Minor role of pregnane-x-receptor for acquired multidrug resistance in head and neck squamous cell carcinoma in vitro

Acquired multidrug resistance (MDR) has been linked to overexpression of drug-metabolising and transporting proteins mediated by pregnane-x-receptor (PXR). The aim of this work was to establish the relevance of PXR for MDR in head and neck squamous cell carcinoma (HNSCC). Using eight HNSCC cell lines, we determined the efficacy of paclitaxel, cisplatin and 5-fluorouracil (5-FU) via proliferation assays and determined the expression and activity of PXR through quantitative real-time polymerase chain reaction, western blotting and luciferase-based reporter gene assay. PXR knockdown approaches using shRNA-encoding vectors were applied to estimate the role of PXR for native MDR. Drug resistance ranged between 5.2 and 620 nM for paclitaxel, varied between 4.5 and 58 mu M for cisplatin, and varied between 1.1 and 5,467 mu M for 5-FU. Lack of PXR mRNA expression was mostly accompanied by the absence of mRNA expression of cytochrome P450 3A4 (CYP3A4) and P-glycoprotein (P-gp, ABCB1) expression. Neither mRNA nor protein expression of PXR correlated with drug resistance. However, PXR activity tended to correlate with IC50 values of paclitaxel (p = 0.08). Knockdown of PXR in one of the cell lines had a slight but not significant impact on paclitaxel efficacy compared to scrambled sequence control. Surprisingly, only in two cell lines, PXR activity was increased by the well-known inductor rifampicin. This study suggests a malfunctioning of PXR and thus a minor relevance for iatrogenic chemotherapy resistance in HNSCC.