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

Human retinoid X receptor alpha (hRXR alpha) plays a critical role in DNA b inding and transcriptional activity through its heterodimeric association w ith several members of the nuclear receptor superfamily, including the vita min D receptor (VDR), Several cancer cell lines derived from different tiss ues 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 pho sphorylation 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 r estores 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]-benz oic acid). In addition, we show that a vitamin D analog with low calcemic a ctivity (EB1089) is more potent than 1,25(OH)(2)D-3 in inhibiting cancer ce ll growth in this system. Targeted therapy with selective analogs such as E B1089, in combination with the inhibition of phosphorylation of the RXR, co uld play a critical role in the development of strategies for cancer treatm ent. (C) 2001 by Radiation Research Society.