Search

TGR5, the G protein-coupled bile acid receptor that transmits bile acid signaling into a cell functional response via the intracellular cAMP signaling pathway, is expressed in human and rodent cholangiocytes. However, detailed information on the localization and function of cholangiocyte TGR5 is limited. We demonstrated that in human (H69 cells) and rat cholangiocytes, TGR5 is localized to multiple, diverse subcellular compartments, with its strongest expression on the apical plasma, ciliary, and nuclear membranes. To evaluate the relationship between ciliary TGR5 and the cholangiocyte functional response to bile acid signaling, we used a model of ciliated and nonciliated H69 cells and demonstrated that TGR5 agonists induce opposite changes in cAMP and ERK levels in cells with and without primary cilia. The cAMP level was increased in nonciliated cholangiocytes but decreased in ciliated cells. In contrast, ERK signaling was induced in ciliated cholangiocytes but suppressed in cells without cilia. TGR5 agonists inhibited proliferation of ciliated cholangiocytes but activated proliferation of nonciliated cells. The observed differential effects of TGR5 agonists were associated with the coupling of TGR5 to Gαi protein in ciliated cells and Gαs protein in nonciliated cholangiocytes. The functional responses of nonciliated and ciliated cholangiocytes to TGR5-mediated bile acid signaling may have important pathophysiological significance in cilia-related liver disorders (i.e., cholangiociliopathies), such as polycystic liver disease. In summary, TGR5 is expressed on diverse cholangiocyte compartments, including a primary cilium, and its ciliary localization determines the cholangiocyte functional response to bile acid signaling.

Primary cilia are multisensory organelles recently found to be absent in some tumor cells, but the mechanisms of deciliation and the role of cilia in tumor biology remain unclear. Cholangiocytes, the epithelial cells lining the biliary tree, normally express primary cilia and their interaction with bile components regulates multiple processes, including proliferation and transport. Using cholangiocarcinoma as a model, we found that primary cilia are reduced in cholangiocarcinoma by a mechanism involving histone deacetylase 6 (HDAC6). The experimental deciliation of normal cholangiocyte cells increased the proliferation rate and induced anchorage-independent growth. Furthermore, deciliation induced the activation of mitogen-activated protein kinase and Hedgehog signaling, two important pathways involved in cholangiocarcinoma development. We found that HDAC6 is overexpressed in cholangiocarcinoma and overexpression of HDAC6 in normal cholangiocytes induced deciliation and increased both proliferation and anchorage-independent growth. To evaluate the effect of cilia restoration on tumor cells, we targeted HDAC6 by short hairpin RNA (shRNA) or by the pharmacologic inhibitor, tubastatin-A. Both approaches restored the expression of primary cilia in cholangiocarcinoma cell lines and decreased cell proliferation and anchorage-independent growth. The effects of tubastatin-A were abolished when cholangiocarcinoma cells were rendered unable to regenerate cilia by stable transfection of IFT88-shRNA. Finally, inhibition of HDAC6 by tubastatin-A also induced a significant decrease in tumor growth in a cholangiocarcinoma animal model. Our data support a key role for primary cilia in malignant transformation, provide a plausible mechanism for their involvement, and suggest that restoration of primary cilia in tumor cells by HDAC6 targeting may be a potential therapeutic approach for cholangiocarcinoma., (©2013 AACR.)