PKA and PKC activation induces opposite glial fibrillary acidic protein (GFAP) expression and morphology changes in a glioblastoma multiform cell line of clonal origin

Possible differentiation mechanisms were investigated in a glioblastoma multiform cell line (GL15) presenting an undifferentiated phenotype with weak glial fibrillary acidic protein (GFAP) and strong vimentin (VIM) expression. Serum-free conditions induced time-dependent increases of GFAP-mRNA and GFAP protein levels, associated with a process-bearing astrocytic morphology. Activation of protein kinase C (PKC) by tumor promoter phorbol 12-myrystate 13-acetate (PMA) induced a rapid morphological differentiation and a decrease in GFAP mRNA, whereas the GFAP level remained unchanged. Such parameters were shown to characterize a physiological differentiation stage in astroglial cultures. Treatment of process-bearing GL15 cells with dibutyryl cyclic AMP (dbcAMP), a protein kinase A (PKA) activator, induced a time-dependent decrease in the GFAP mRNA and GFAP protein levels and reverted morphological changes induced by serum-free conditions. Neither PMA nor dbcAMP influenced the VIM mRNA expression. In GL15 cells, PKC and PKA activation have opposite effects. Understanding the role of these kinases in malignant transformation and in the in vitro differentiation process is of both basic and clinical interest.