HEPARIN INHIBITS MITOGEN-ACTIVATED PROTEIN KINASE-DEPENDENT AND KINASE-INDEPENDENT C-FOS INDUCTION IN MESANGIAL CELLS

Heparin suppresses mitogenic responses in renal mesangial cells, and w hen quiescent mesangial cells are stimulated with serum, heparin block s the induction of c-fos seen at 15 min, Because heparin is taken up b y cells over a much longer time course, we addressed mechanisms whereb y extracellular heparin might suppress c-fos induction at such early t imes, Quiescent cells were treated with serum, 12-O-tetradecanoylphorb ol-13-acetate, or low concentrations of Ca2+ ionophores that produced increases in intracellular Ca2+ concentration ([Ca2+](i)) in the physi ological range, Each treatment caused an increase in c-fos mRNA, but t hey did so by different mechanisms. Serum activated mitogen-activated protein kinase (MAPK) and increased [Ca2+](i) without affecting protei n kinase C, Activation of protein kinase C with phorbol ester activate d MAPK without much effect on [Ca2+](i). Ionophores increased [Ca2+](i ) without affecting basal levels of protein kinase C or MAPK, Heparin (1 mu g/ml) suppressed the induction of c-fos initiated by all three t reatments, It did not affect the activity of protein kinase C, but inh ibited activation of MAPK by either serum or phorbol ester, suggesting a common site of action at or below the probable convergence of the i nduced signals at Ras/Raf-1 activation. Heparin also inhibited the ser um-stimulated entry of extracellular Ca2+ to the same extent as verapa mil, consistent with the ability of verapamil to block L-type Ca-2+ ch annels and the known presence of these channels in mesangial cells, Ho wever, this effect does not appear to be related to heparin's ability to inhibit induction of c-fos. First, verapamil had no effect on induc tion of c-fos by serum, Second, heparin had no effect on changes in [C a2+](i) achieved by ionophores, we conclude that heparin suppresses in duction of c-fos in mesangial cells by blocking at least two different points in signal transduction cascades, one upstream of MAPK and the other independent of MAPK but dependent on intracellular Ca2+.