Scientists in Dr. Hongbing Wang’s lab are currently focused on three major research initiatives.
Regulation of Drug-metabolizing Enzymes by Xenobiotic Receptors
Drug-metabolizing enzymes (DMEs) and transporters play pivotal roles in the disposition and detoxification of numerous endogenous and xenobiotic chemicals. To accommodate chemical challenges, the expression of many DMEs and transporters are up-regulated by a group of ligand-activated transcription factors. The importance of such transcription factors in xenobiotic metabolism and clearance is best exemplified by the promiscuous xenobiotic receptors: the pregnane X receptor (PXR, NR1I2), the constitutive androstane receptor (CAR, NR1I3), and the aryl hydrocarbon receptor (AhR). Together, these receptors govern the inductive expression of a large array of target genes encoding phase I and II DMEs, and drug transporters. Moreover, these receptors also exhibit distinctive mechanisms of activation: ligand-dependent (direct) and ligand-independent (indirect) activation. Over the years, we have investigated the transcriptional regulation of CYP2B6, CYP3A4, BCRP, and SLC13A5 in human liver by different xenobiotic receptors, with CYP2B6 as a primary target used to illustrate the mechanisms and contribution of CAR and PXR in hepatic P450 induction. We have shown that in addition to CAR and PXR, other liver-enriched transcriptional factors such as HNF4α, C/EBPα, and HNF3β also contribute to the optimal transcription of CYP2B6 in the liver.
The Role of CAR in Cyclophosphamide-based Chemotherapy
Cyclophosphamide (CPA), an alkylating prodrug, has been used extensively in the treatment of hematologic malignancies, and particularly as an important component in the front-line regimens for non-Hodgkin lymphoma and chronic lymphocytic leukemia. Unfortunately, despite aggressive chemotherapy, a significant number of patients remain uncured due to development of drug resistance and/or intolerable toxicities. The need for further optimization of the current regimens are evident. We strive to improve the therapeutic efficacy of CPA-based chemotherapy by enhancing metabolic conversion of CPA to the pharmacologically active 4-hydroxylcyclophosphamide (4-OH-CPA) via CYP2B6, but not to the N-dechloroethyl-cyclophosphamide and the toxic chloroacetaldehyde by CYP3A4. Towards this end, we have shown that activation of human CAR preferentially induces the expression of hepatic CYP2B6 over CYP3A4 and increased the formation of 4-OH-CPA. We have also developed a unique human primary hepatocyte (HPH)-leukemia co-culture model and demonstrated that co-administration of CPA with a selective hCAR activator significantly enhances apoptosis in leukemia cells without increasing hepatotoxicity. Most recently, utilizing an improved multi-tissue co-culture model, we have shown the beneficial effects of including a hCAR activator in the clinically-used lymphoma treatment regimen (CHOP). Eventually, we expect to establish hCAR as a novel therapeutic target facilitating CPA-based chemotherapy for hematopoietic malignancies.
Function and Regulation of SLC13A5 in the Liver
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. 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. 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 findings, we expect that further studies will yield novel knowledge regarding the transcriptional regulation of SLC13A5 in the liver, and offer experimental evidence that targeted disruption of SLC13A5 as a nutrient regulator will alter the proliferation of hepatoma cells by modulating AMPK/mTOR signaling pathways.