Colon OCTN2 gene expression is up-regulated by peroxisome proliferator-activated receptor gamma in humans and mice and contributes to local and systemic carnitine homeostasis

In the large intestine organic cation transporter type-2 (OCTN2) is recognized as a transporter of compounds such as carnitine and colony sporulation factor, promoting health of the colon intestinal epithelium. Recent reports suggest that OCTN2 expression in small intestine is under control of peroxisome proliferator-activated receptor-alpha (PPAR alpha). However, PPAR alpha contribution to colonic OCTN2 expression remains controversial. Here we examined the transcriptional regulation of colon OCTN2 gene by PPAR alpha. To exclude any additional modulation of other PPAR to OCTN2 expression, we used both in vivo and in vitro PPAR-null models and specific PPAR inhibitors. The PPAR gamma agonists thiazolidinediones increased both OCTN2 mRNA and protein expression in colonic epithelial cell lines independently by PPAR alpha expression. The induction was blocked only by PPAR gamma antagonists or by gamma ORF4, a PPAR gamma isoform with dominant negative activity, suggesting a PPAR gamma-dependent mechanism. A conserved noncanonical PPAR-responsive element was found by computational analysis in the first intron of human OCTN2 gene and validated by EMSA assay. Promoter-reporter assays further confirmed transcriptional functionality of the putative PPAR response element, whereas selective mutation caused complete loss of responsiveness to PPAR gamma activation. Finally, adenovirus-mediated overexpression of constitutively active PPAR gamma mutant increased colon OCTN2 expression in PPAR alpha(-/-) mice. Interestingly, animals overexpressing colon PPAR gamma showed a significant increase in plasma carnitine, thus demonstrating the functional contribution of large intestine to systemic carnitine homeostasis. This study reveals a PPAR gamma-dependent absorption machinery in colon that is likely involved in the health of colon epithelium, in the microbiota-host interactions and in the absorption of nutraceuticals and drugs.