ALKYLCATECHOLS REGULATE NGF GENE-EXPRESSION IN ASTROGLIAL CELLS VIA BOTH PROTEIN KINASE-C-INDEPENDENT AND CAMP-INDEPENDENT MECHANISMS

It has been previously demonstrated that, in mouse astroglial cells and fibroblast cells in culture, alkylcatechols cause a rapid increase in the nerve growth factor (NGF) mRNA level followed by a marked increase in the amount of NGF released into the medium. To understand the mechanism of this alkylcatechol effect on NGF gene expression in astroglial cells, we examined the effects of protein kinases that influence intracellular signal transduction and of their inhibitors. The reagents to increase the intracellular content of cyclic AMP (cAMP) such as dibutyryl cyclic AMP (Bt2cAMP), forskolin, or cholera toxin did not mimic alkylcatechol induction of NGF gene expression. Phorbol ester, a direct activator of protein kinase C (PKC), caused an increase in the NGF synthesis/secretion. The stimulatory effect of homocatechol (4-methylcatechol) on NGF synthesis was not completely inhibited by staurosporine, an inhibitor of PKC. The treatment of cells with homocatechol resulted in the translocation of PKC from cytosol to membrane-associated fractions, although the levels of the subcellular location of PKC were not correlated with the level of the induction of NGF gene expression. The concomitant administration of homocatechol (10(-4) M) and PMA (10(-8)-10(-6) M) evoked a drastic and prolonged increase in the NGF mRNA level, and also markedly increased the amounts of NGF secreted by the cells (approximately 150-fold). This synergism was inhibited in part by staurosporine, but the level of increase in NGF mRNA and in NGF protein was rather greater than that of activation of PKC. These results suggest that neither PKC-dependent pathway nor cAMP-dependent pathway is dominant in the stimulatory effect of alkylcatechol on NGF synthesis. (C) 1993 Wiley-Liss, Inc.