Retinoic Acid Metabolites and Isomers have Differential Effects on Lipid Metabolism

Abstract

All‐trans retinoic acid (atRA) and 13‐cisRA, and their metabolites 4‐oxo‐atRA and 4‐oxo‐13‐cisRA are found in circulation in humans as endogenous compound and following clinical doses of atRA and 13‐cisRA. Of these four compounds, only atRA is believed to be biologically important due to its high affinity towards RARs and activation of PPARδ. 13‐cisRA and 4‐oxo‐atRA also activate RARs but less is known about the 4‐oxo‐13‐cisRA. The activation of PPARs by 13‐cisRA and 4‐oxo‐metabolites has not been studied. The aim of this study was to determine whether atRA, 4‐oxo‐atRA, 13‐cisRA and 4‐oxo‐13‐cisRA have biological effects via RAR and other non‐RAR mediated pathways in human hepatocytes, HepG2 cells and adipocytes. At 1 μM concentration atRA, 4‐oxo‐atRA, 13‐cisRA and 4‐oxo‐13‐cisRA each significantly increased the mRNA levels of RARβ demonstrating that all four compounds activate RARs. atRA treatment increased expression of PPARδ targeted genes PGC1α, PGC1β and NRF1 and fatty acid oxidation, but had no effect on fatty acid and triglyceride synthesis genes SREBP1, MTTP, DGAT2 and PPARγ. In contrast, 13‐cisRA, 4‐oxo‐atRA, and 4‐oxo‐13‐cisRA had no effect on PPARδ targeted genes PGC1α, PGC1β and NRF1 and fatty acid oxidation, but significantly increased the expression of SREBP1, MTTP, DGAT2 and PPARγ. This suggests that 13‐cisRA, 4‐oxo‐atRA, and 4‐oxo‐13‐cisRA increase triglyceride synthesis and secretion. In addition, 4‐oxo‐atRA antagonized the induction of PGC1α, PGC1β and NRF1 and fatty acid oxidation by atRA demonstrating an independent activity of the 4‐oxo‐atRA when compared to atRA. Collectively, these results indicate that atRA promotes lipid catabolism via PPARδ, and 13‐cisRA and 4‐oxo‐RAs increase triglyceride synthesis, possibly via SREBP1 and PPARγ. Our findings regarding 13‐cisRA and its main circulatory metabolite 4‐oxo‐13‐cisRA explain the clinical data of increased incidence of hypertriglyceridemia and hypercholesterolemia following 13‐cisRA dosing. In addition, our data supports the mouse findings that atRA increases fatty acid oxidation in vivo. This study demonstrates that the 13‐cisRA isomer and 4‐oxo‐metabolites of RA found in circulation regulate lipid metabolism independent of atRA and RAR activation, and may play an important role in vivo in modulating lipid homeostasis in human tissues.Support or Funding InformationNIH R01 GM111772, R01 GM081569, R01 GM081569‐S1 and P30 DK035816