Binding of protein kinase C to napthalenesulfonamide- and phenothiazine-agarose columns: evidence for direct interactions between protein kinase C and cationic amphiphilic inhibitors of the enzyme

Modulation of Calmodulin and Protein Kinase C Activities by Pencillium Mycotoxins

Calmodulin (CaM), a calcium-binding protein, is found in high concentrations in mammalian brain where it plays a pivotal role in a large number of cellular functions. Protein kinase C (PKC), a multifunctional cytosolic enzyme, in the presence of both Ca2+ and phospholipids, transduce extracellular signals into intracellu-lar events. Both CaM and PKC are partially involved in maintaining Ca2+ homeostasis in the cell. Any fluctuations in the intracel-lular Ca2+ can modulate cellular functions and may contribute to neuronal dysfunction. Hence, the present investigation was initiated to study the effects of some selected penicillium (naturally occurring tremorgenic) mycotoxins like secalonic acid, citreoviridin, and verruculogen on CaM activity, active conformation of CaM and PKC activity. Stimulation of CaM-deflcient bovine brain 3′-5′ phosphodieste rase (PDE) indicated CaM activity. The modification of CaM active conformation was studied by the binding of fluorescent probe N-phenyl-1-napthylamine (NPN) to CaM. Alterations in the fluorescence of dansyl-CaM was used to study the effect of these compounds on complex formation between CaM and PDE. Rat brain cytosolic PKC was studied using 32P-ATP as a measure of altered protein phosphorylation. The concentrations of mycotoxins used were in the range of 10 to 50 μM. All three mycotoxins inhibited CaM-stimulated PDE activity in a concentration-dependent manner. Citreoviridin and secalonic acid inhibited NPN fluorescence and Ca2+-dependent complex formation of dansyl-CaM and PDE. The IC50 values for NPN fluorescence of citreoviridin and secalonic acid were 13 μM and 19 μM respectively. However, verruculogen showed little effect on NPN fluorescence and the Ca2+-dependent complex formation of dansyl-CaM and PDE. These mycotoxins also inhibited PKC activity in a concentration-dependent manner with IC50 values of 19.8, 25.7, and 38.4 μM for secalonic acid, citreoviridin, and verruculogen, respectively. The results of our study suggest that these mycotoxins at very low concentrations are interacting with CaM and PKC. Such an effect could lead to impairment of neurotransmission and result in neurotoxicity.

Biology of the protein kinase C family

Protein kinase C (PKC) is composed of a family of isozymes that transduce signals of certain hormones, growth factors, lectins, and neurotransmitters. This review addresses the role of PKC in the regulation of cellular proliferation and its disorders. PKC is directly activated in vivo by the second messenger diacylglycerol, a lipid produced by phospholipase C-catalyzed hydrolysis of phosphatidylinositol and polyphosphoinositides. Diacylglycerol activates PKC by reducing the enzyme's requirement for Ca2+. Phorbol ester tumor promoters and related agents potently activate PKC by a mechanism analogous to that of diacylglycerol, providing evidence that PKC activation is a critical event in tumor promotion. However, the role of PKC activation in tumor promotion is not entirely clear. For example, bryostatin is a potent PKC activator that antagonizes phorbol ester-mediated tumor promotion, and mezerein is a second-stage tumor promoter that potently activates PKC. In addition to studies concerned with tumor promotion, studies of oncogene action also indicate a role for PKC in carcinogenesis. A number of plasma membrane-associated oncogene products and related proteins are PKC substrates, and PKC activation leads to induction of the expression of oncogenes that code for nuclear proteins. PKC is implicated in human breast and colon carcinogenesis. Tumor-promoting bile acids activate PKC, and PKC expression studies in rat colonic epithelial cells and human breast cancer cells indicate a positive role for PKC in the proliferation of the cells. Altered expression of PKC in human colon and breast tumors indicates that PKC isozymes may be useful markers for these diseases.

Binding of protein kinase C to N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide through its ATP binding site

Protein kinase C (PKC) is a Ca2+- and phospholipid-dependent protein kinase which has been implicated as a key enzyme in the regulation of cellular growth. The naphthalenesulfonamide W7 [N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide] is representative of a number of cationic amphiphilic inhibitors of PKC which appear to inhibit PKC by interacting with the acidic phospholipid cofactor of the enzyme, according to kinetic studies. In a previous report, we demonstrated that PKC binds directly to W7 when the naphthalenesulfonamide is immobilized on agarose. In the present report, we have defined the mechanism of the binding of PKC to W7-agarose, and its relevance to the inhibitory mechanism of the naphthalenesulfonamide. We demonstrate that PKC bound W7-agarose through the catalytic domain of the enzyme. An active catalytic fragment of PKC was generated by limited proteolysis, and we found that this fragment bound W7-agarose and coeluted with intact PKC upon the addition of Triton X-100. W7 inhibited PKC activity by two different mechanisms. As previously reported, W7 inhibited PKC by interacting with the phospholipid cofactor of the enzyme (IC50 = 260 microM). However, at higher concentrations of W7, we found that this naphthalenesulfonamide inhibited PKC by serving as a competitive inhibitor with respect to the substrate ATP, according to a kinetic analysis of the inhibition of the active catalytic fragment of PKC by W7. W7 inhibited the active catalytic fragment of PKC as well as PKC-catalyzed phosphorylation of protamine sulfate, a reaction which is independent of Ca2+ and phospholipid, with similar potencies. Consistent with the kinetic evidence that W7 serves as a competitive inhibitor of PKC with respect to ATP, we found that, in the presence of 10 mM MgCl2, 1 mM ATP was sufficient to elute PKC from W7-agarose. Thus, naphthalenesulfonamide PKC inhibitors may include both agents which primarily function by interacting with the phospholipid cofactor of the enzyme and agents which primarily serve as active site inhibitors of PKC.