Recently, the JQ1 buy gut specific homeodomain transcription factor ISX has been identified as a putative repressor of intestinal SR-BI expression (15). SR-B1 is normally found on the apical surfaces of absorptive epithelial cells, and its levels decrease from the duodenum to ileum (6, 16, 17), in contrast to the increasing duodenum-ileum gradient for ISX (15). In ISX-deficient mice, SR-BI expression is significantly enhanced and its expression extends to more distal parts of the intestine (15). ISX also has been shown to repress the intestinal expression of the carotenoid-15,15��-monooxygenase, BCMO1 (18). In intestinal enterocytes, BCMO1 acts downstream of SR-B1 and converts absorbed ��,��-carotene to vitamin A-aldehyde (for recent review, see ref. 19).
This compound can be metabolized into the unique series of endogenous vitamin A metabolites, including retinoic acid (RA). RA is a hormone-like compound that regulates gene expression by activating nuclear receptors termed retinoic acid receptors (RARs), which are ligand-controlled transcription factors that function as heterodimers with the retinoid X receptor (RXR). RAR-RXR heterodimers bind to regulatory regions of target genes harboring response elements (REs) composed of two direct repeats of the motif 5��-PuG(G/T)TCA spaced by 2 or 5 bp (DR-2, DR-5), and they activate gene expression on ligand binding (20). Animal model data also suggest that dietary ��,��-carotene and its retinoid metabolites repress intestinal BCMO1 enzymatic activity and that this regulation, involving RA and RARs, appears to be exerted at the transcriptional level (21, 22).
ISX expression also is influenced by dietary retinoids, being low in vitamin A deficiency and high in vitamin A sufficiency (18). These findings indicate that the transcription factor ISX lies at the intersection between the retinoid signaling pathway and the regulation of intestinal lipid absorption, thus making it a promising therapeutic target for treating patients with dyslipidemia. However, the molecular mechanisms involved in the crosstalk between retinoid signaling and ISX activity have yet to be elucidated in functional detail. In addition, the putative role of ISX in controlling lipid absorption via SR-BI and vitamin A homeostasis lacks experimental testing in animal models.
To address these questions, we analyzed the role of ISX and retinoid signaling for the regulation of intestinal lipid absorption using both human colonic cell lines and mouse models with impaired ��,��-carotene and retinoid metabolism. MATERIALS AND METHODS All reagents unless indicated were purchased from Sigma Chemical Co. (Portland, OR, USA). Platinum Pfx polymerase, Prolong Dacomitinib Gold antifade mounting medium, mammalian expression vector pCDNA 3.1 V5/His-TOPO, and TOP10 competent cells were obtained from Invitrogen/Molecular Probes (Carlsbad, CA, USA).