Farnesoid X receptor knockdown provides significant growth inhibition in hepatocellular carcinoma cells while it does not interfere with the proliferation of primary human hepatocyte-derived cells

Transient Receptor Potential Melastatin 8, a sensor of cold temperatures mediates expression of cyclin-dependent kinase inhibitor, p21/Cip1, a regulator of epidermal cell proliferation

Transient Receptor Potential Melastatin 8 (TRPM8) is a calcium-permeable, non-selective cation channel of the transient receptor potential superfamily, required for the transduction of moderate cold temperatures. TRPM8 is also known to regulate proliferation of prostate, pancreatic, breast, and melanoma carcinoma cells. Here, we examined a key factor in the regulation of TRPM8-mediated proliferation of epidermal cells, which are directly affected by cold temperatures. Experiments involving knockdown and ectopic expression of TRPM8 in normal keratinocyte HaCaT and squamous carcinoma SAS cells suggest that TRPM8 inhibits cell proliferation by upregulating the expression of cyclin-dependent inhibitor p21/Cip1. Whereas these findings were observed in the absence of an endogenous agonists, additions of the synthetic TRPM8 agonist icilin reduced DNA synthesis in HaCaT cells but stimulated that in SAS cells by altering p21/Cip1 levels in a TRPM8-independent manner, indicating that icilin poses a risk of stimulating carcinoma cell proliferation. Unexpectedly, the TRPM8 blocker, used for the treatment of overactive bladder and bladder pain, N-(3-aminopropyl)-2-{[(3-methylphenyl) methyl] oxy}-N-(2-thienylmethyl) benzamide hydrochloride salt (AMTB) reduced DNA synthesis by upregulating p21/Cip1 expression. However, another TRPM8 blocker, N-(4-Tertiarybutylphenyl)-4-(3-chloropyridin- 2-yl) tetrahydropyrazine-1 (2H)-carbox-amide (BCTC), stimulated DNA synthesis by downregulating p21/Cip1 expression, indicating that it may pose a risk of carcinogenesis associated with dysregulated cell cycles when used to treat overactive bladder and bladder pain.

Farnesoid X receptor and liver X receptors regulate Oct3/4 expression by multiple feedback regulating system in normal renal-derived cells and renal adenocarcinoma cells

In this study, we found that nuclear receptors FXR and LXR (originally characterized as regulatory factors involved in cholesterol/bile acid homeostasis) regulate the expression of Oct3/4, a marker for cell differentiation, in both normal renal-derived cell line HK-2 and renal adenocarcinoma cell line ACHN. Down-regulation of Oct3/4 expression by activating FXR and LXR occurs only in normal renal cell-derived HK-2 cells. We also found that the RNA-binding protein, ELAVL2, oppositely regulates Oct3/4 expressions in HK-2 and ACHN cells. Moreover, we revealed that LXR-alpha and LXR-beta regulate each other's expression. Although an LXR-beta-specific agonist is assumed to be the basis for an anti-arteriosclerotic drug that only stimulates reverse cholesterol transport, our findings show that the development of such an anti-arteriosclerotic drug would require further elucidation of the complex mechanism of LXR-alpha and LXR-beta regulation.

Farnesoid X receptor regulates the growth of renal adenocarcinoma cells without affecting that of a normal renal cell-derived cell line

The farnesoid X receptor (FXR) is a bile acid-activated nuclear receptor which is abundant in the liver, intestine, and kidney. FXR is a pivotal factor in cholesterol/bile acid homeostasis but is involved in the growth of hepatocellular carcinoma cells. In the present study, we investigated whether FXR is also involved in the growth of renal adenocarcinoma cells. The cell growth of renal adenocarcinoma cell line ACHN was inhibited by FXR knockdown and stimulated by FXR ligand, while that of a normal renal cell-derived cell line, HK-2, was not affected. The carcinoma-specific stimulation of cell growth by FXR was found to arise from down-regulation of p53 and p21/Cip1 mRNA expression. Our study showed that FXR stimulates proliferation of renal adenocarcinoma cells and that FXR knockdown is useful for growth suppression of renal adenocarcinoma without cytotoxicity to normal renal cells.

AU-1 from Agavaceae plants causes transient increase in p21/Cip1 expression in renal adenocarcinoma ACHN cells in an miR-34-dependent manner

Here, we show that AU-1, spirostanol saponin isolated from Agavaceae plants, causes a transient increase in cyclin-dependent kinase inhibitor (CDKI) p21/Cip1 through the upregulation of miRNAs, miR-34 and miR-21. AU-1 stimulated p21/Cip1 expression without exerting cytotoxicity against different types of carcinoma cell lines. In renal adenocarcinoma ACHN cells, AU-1 transiently elevated the expression level of p21/Cip1 protein without marked increases in p21/Cip1 mRNA levels. Rapid and transient increases in miR-34 and miR-21, both of which are known to upregulate p21/Cip1, were observed in AU-1-treated cells. Inhibitor for miR-34 and for miR-21 significantly blocked the AU-1-caused increase in p21/Cip1, indicating that elevation of p21/Cip1 protein by AU-1 is dependent on these microRNAs. We further clarified that NAD-dependent deacetylase SIRT1, a direct target of miR-34, is decreased by the treatment with AU-1. Furthermore, we found that SIRT1-knockdown increases p21/Cip1 protein levels in an miR-21-dependent manner. On the other hand, ectopic expression of p21/Cip1 resulted in the lowered expression of miR-34 and miR-21, suggesting that reciprocal regulation exists between p21/Cip1 and these miRNAs. We propose that the following feedback network composed of miR-34/SIRT1/miR-21/p21 is triggered by the treatment with AU-1: in cells treated with AU-1, transient elevation of miR-34 leads to the downregulation of SIRT1, thereby miR-21 is freed from SIRT1-dependent suppression. Then, elevated miR-21 upregulates p21/Cip1 protein, followed by the suppression of miR-34 expression.

The expanding role of the bile acid receptor farnesoid X in the intestine and its potential clinical implications

Knowledge about the role of farnesoid X receptor (FXR) in the intestine is rapidly expanding. In pre-clinical animal models of inflammatory bowel disease and bile duct ligation, FXR activation has proven to directly target the three pillars of intestinal homeostasis: intestinal permeability, inflammation and bacterial translocation. The protective role of FXR-ligands on this homeostasis has implications for many intestinal pathologies like inflammatory bowel disease, ischemia reperfusion injury, the metabolic syndrome, colon cancer and even diarrhea. In this review, we summarize the mechanisms by which FXR-activation exerts these protective effects and we discuss its potential clinical applications.