Hypoxia Compromises the Differentiation of Human Osteosarcoma Cells to CAR-R, a Hydroxylated Derivative of Lithocholic Acid and Potent Agonist of the Vitamin D Receptor.

The active metabolite of vitamin D3, calcitriol (1,25D), is widely recognised for its direct anti-proliferative and pro-differentiation effects. However, 1,25D is calcaemic, which restricts its clinical use for cancer treatment. Non-calcaemic agonists of the vitamin D receptor (VDR) could be better candidates for cancer treatment. In this study, we examined the influence of the hydroxylated lithocholic acid derivative CAR-R on osteosarcoma (OS) cell (MG63) growth and differentiation. Treatment of MG63 cells with CAR-R inhibited growth under conventional and hypoxic conditions. Co-treating cells with CAR-R and a lysophosphatidic acid (LPA) analogue resulted in their differentiation, as supported by synergistic increases in alkaline phosphatase (ALP) activity. Under hypoxic conditions, however, this differentiation response was attenuated. The importance of observed increases in hypoxia inducible factors (HIFs) were investigated through targeted disruption using pharmacological and genetic approaches. Disruption elicited a reduction in ALP activity, suggesting an important role for HIFs in OS differentiation. Finally, we examined the expression of the VDR protein. Hypoxic MG63s expressed less VDR, with the levels increasing with CAR-R exposure. Whilst these findings are encouraging, future studies aimed at bolstering the pro-differentiating effect of CAR-R under hypoxic conditions are warranted if this agent is to gain traction in the treatment of OS.