Question:

The multidrug resistance (MDR) P-glycoprotein ABCB1 plays a major role in MDR of malignant cells and is regulated by various transcription factors, including Wnt/β-catenin/TCF4. The transcription factor PITX2 (Pituitary homeobox-2) is essential for embryonic development. PITX2 operates by recruiting and interacting with β-catenin to increase the expression of growth-regulating genes, such as cyclin D1/2 and c-Myc. The importance of PITX2 in malignancy is not yet known.

Methods & Results:

Here we demonstrate that in the renal cancer cell lines ACHN and A498, the level of ABCB1 expression and function correlate with nuclear PITX2 localization and PITX2-luciferase reporter gene activity (A498 > ACHN). In A498, doxorubicin toxicity is augmented by the ABCB1 inhibitor, PSC833. PITX2 overexpression increases ABCB1 expression and cell survival in ACHN cells. Silencing of PITX2 by siRNA downregulates ABCB1 and induces a greater chemotherapeutic response to doxorubicin in A498 cells, as determined by MTT cell viability and clonogenic survival assays. Two PITX2 binding sequences were identified in the ABCB1 promoter sequence. PITX2 binding was confirmed by chromatin immunoprecipitation. Beta-catenin is not required for PITX2 upregulation of ABCB1 since ABCB1 mRNA increased and doxorubicin toxicity decreased upon PITX2 overexpression in β-catenin^-/- cells.

Conclusions:

The data show for the first time that ABCB1 is a target gene of PITX2 transcriptional activity, promoting MDR and cell survival of cancer cells.

Funded by the DFG (TH345/10-1 to F.T.) and the Westermann-Westerdorp Foundation (to W.K.L.).