BIPHASIC MODULATION OF PROTEIN-KINASE-C (PKC) ACTIVITY BY POLYCHLORINATED DIBENZO-P-DIOXINS (PCDDS) IN SERUM-DEPRIVED RAT AORTIC SMOOTH-MUSCLE CELLS

Previous studies in this laboratory have shown that benzo(a)pyrene (Ba P) modulates protein kinase C (PKC)-mediated phosphorylation of aortic smooth muscle cell (SMC) proteins. This observation is consistent wit h the ability of other aromatic hydrocarbons (AHs), such as 2,3,7,8-te trachlorodibenzo-p-dioxin (TCDD), to modulate kinase activities in cel ls of hepatic, testicular, and thymic origin. Because all these chemic als share the ability to bind the aryl hydrocarbon receptor (AhR), the present studies were conducted to determine if changes in PKC activit y by AHs conform with established structure-activity relationships. Ex periments were conducted to examine the effects of TCDD, 2,3,7,8-tetra chlorodibenzofuran (TCDF), and 2,8-dichlorodibenzodioxin (DCDD) on the phosphorylation of exogenous histone type-III under basal and PKC-act ivating conditions. These congeners exhibit both high (TCDD and TCDF) and low (DCDD) AhR agonist activities. Measurements of kinase activity were conducted in the cytosolic and particulate fractions of growth-a rrested (i.e., serum-deprived) cultured rat aortic SMCs incubated with 10 nM TCDD, TCDF, and DCDD for 0.5, 12, or 24 hours. No changes in ba sal kinase activity were induced by these chemicals at any of the time s tested. Significant decreases in cytosolic and particulate PKC activ ity relative to controls were observed upon exposure of SMCs for 0.5 h ours to 10 nM TCDD, TCDF, and DCDD. In contrast, SMCs exposed to TCDD and TCDF for 12 hours exhibited a significant increase in PKC activity in both cytosolic and particulate fractions. The PKC activity in cell s exposed to DCDD for 12 hours was not altered. Prolonged exposure of SMCs to 10 nM TCDD, TCDF, and DCDD for 24 hours decreased PKC activity in the cytosolic fraction, while only TCDD and TCDF decreased particu late PKC activity. These data show that PKC activity is modulated diff erentially as a function of time in SMCs exposed to TCDD and related c ompounds. Collectively, the patterns of histone phosphorylation induce d by these chemicals in rat aortic SMCs suggest that modulation of C-k inase activity involves both receptor-independent and receptor-related events.