Modulation of expression of the nuclear receptor NR0B2 (small heterodimer partner 1) and its impact on proliferation of renal carcinoma cells.

Mammalian nuclear receptors (NRs) are transcription factors regulating the expression of target genes that play an important role in drug metabolism, transport, and cellular signaling pathways. The orphan and structurally unique receptor small heterodimer partner 1 (syn NR0B2) is not only known for its modulation of drug response, but has also been reported to be involved in hepatocellular carcinogenesis. Indeed, previous studies show that NR0B2 is downregulated in human hepatocellular carcinoma, suggesting that NR0B2 acts as a tumor suppressor via inhibition of cellular growth and activation of apoptosis in this tumor entity. The aim of our study was to elucidate whether NR0B2 may also play a role in other tumor entities. Comparing NR0B2 expression in renal cell carcinoma and adjacent nonmalignant transformed tissue revealed significant downregulation in vivo. Additionally, the impact of heterologous expression of NR0B2 on cell cycle progression and proliferation in cells of renal origin was characterized. Monitoring fluorescence intensity of resazurin turnover in RCC-EW cells revealed no significant differences in metabolic activity in the presence of NR0B2. However, there was a significant decrease of cellular proliferation in cells overexpressing this NR, and NR0B2 was more efficient than currently used antiproliferative agents. Furthermore, flow cytometry analysis showed that heterologous overexpression of NR0B2 significantly reduced the amount of cells passing the G1 phase, while on the other hand, more cells in S/G2 phase were detected. Taken together, our data suggest that downregulation of NR0B2 may also play a role in renal cell carcinoma development and progression.

Cell-specific expression of uptake transporters--a potential approach for cardiovascular drug delivery devices.

Enhanced proliferation of human coronary artery smooth muscle cells (HCASMCs) and thereby formation of neointima is one of the factors contributing to failure of coronary stents. Even if the use of drug eluting stents (DES) and thereby the local delivery of cytotoxic compounds has significantly improved the clinical outcome, unselective cytotoxic effects are assumed to hamper clinical success. Novel pharmacological approaches are required to enhance cellular selectivity of locally delivered drugs. Cell specific overexpression of a drug transporter could be used to enhance cellular accumulation and therefore cell specificity. In the herein reported study we tested the possibility of cell specific transporter expression to enhance drug effects in HCASMCs. We generated adenoviral constructs to overexpress the organic cation transporter 1 (OCT1) under control of the promoter of SM22α, which had been previously reported as muscle cell specific gene. First the activity of the SM22α-promoter was assessed in various cell types supporting the notion of muscle cell specificity. Subsequently, the activity of the transporter was compared in infected HCAECs and HCASMCs revealing enhanced accumulation of substrate drugs in HCASMCs in presence of the SM22α-promoter. Testing the hypothesis that this kind of targeting might serve as a mechanism for cell-specific drug effects, we investigated the impact on paclitaxel treatment in HCASMC and HCAECs, showing significantly increased antiproliferative activity of this substrate drug on muscle cells. Taken together, our findings suggest that cell-specific expression of transport proteins serves as mechanism governing the uptake of cytotoxic compounds for a selective impact on targeted cells.

Liver X receptor α and farnesoid X receptor are major transcriptional regulators of OATP1B1.

UNLABELLED: Organic anion transporting polypeptide 1B1 (OATP1B1) is a liver-enriched transporter involved in the hepatocellular uptake of many endogenous molecules and several structurally divergent drugs in clinical use. Although OATP1B1 coding region polymorphisms are known to make an impact on substrate drug disposition in humans, little is known regarding the mechanisms underlying the transcriptional regulation of this transporter. In this study, we note that messenger RNA (mRNA) expression of OATP1B1 in a large human liver bank exhibited marked interindividual variability that was not associated with coding region polymorphisms. Accordingly, we hypothesized that such variability in expression is reflective of nuclear receptor-mediated transcriptional regulation of this transporter. We tested prototypical ligands for the nuclear receptors pregnane X receptor (PXR), constitutive androstane receptor (CAR), liver X receptor (LXR) α, and farnesoid X receptor (FXR) in a human hepatoma-derived cell line and noted induction of OATP1B1 mRNA when the cells were treated with LXRα or FXR ligands. To confirm a direct role for LXRα and FXR to OATP1B1 expression, we performed detailed promoter analysis and cell-based reporter gene assays resulting in the identification of two functional FXR response elements and one LXRα response element. The direct interaction between nuclear receptors with the identified response elements was assessed using chromatin immunoprecipitation assays. Using isolated primary human hepatocytes, we show that LXRα or FXR agonists, but not PXR or CAR agonists, are capable of OATP1B1 induction. CONCLUSION: We note that OATP1B1 transcriptional regulation is under dual nuclear receptor control through the oxysterol sensing LXRα and the bile acid sensor FXR. Accordingly, the interplay between OATP1B1 and nuclear receptors may play an important and heretofore unrecognized role during cholestasis, drug-induced liver injury, and OATP1B1 induction-related drug interactions.