Retinoids Upregulate Phosphoenolpyruvate Carboxykinase and Glyceroneogenesis in Human and Rodent Adipocytes1,2

Glyceroneogenesis is an important metabolic pathway for fatty acid reesterification in adipose tissue, thereby reducing fatty acid, release. Glyceroneogenesis and cytosolic phosphoenolpyruvate carboxykinase (PEPCK-C), which is the key enzyme in this pathway, are both regulated by a series of hormones and nutrients, among which all-trans retinoic acid (all-trans RA) is a transcriptional inducer of the PEPCK-C gene (Pck1). All-trans RA binds to the retinoic acid receptor (RAR) and activates it, whereas its stereoisomer 9-cis retinoic acid (9-cis RA) is a ligand for the 9-cis RA receptor (RXR). Three RXR-binding elements [retinoic acid response element (RARE)1/PCK1, RARE2, and RARE3/PCK2] were previously located in the promoter of Pck1. Using 3T3-F442A adipocytes, we demonstrated that Pck1 expression was 10-fold more sensitive to 9-cis RA (EC^sub 50^: 10 nmol/L) than to all-trans RA. We then analyzed the respective involvement of RARE1/PCK1, RARE2, and RARE3/PCK2 in the response of Pck1 to 9-cis RA and all-trans RA in adipocytes. The response to 9-cis RA mainly involved the RARE1/PCK1 element, whereas RARE2 was mainly responsive to all-trans RA. In contrast, the full response to both RA isomers involved these 2 elements and included RARE3/PCK2 as well. Furthermore, 9-cis RA, but not all-trans RA, selectively induced PCK1 in ex-vivo-treated human adipose tissue expiants, with a concomitant induction of glyceroneogenesis monitored by [1-^sup 14^C)-pyruvate incorporation into neutral lipids. The concomitant 9-cis RA-induced reduction in fatty acid output indicates an important role for this RA stereoisomer in lipid homeostasis through stimulation of PEPCK-C and glyceroneogenesis in adipose tissue. [PUBLICATION ABSTRACT]