ABSTRACT: Like vascular smooth-muscle cells, rat mesangial cells (RMCs) display an anti-mitogenic response to heparin. In particular, heparin partially suppresses the ability of quiescent RMCs to enter the cell cycle and induce c-fos expression. When the mitogenic stimulus is serum, phorbol ester or platelet-derived growth factor, this response appears to result from the ability of heparin to suppress activation of the extracellular-signal-regulated kinase family of mitogen-activated protein kinases. However, we have also shown that heparin suppresses c-fos expression in response to ionophores such as ionomycin, an event independent of mitogen-activated protein kinase [Miralem, Wang, Whiteside and Templeton (1996) J. Biol. Chem. 271, 17100-17106]. Here we identify this second heparin-sensitive pathway as involving Ca2+/calmodulin-dependent kinase (CaMK) II. Ionomycin (100 nM) caused a transient rise in intracellular Ca2+ concentration ([Ca2+]i) in quiescent RMCs to 386+/-55 nM, with an increase in CaMK II activity that peaked 30 s later. The accumulation of c-fos mRNA that ensued 30 min later was prevented when the increase in [Ca2+]i was prevented with the intracellular Ca2+ chelator, 1,2-bis-(2-aminophenyoxy)ethane-N,N,N',N'-tetra-acetic acid. The broad-specificity CaMK inhibitor, KT 5926, inhibited ionomycin-dependent c-fos induction at a concentration at which it was without effect on induction by serum or phorbol ester. The CaMK II-specific inhibitor, KN-93, likewise inhibited c-fos induction by ionomycin, but not by serum or phorbol ester. ML-7, an inhibitor of the CaMK-related myosin light-chain kinase (MLCK), was without effect. Heparin (1 microg/ml) suppressed ionomycin-dependent c-fos induction. It was without effect on [Ca2+]i, but inhibited the development of autonomous CaMK II activity. However, when heparin was added to the CaMK II assay solution in vitro, it was without effect on autonomous activity. Furthermore, heparin did not prevent full activation of CaMK II by the Ca2+-calmodulin complex in vitro. Heparin did not affect myosin light-chain phosphorylation or RMC contraction, processes mediated by MLCK. We conclude that ionomycin induces c-fos in RMCs through the CaMK II pathway, and that heparin prevents CaMK II activation by an indirect process mediated by other cell components. Heparin does not affect activation of the closely related CaMK, MLCK.
