A monocarboxylate transporter required for hepatocyte secretion of ketone bodies during fasting

Abstract

To find new genes that influence liver lipid mass, we performed a genetic screen for zebrafish mutants with hepatic steatosis, a pathological accumulation of fat. The red moon (rmn) mutant develops hepatic steatosis as maternally deposited yolk is depleted. Conversely, hepatic steatosis is suppressed in rmn mutants by adequate nutrition. Adult rmn mutants show increased liver neutral lipids and induction of hepatic lipid biosynthetic genes when fasted. Positional cloning of the rmn locus reveals a loss-of-function mutation in slc16a6a (solute carrier family 16a, member 6a), a gene that we show encodes a transporter of the major ketone body b-hydroxybutyrate. Restoring wild-type zebrafish slc16a6a expression or introducing human SLC16A6 in rmn mutant livers rescues the mutant phenotype. Radiotracer analysis confirms that loss of Slc16a6a function causes diversion of livertrapped ketogenic precursors into triacylglycerol. Underscoring the importance of Slc16a6a to normal fasting physiology, previously fed rmn mutants are more sensitive to death by starvation than are wild-type larvae. Our unbiased, forward genetic approach has found a heretofore unrecognized critical step in fasting energy metabolism: hepatic ketone body transport. Since b-hydroxybutyrate is both a major fuel and a signaling molecule in fasting, the discovery of this transporter provides a new direction for modulating circulating levels of ketone bodies in metabolic diseases.

The Retinal Pigment Epithelium Utilizes Fatty Acids for Ketogenesis

“…Two promising candidates for facilitated ketone transporter are MCT isoforms 1 (MCT1, encoded by Slc16a1) and 7 (MCT7, encoded by Slc16a6), both of which are known to transport ␤-HB (12,15). Although MCT1 is expressed exclusively at the RPE apical membrane and enriched at the photoreceptor inner segment (16), MCT7 expression in the RPE and retina has not been studied.…”

“…In addition to intracellular metabolites, we also analyzed the Ringer's solution in the apical and basolateral compartments of these hfRPE for 13 C metabolites. We found that hfRPE cells incubated with [1][2][3][4][5][6][7][8][9][10][11][12][13] C]palmitate alone released significantly more (7-fold) labeled ␤-HB (75.2 Ϯ 49.7 pmol/cm 2 RPE in a 500-l volume over 3 h) into the apical chamber compared with the basal chamber (10.5 Ϯ 18.3 pmol/cm 2 RPE in a 1500-l volume over 3 h) (Fig. 3E).…”

“…The transporter for ␤-HB was identified recently in the zebrafish model to be monocarboxylate transporter isoform 7 (MCT7, encoded by Slc16a6) (12). MCT7 expression in the RPE and retina has not been reported, but a search of microarray data published recently of individual mouse retina cell types (from the work of Siegert et al (13)) revealed that MCT7 is expressed in photoreceptor cells, suggesting that photoreceptor cells can take up and metabolize ␤-HB.…”

Clinical and Functional Relevance of the Monocarboxylate Transporter Family in Disease Pathophysiology and Drug Therapy

“…Although functional investigations have yet only been performed in zebrafish,11 this study provides first evidence for a contribution of MCT7 in hepatic lipid metabolism by secreting ketone bodies, especially β‐hydroxybutyrate. Zebrafish mutants with hepatic steatosis were identified to harbor a loss‐of‐function mutation in the slc16a6 locus, which causes lipid accumulation in fasted livers.…”

“…Human MCT7 has not yet been systematically characterized. Mct7/s lc16a6 from zebrafish has been demonstrated to selectively export ketone bodies from the liver, especially β‐hydroxybutyrate, and transgenic expression of human SLC16A6 rescued a mutant s lc16a6 zebrafish phenotype with hepatic steatosis, indicating a similar function for human MCT7 11. MCT11 has recently been shown to mediate H + ‐coupled transport of pyruvate, indicating similar substrate specificities as MCT1–MCT4; however, transport of other monocarboxylates was not tested.…”