Gap junctional
intercellular communication (GJIC) plays a critical role in the regulation of tissue homeostasis and
carcinogenesis and is modulated by the levels,
subcellular localization, and
posttranslational modification of
gap junction proteins, the
connexins (Cx). Here, using oval cell-like rat
liver epithelial cells, we demonstrate that the
RNA-binding protein HuR promotes GJIC through two mechanisms. First, HuR silencing lowered the levels of
Cx43 protein and
Cx43 messenger RNA (
mRNA), and decreased
Cx43 mRNA half-life. This regulation was likely due to the direct stabilization of
Cx43 mRNA by HuR, because HuR associated directly with
Cx43 mRNA, a transcript that bears signature adenylate-uridylate-rich (AU-rich) and uridylate-rich (U-rich) sequences in its 3'-untranslated region. Second, HuR silencing reduced both
half-life and the levels of
beta-catenin mRNA, also a target of HuR; accordingly, HuR silencing lowered the levels of whole-cell and membrane-associated
beta-catenin.
Coimmunoprecipitation experiments showed a direct interaction between
beta-catenin and
Cx43.
Small interfering RNA (siRNA)-mediated depletion of
beta-catenin recapitulated the effects of decreasing HuR levels: it
attenuated GJIC, decreased
Cx43 levels, and redistributed
Cx43 to the
cytoplasm, suggesting that depletion of
beta-catenin in HuR-silenced cells contributed to lowering
Cx43 levels at the membrane. Finally, HuR was demonstrated to support GJIC after exposure to a
genotoxic agent,
doxorubicin, or an
inducer of differentiation processes, retinoic acid, thus pointing to a crucial role of HuR in the cellular response to stress and in physiological processes modulated by GJIC. Conclusion: HuR promotes gap junctional
intercellular communication in rat
liver epithelial cells through two related regulatory processes, by enhancing the expression of
Cx43 and by increasing the expression of
beta-catenin, which, in turn, interacts with
Cx43 and is required for proper positioning of
Cx43 at the
plasma membrane. (HEPATOLOGY 2009.).
