Microbiome-associated metabolites, valerobetaine and homocarnitine, inhibit carnitine transport via OCTN2.

The intestinal microbiome-derived metabolite, valerobetaine (δ-valerobetaine), promotes obesity, hepatic steatosis, and impaired cognition in mice and is associated with obesity, fatty liver disease, diabetes, and cardiovascular disease in humans. Mechanistic studies show that valerobetaine decreases systemic carnitine and inhibits mitochondrial fatty acid oxidation. Valerobetaine and its mammalian hydroxylation product, homocarnitine, share close structural homology with carnitine, which is mainly transported by the organic cation transporter 2 (OCTN2). To determine whether reductions in systemic carnitine induced by valerobetaine and homocarnitine result from interactions with OCTN2, we performed in vitro uptake studies using HEK293 cells overexpressing human OCTN2 coupled with metabolite measurement by mass spectrometry. OCTN2 overexpression increased the uptake rates of both homocarnitine and valerobetaine relative to control cells. Meldonium, an OCTN2 substrate and inhibitor, reduced uptake of both metabolites in a concentration-dependent manner. Saturating uptake kinetics were observed for valerobetaine, whereas homocarnitine exhibited linear uptake across the concentration range tested (1-100 µM). Both metabolites exhibited lower transport efficiency compared with the carnitine precursor γ-butyrobetaine and showed relatively lower potency in inhibition of carnitine uptake. Together, these findings identify homocarnitine and valerobetaine as modulators of carnitine transport and provide a mechanistic basis by which these microbiome-derived metabolites lower systemic carnitine levels and impair mitochondrial fatty acid oxidation.NEW & NOTEWORTHY This study uses transport physiology, ectopic gene expression, and mass spectrometry-based detection methods to show that the microbiome-derived valerobetaine and its mammalian metabolic product, homocarnitine, are transported by OCTN2, an important carnitine transporter. Moreover, both valerobetaine and homocarnitine inhibit carnitine transport, providing a mechanism to explain the systemic decrease in carnitine, global impairment of mitochondrial fatty acid oxidation, and association of valerobetaine with several pathophysiological processes, including obesity, hepatic steatosis, and cognitive decline.