Shedding Light on FoxO Function

Science Signaling  28 Sep 2010:
Vol. 3, Issue 141, pp. ec295
DOI: 10.1126/scisignal.3141ec295

Vitamin D, the “sunshine vitamin” long known for its role in calcium homeostasis, has more recently been implicated in protecting against some forms of cancer. Noting that the target genes regulated by FoxO-family transcription factors, which act as tumor suppressors, overlap with those regulated by the hormonal form of Vitamin D [1,25-dihydroxyvitamin D (1,25D)] through its nuclear receptor (VDR), An et al. investigated the relationship between 1,25D- and FoxO-mediated signaling. Analyses of a squamous cell carcinoma line that undergoes growth arrest in response to 1,25D (SCC25 cells) revealed that 1,25D acted through the VDR to increase or decrease the abundance of the transcripts of various FoxO target genes in a manner consistent with their regulation by FoxO proteins. 1,25D slowly increased the abundance of FoxO proteins and inhibited insulin-dependent export of FoxO3a from the nucleus. Chromatin immunoprecipitation analysis revealed that 1,25D enhanced association of FoxO3a with the promoters of target genes; this was more rapid than the increase in FoxO3a protein but paralleled a decrease in FoxO3a phosphorylation. FoxO dephosphorylation is enhanced by its deacetylation, and a combination of coimmunoprecipitation analysis and glutathione S-transferase (GST) pull-down assays revealed ligand-independent association of VDR with FoxO proteins and protein phosphatase 1, as well as its partially 1,25D-dependent association with the deacetylase sirtuin 1 (Sirt1). Pharmacological inhibition of phosphatase activity inhibited 1,25D-dependent regulation of genes targeted by both VDR and FoxO (VDR/FoxO target genes), as did Sirt1 knockdown; indeed, the latter blocked a 1,25D-dependent increase in VDR binding to VDR/FoxO target gene promoters. Whereas knockdown of FoxO proteins failed to affect 1,25D-dependent regulation of VDR-specific target genes, it attenuated 1,25D-dependent regulation of VDR/FoxO target genes. Moreover, FoxO3a knockdown blocked the ability of 1,25D to inhibit SCC25 cell proliferation. Thus, 1,25D acts through the VDR to regulate FoxO function, a role that may be pertinent to its ability to protect against cancer.

B.-S. An, L. E. Tavera-Mendoza, V. Dimitrov, X. Wang, M. R. Calderon, H.-J. Wang, J. H. White, Stimulation of Sirt1-regulated FoxO protein function by the ligand-bound vitamin D receptor. Mol. Cell. Biol. 30, 4890–4900 (2010). [Abstract] [Full Text]