Background: Gap junctional intercellular communication (GJIC) plays an important role in the regulation of cell growth, migration, and differentiation. Ultrastructural and histochemical studies indicate the existence of a high density of gap junctions among mesangial cells (MCs), but little is known about their regulation. Because of the close link between growth and GJIC, we examined how platelet-derived growth factor (PDGF) may affect GJIC in cultured MCs.
Methods: MCs were exposed to PDGF in the presence or absence of phosphatidylinositol 3' kinase (PI3K) inhibitors, and GJIC was evaluated by the transfer of Lucifer yellow. The gap junction protein connexin43 (Cx43) was examined by immunohistochemistry, immunoprecipitation, and Western blot.
Results: The addition of PDGF into MC culture caused a rapid and transient inhibition of GJIC, with maximal inhibition (80%) occurring 15 minutes after PDGF exposure and returning to control levels after 90 minutes. This action of PDGF could be largely prevented by pretreatment of MCs with the PI3K inhibitor LY294002. Immunochemical staining showed that PDGF did not alter the localization and distribution of Cx43. Immunoprecipitation studies demonstrated that PDGF induced a rapid and transient increase of tyrosine phosphorylation of Cx43 protein, which was dose dependent and in accordance with the time course of the disruption of GJIC. PDGF also elicited activation of extracellular signal-regulated kinase (ERK). Using two structurally unrelated PI3K inhibitors, wortmanin and LY294002, both tyrosine phosphorylation of Cx43 and activation of ERK stimulated by PDGF were largely blocked.
Conclusion: These results suggest that PDGF abrogates GJIC function in MCs via the PI3K-dependent signaling pathway. Disruption of GJIC by PDGF could be one mechanism by which PDGF modulates MC behavior. Participation of PI3K in the regulation of GJIC demonstrates the complex coordination of molecular events that accompany MC mitogenesis.