Phorbol‐12‐myristate‐13‐acetate (PMA) and inhibitors of protein kinase C alter glial fibrillary acidic protein (GFAP) mRNA levels

Glial fibrillary acidic protein (GFAP) mRNA levels in the human astrocytoma line U‐373MG were examined to explore further the effects of agents that regulate protein kinase C. U‐373MG cells exhibit a biphasic change in steady‐state GFAP mRNA in the presence of the phorbol ester phorbol‐12‐myristate‐13‐acetate (PMA). Short‐term treatment with PMA results in increased GFAP mRNA, and long‐term treatment results in decreased GFAP mRNA. Nuclear run‐off experiments demonstrate that the PMA‐induced decrease in GFAP mRNA levels is not at the level of GFAP gene transcription. PMA exerts its effect in the presence of protein synthesis inhibitors, demonstrating that de novo protein synthesis is not required for the PMA‐induced changes in GFAP mRNA. Staurosporine, a protein kinase C inhibitor, reduces GFAP mRNA expression in a dose‐dependent manner; in the presence of PMA the effect is additive. By contrast HA1004, an inhibitor of cAMP‐dependent protein kinase, is not inhibitory to GFAP steady‐state mRNA. Total protein kinase C activity was determined to be 2,398.8 ± 94.3 pmol/min/mg protein, with most of the activity in the cytosol. Short‐term PMA treatment results in the translocation of the cytosolic protein kinase C activity to the membrane. Long‐term PMA treatment results in a decrease in total protein kinase C activity indicating that downregulation occurs. These studies demonstrate that in the U‐373MG cells, protein kinase C inhibitors and long treatment with PMA result in a decrease in steady‐state GFAP mRNA.