Disruption of gap junctional communication by the platelet-derived growth factor is mediated via multiple signaling pathways

The platelet-derived growth factor (PDGF) mediates its cellular functions v ia activation of its receptor tyrosine kinase followed by the recruitment a nd activation of several signaling molecules. These signaling molecules the n initiate specific signaling cascades, finally resulting in distinct physi ological effects. To delineate the PDGF signaling pathway responsible for t he disruption of gap junctional communication (GJC), wild-type PDGF recepto r beta (PDGFR beta) and a series of PDGFR beta mutants were expressed in T5 1B rat liver epithelial cells. In cells expressing wild-type PDGFR beta, PD GF induced disruption of GJC and phosphorylation of a gap junctional protei n, connexin-43 (Cx43), which required activation of mitogen-activated prote in kinase, although involvement of additional factors was also evident. In the F5 mutant lacking binding sites for phosphatidylinositol 3-kinase, GTPa se-activating protein, SHP-2, and phospholipase C gamma 1 (PLC gamma 1), PD GF induced mitogen-activated protein kinase, but failed to affect GJC or Cx 43, indicating involvement of additional signals presumably initiated by on e or more of the mutated binding sites. Examination of the single-site muta nts revealed that PDGF effects were not mediated via a single signaling com ponent. This was confirmed by the "add-back" mutants, which showed that res toration of either SHP-2 or PLC gamma 1 binding was sufficient to propagate the GJC inhibitory actions of PDGF. Further analysis showed that activatio n of PLC gamma 1 is involved in Cx43 phosphorylation, which surprisingly fa iled to correlate with GJC blockade. The results of our study demonstrate t hat PDGF-induced disruption of GJC can be mediated by multiple signaling pa thways and requires participation of multiple components.