THE INHIBITORY EFFECTS OF BOLDINE, GLAUCINE, AND PROBUCOL ON TPA-INDUCED DOWN-REGULATION OF GAP JUNCTION FUNCTION - RELATIONSHIPS TO INTRACELLULAR PEROXIDES, PROTEIN-KINASE-C TRANSLOCATION, AND CONNEXIN-43 PHOSPHORYLATION

The naturally occurring antioxidant boldine and its di-methoxy analogue glucine, as well as the drug antioxidant probucol, all inhibit TPA-induced downregulation of gap junctional intercellular communication in WB-F344 rat liver epithelial cells in dose-dependent manners. The compounds were essentially 100% inhibitory to the effect of TPA (10 nM) at 50 mu M each. Analysis of the mechanism of the antitumor promotive action of these agents in vitro revealed that boldine and probucol (both at 10 mu M) totally inhibited the TPA-induced accumulation of intracellular oxidants. Additionally, boldine, glaucine, and probucol, each at 50 mu M, inhibited TPA-induced translocation of protein kinase C (PKC) to the particulate fraction of the cells, with concomitant inhibition of TPA-induced hyperphosphorylation of gap junctional connexin 43 (cx43) and TPA-induced internalisation of cx43 protein from the plasma membrane of the cells. None of the compounds inhibited the binding of (H-3)-PDBu to TPA-specific binding sites in the cells. The results indicate that antioxidant molecules, irrespective of structure, possess common antitumor promotive potential in this model of gap junctional intercellular communication. The data also indicate that the compounds may interfere with the promotive function of TPA, at least in part, by the destruction of oxidants within the cells. Xanthine oxidase was excluded as a major source of such intracellular oxidants because allopurinol (50 mu M) did not significantly affect either the accumulation of oxidants in the cells or the downregulation of gap junctional communication in response to TPA, Taken together, these data also suggest that TPA-induced oxidants play a role in the translocation of PKC to cellular membranes and it is at this level where the antioxidants may interfere in TPA-induced downregulation of gap junctional function.