Vitamin D resistance in RAS-transformed keratinocytes:: Mechanism and reversal strategies

Human retinoid X receptor alpha (hRXR alpha) plays a critical role in DNA binding and transcriptional activity through its heterodimeric association with several members of the nuclear receptor superfamily, including the vitamin D receptor (VDR), Several cancer cell lines derived from different tissues have been shown to be resistant to the growth-inhibitory action of 1,25-dihydroxyvitamin D-3 [1,25(OH)(2)D-3], the biologically active metabolite of vitamin D-3, Here we show that in RAS-transformed keratinocytes, Ser260 of hRXRa is phosphorylated through the RAS-RAF-MAP kinase cascade. This phosphorylation event results in the inhibition of vitamin D signaling via VDR/hRXR alpha heterodimers, Strategies to reverse this resistance include the use of the MAP kinase inhibitor, PD098059, and a non-phosphorylatable hRXR alpha mutant, Ala260, which completely abolishes RXR phosphorylation and restores the function of both 1,25(OH),D, and a specific RXR ligand, LG1069 (4-[1-(5,6,7,8-tetrahydro-3,5,5,8,8-pentmethyl- 2-naphtalenyl)ethenyl]-benzoic acid). In addition, we show that a vitamin D analog with low calcemic activity (EB1089) is more potent than 1,25(OH)(2)D-3 in inhibiting cancer cell growth in this system. Targeted therapy with selective analogs such as EB1089, in combination with the inhibition of phosphorylation of the RXR, could play a critical role in the development of strategies for cancer treatment. (C) 2001 by Radiation Research Society.