The tumor suppressor Fbw7 and the Vitamin D receptor are mutual cofactors in protein turnover and transcriptional regulation

Abstract

The E3 ligase and tumor suppressor FBW7 targets drivers of cell-cycle progression such as the oncogenic transcription factor c-MYC, for proteasomal degradation. Vitamin D signaling regulates c-MYC expression and turnover in vitro and in vivo, which is highly significant as epide-miologic data link vitamin D deficiency to increased cancer incidence. We hypothesized that FBW7 and the vitamin D receptor (VDR) controlled each other's function as regulators of protein turnover and gene transcription, respectively. We found that hormonal 1,25-dihy-droxyvitamin D3 (1,25D) rapidly enhanced the interaction of FBW7 with VDR and with c-MYC, whereas it blocked FBW7 binding to c-MYC antagonist MXD1. 1,25D stimulated the recruitment of FBW7, SCF complex subunits, and ubiquitin to DNA-bound c-MYC, consistent with 1,25D-regulated c-MYC degradation on DNA. 1,25D also accelerated the turnover of other FBW7 target proteins such as Cyclin E, c-JUN, MCL1, and AIB1, and, importantly, FBW7 depletion attenuated the 1,25D-induced cell-cycle arrest. Although the VDR contains a consensus FBW7 recognition motif in a VDR-specific insertion domain, its mutation did not affect FBW7–VDR interactions, and FBW7 ablation did not stabilize the VDR. Remarkably, however, FBW7 is essential for optimal VDR gene expression. In addition, the FBW7 and SCF complex subunits are recruited to 1,25D-induced genes and FBW7 depletion inhibited the 1,25D-dependent transactivation. Collectively, these data show that the VDR and FBW7 are mutual cofactors, and provide a mechanistic basis for the cancer-preventive actions of vitamin D.