Citrate is a key energy sensor that plays a central role in carbohydrate metabolism, energy production, and histone acetylation. The intracellular level of citrate is tightly controlled through a balance of biosynthesis and transport. In the liver, the solute carrier family 13 member 5 (SLC13A5), a sodium-coupled citrate transporter, is essential for the import of citrate from the circulation to hepatocytes, a process that can be perturbed by both xenobiotic and endobiotic stimuli. Recent studies have shown that expression of SLC13A5 was increased in obese, non-alcoholic fatty liver disease (NAFLD) patients, high-fat diet (HFD)-treated rhesus monkeys, and in xenobiotic-treated human and rat hepatocytes, suggesting upregulation of SLC13A5 can be a risk factor for metabolic disorders. In contrast, deletion of SLC13A5 protects mice from HFD-induced hepatic steatosis and mutations of the SLC13A5 ortholog in D. melanogaster promote longevity. However, despite the emerging importance of SLC13A5 in energy homeostasis, the mechanism(s) by which the SLC13A5 gene is transcriptionally regulated and whether clinically used drugs disturb the expression of this transporter are not well characterized. Moreover, whether SLC13A5 affects hepatic functions beyond lipid homeostasis is largely unknown. The overall objective of this proposal is to understand the molecular mechanisms governing hepatic SLC13A5 gene expression and to delineate the role of SLC13A5 in human liver cell proliferation. To this end, we have shown that 1) prototypical activators of the constitutive androstane receptor (CAR) and the pregnane X receptor (PXR) robustly induce expression of human SLC13A5; 2) knockdown of SLC13A5 attenuates the proliferation of hepatocellular carcinoma cells; and 3) expression of SLC13A5 is inversely correlated with the activation of AMPK signaling. Building on these preliminary results, we hypothesize that CAR and PXR are key regulators of the inductive expression of SLC13A5 in the liver, and SLC13A5 functions as a nutrient regulator altering the proliferation of hepatoma cells by modulating AMPK/mTOR signaling pathways. This central hypothesis will be tested in the following specific aims:
Aim 1. Define the role of CAR and PXR in xenobiotic- induced expression of SLC13A5;
Aim 2. Elucidate the mechanism(s) underlying CAR- and PXR-mediated induction of SLC13A5;
Aim 3. Determine the effects of SLC13A5 on hepatoma cell proliferation. The outcomes are expected to provide fundamental novel knowledge on the transcriptional regulation of SLC13A5 in the liver, and to delineate a crucial role of SLC13A5 in bridging energy metabolism with liver cancer progression.
The proposed studies will yield fundamental and novel knowledge regarding the regulation of SLC13A5 expression through specific interactions with CAR and PXR, which will considerably improve our understanding of xenobiotic-mediated alteration of energy homeostasis in the liver. The anticipated outcome will offer proof- of-concept evidence that targeted disruption of SLC13A5 affects the proliferation of human hepatoma cells. This research may eventually lead to novel strategies in controlling metabolic disorders and liver cancer.
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