Amino acid insufficiency impairs hepatic vitamin A mobilization in mice

Abstract

Retinol-binding protein 4 (RBP4) is the sole specific serum carrier of vitamin A, delivering it from the hepatic stores to the peripheral organs in a complex with retinol (ROH) and transthyretin (TTR). Regulators of hepatic mobilization aside from vitamin A status itself are ill-defined. Here, we show that amino acid (AA) insufficiency by diet (low-protein, leucine-devoid diet) or drug (asparaginase, ASNase) elevated liver RBP4 without depleting hepatic retinoid stores. In addition, ASNase reduced liver TTR protein. Furthermore, circulating ROH–RBP4–TTR levels were attenuated, and retinoid levels in peripheral organs were perturbed. To understand the basis for elevated RBP4 in the liver, we isolated total and polysomal mRNA to assess gene-specific translation. ASNase significantly reduced Rbp4 mRNA translation, indicating that elevated hepatic RBP4 protein was not due to increased protein synthesis. In contrast, ASNase reduced Ttr mRNA abundance but not its translation. Global deletion of the AA insufficiency sensor, general control nonderepressible 2 (GCN2), lessened ASNase-induced RBP4 protein accumulation in the liver but did not rescue circulating ROH–RBP4 levels. These effects were replicated by chemical inhibition of GCN2 in ASNase-exposed primary hepatocytes. Finally, hepatocyte-specific knockout of autophagy-related 7, an enzyme involved in autophagy and protein secretion, fully rescued circulating ROH–RBP4–TTR and normalized liver RBP4 and TTR during ASNase. Overall, our findings identify AA insufficiency to modulate hepatic ROH–RBP4 mobilization independent of vitamin A status.