Vitamin A, which is also known as retinol, is an essential vitamin and needed for the functionality of vision, immune cells, reproduction, embryonic development, and regulation of cell proliferation and differentiation. Retinol is mobilized from the liver while bound to retinol binding protein 4 (RBP4) and transthyretin (TTR) in order to ensure a successful transport to extrahepatic tissues and to maintain these physiological functions. During fasting, the dietary intake of essential retinol is lacking, which was shown in a preliminary study in mice to induce a shift in the retinoid homeostasis in order to ensure retinoid availability for the organism. It was hypothesized that retinol is reversely transported from adipose tissue to replenish hepatic stores. Moreover, it was suggested that fibroblast growth factor 21 (FGF21) is secreted by the liver to induce this reverse transport. This study aimed to illuminate which mechanism underlies the changes of hepatic mRNA expression and protein levels of RBP4 and TTR in the fasted state observed in a preliminary study in mice. Therefore, primary murine hepatocytes were used as a model to investigate hepatic regulations. Insulin, glucagon, and their related signaling pathways were identified to regulate RBP4 and TTR. In addition, the question whether there are alterations in the concentrations of apo-RBP4 and holo-RBP4 in serum in times of fasting was investigated, since the export of hepatic retinol was hypothesized to be reduced and repartitioning thereof from adipose tissue was suggested to be enhanced. This was done by using non-denaturing immunoblotting and immunoprecipitation. Also, it was investigated whether FGF21 can mediate a signal and its enhanced secretion can induce reverse transport from adipose tissue to the liver. For this reason, an adeno-associated virus (AAV) was injected into mice, resulting in liver-specific overexpression of FGF21, which led to minor changes in the retinoid homeostasis. The amount of retinoids in the liver and subcutaneous white adipose tissue did not change and the expression of retinol metabolism-associated target genes was induced. In epididymal white adipose tissue, retinyl ester stores were found to have reduced while retinol metabolism-associated target genes remained unaltered. Moreover, hepatic FGF21 overexpression was found to promote the gene and protein expression of uncoupling protein 1 (UCP1) and, thus, so-called browning in subcutaneous white adipose tissue. Overall, this study found that the ratio of insulin and glucagon, which shifts during the feeding-fasting-transition, can regulate the gene and protein expressions of RBP4 and TTR. Moreover, it was shown that FGF21 can induce minor changes in retinoid homeostasis, but was not identified as the primary regulator of the fasting-induced influences affecting retinoid homeostasis.
