An Alternative Retinoic Acid Responsive Stra6 Promoter Regulated in Response to Retinol Deficiency [Gene Regulation]

December 28th, 2014 by Laursen, K. B., Kashyap, V., Scandura, J., Gudas, L. J.

Cellular uptake of vitamin A (retinol) is essential for many biological functions. The Stra6 protein binds the serum retinol binding protein, RBP4, and acts in conjunction with the enzyme lecithin:retinol acyltransferase (LRAT) to facilitate retinol uptake in some cell types. We show that in embryonic stem (ES) cells and in some tissues the Stra6 gene encodes two distinct mRNAs transcribed from two different promoters. While both are all-trans retinoic acid (RA) responsive in ES cells, the downstream promoter contains a half-site RA response element (RARE) and drives an approximately 13-fold, RA-associated increase in luciferase reporter activity. We employed CRISPR-Cas9 genome editing to show that the endogenous RARE is required for RA-induced transcription of both Stra6 isoforms. We further demonstrate that in ES cells 1) both RARγ and RXRα are present at the Stra6 RARE; 2) RA increases co-activator p300 (KAT3B) binding and histone H3K27 acetylation at both promoters; 3) RA decreases Suz12 levels and histone H3K27 trimethylation epigenetic marks at both promoters; and 4) these epigenetic changes are diminished in the absence of RARγ. Both Stra6 transcripts are highly expressed in brain in WT, but at low levels in RARγ-/- mice. In the brain both Stra6L and Stra6S levels decrease in VAD mice. In contrast, the longer Stra6 transcript (Stra6L) levels are greatly increased in the kidney upon vitamin A deficiency (VAD), whereas shorter transcript (Stra6S) levels are decreased. Our data show that kidneys respond to retinol deficiency by differential Stra6 promoter usage, which may play a role in the retention of retinol when vitamin A is low.