GAP-JUNCTION DISASSEMBLY AND CONNEXIN-43 DEPHOSPHORYLATION INDUCED BY18-BETA-GLYCYRRHETINIC ACID

Cap-junction channels connect the interiors of adjacent cells and tan be arranged into aggregates or plaques consisting of hundreds to thous ands of channel particles. The mechanism of channel aggregation into p laques and whether plaques can disaggregate are not known. Many carcin ogenic and tumor-promoting chemicals have been identified that inhibit cell-cell gap-junctional coupling. Here, we provide morphological evi dence that 18 beta-glycyrrhetinic acid (18 beta-GA), a saponin isolate d from licorice root that is an inhibitor of gap-junctional communicat ion, caused the disassembly of gap-junction plaques in WB-F344 rat liv er epithelial cells. This effect was dose (5-40 mu M) and time depende nt (1-4 h treatment). Gap-junction channels in WB-F344 cells are compr ised of connexin 43 (Cx43), and the protein is phosphorylated to a spe cies known as Cx43-P2 coincident with the assembly of channels into pl aques. Consistent with this, the disassembly of plaques induced by 18 beta-GA was correlated with decreases in Cx43-P2 levels and increases in nonphosphorylated Cx43. Biochemical evidence indicated that these c hanges in the P2 and NP forms of Cx43 represented 18 beta-GA-induced d ephosphorylation of Cx43-P2 and not its degradation or the inhibition of Cx43-NP phosphorylation. Okadaic acid and calyculin A, which are in hibitors of type 1 and type 2A protein phosphatases, prevented the dep hosphorylation of Cx43, suggesting that one or both of these phosphata ses were involved in Cx43 dephosphorylation. These data indicate that 18 beta-GA causes type 1 or type 2 A protein phosphatase-mediated Cx43 dephosphorylation coincident with the disassembly of gap-junction pla ques. (C) 1996 Wiley-Liss, Inc.