059 (10-30 microM). Western blot analysis with an antibody specific for phosphorylated ERK1/2 revealed that PMA, but not dBcAMP, induced phosphorylation of ERK1/2 in a time- and concentration-dependent manner. The PMA-induced astrocyte stellation and ERK1/2 phosphorylation were blocked by specific PKC inhibitors, GF-109203X (0.01-1 microM) and calphostin C (1 microM). In addition, when U0126 or PD98059 was added after treatment with PMA, stellate astrocytes returned to polygonal. These results suggest that the MEK/ERK cascade is involved in the induction and maintenance of astrocyte stellation mediated by PKC, but not by cyclic AMP signaling.