PROTEIN-KINASE-C AND PROTEIN-KINASE-A INHIBIT CALCIUM-DEPENDENT BUT NOT STRESS-DEPENDENT C-JUN N-TERMINAL KINASE ACTIVATION IN RAT-LIVER EPITHELIAL-CELLS

In rat liver epithelial cells (GN4), angiotensin II (Ang II) and thaps igargin stimulate a novel calcium-dependent tyrosine kinase (CADTK) al so known as PYK2, CAK beta, or RAFTK. Activation of CADTK by a thapsig argin-dependent increase in intracellular calcium failed to stimulate the extracellular signal-regulated protein kinase pathway but was well correlated with a 30-50-fold activation of c-Jun N-terminal kinase (J NK). In contrast, Ang II, which increased both protein kinase C (PKC) activity and intracellular calcium, stimulated extracellular signal-re gulated protein kinase but produced a smaller, less sustained, JNK act ivation than thapsigargin. 12-O-Tetradecanoylphorbol 13-acetate (TPA), which slowly activated CADTK, did not stimulate JNK. These findings s uggest either that CADTK is not involved in JNK activation or PKC acti vation inhibits the CADTK to JNK pathway. A 1-min TPA pretreatment of GN4 cells inhibited thapsigargin-dependent JNK activation by 80-90%. I n contrast, TPA did not inhibit the >50-fold JNK activation effected b y anisomycin or UV. The consequence of PKC-dependent JNK inhibition wa s reflected in c-Jun and c-Fos mRNA induction following treatment with thapsigargin and Ang II. Thapsigargin, which only minimally induced c -Fos, produced a much greater and more prolonged c-Jun response than A ng II. Elevation of another intracellular second messenger, cAMP, for 5-15 min also inhibited calcium-dependent JNK activation by similar to 80-90% but likewise had no effect on the stress-dependent JNK pathway . In summary, two pathways stimulate JNK in cells expressing CADTK, a calcium-dependent pathway modifiable by PKC and cAMP-dependent protein kinase and a stress-activated pathway independent of CADTK, PKC, and cAMP-dependent protein kinase; the inhibition by PKC can ultimately al ter gene expression initiated by a calcium signal.