Previous studies from our laboratory have found glypican 3 (GPC3) as a negative regulator of growth. CD81 was found to be a binding partner for GPC3, and its expression and co-localization with GPC3 increased at the end of hepatocyte proliferation. However, the mechanisms through which these two molecules might regulate liver regeneration are not known. We tested the hypothesis that GPC3 down-regulates the hedgehog (HH) signaling pathway by competing with patched-1 for HH binding. We found decreased GPC3-Indian HH binding at peak proliferation in mice followed by increase in glioblastoma 1 protein (effector of HH signaling). We performed a yeast two-hybrid assay and identified hematopoietically expressed homeobox (Hhex, a known transcriptional repressor) as a binding partner for CD81. We tested the hypothesis that Hhex binding to CD81 keeps it outside the nucleus. However, when GPC3 binds to CD81, CD81-Hhex binding decreases, resulting in nuclear translocation of Hhex and transcriptional repression. In support of this, we found decreased GPC3-CD81 binding at hepatocyte proliferation peak, increased CD81-Hhex binding, and decreased nuclear Hhex. GPC3 transgenic mice were used as an additional tool to test our hypothesis. Overall, our data suggest that GPC3 down-regulates cell proliferation by binding to HH and down-regulating the HH signaling pathway and binding with CD81, thus making it unavailable to bind to Hhex and causing its nuclear translocation.
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