A TBX2-driven signaling switch from Androgen Receptor to Glucocorticoid Receptor confers therapeutic resistance in Prostate Cancer
Recent studies suggest that glucocorticoid receptor (GR) activation can cause enzalutamide resistance in advanced prostate cancer (PCa) via functional bypass of androgen receptor (AR) signaling. However, the specific molecular mechanism(s) driving this process remain unknown. We previously reported that the transcription factor TBX2 is over-expressed in castrate resistant prostate cancer (CRPC). Our current study demonstrates that TBX2, which has known repressor and activator functions, is a molecular switch that represses AR levels while inducing GR expression to bypass AR-signaling. Mechanistically, our studies revealed that TBX2 binds to the promoters of both AR and GATA2, an AR coregulator, thereby resulting in a two-tiered repression of AR expression.  Concurrently, TBX2 upregulated the GR via direct GR promoter binding and TBX2-GR protein-protein interaction. Together, TBX2-driven repression of the AR and activation of GR resulted in enzalutamide resistance. Significantly, we report that SP2509, an allosteric inhibitor of the demethylase-independent function of LSD1, a TBX2-interacting protein in the COREST complex, disrupted both TBX2-LSD1 and TBX2-GR protein-protein interactions thereby uncovering a unique mode of SP2509 action in CRPC. Taken together, our study provides key insights into a potential therapeutic modality for targeting the AR- to GR- signaling switch via disruption of TBX2-GR and TBX2-LSD1 protein-protein interactions. Overall design: To investigate the effects of genetic modulation of TBX2 in human prostate cancer cells,  we blocked endogenous TBX2 in PC3 human prostate cancer cell lines using a dominant negative construct (TBX2DN) resulting in PC3TBX2DN cells and compared gene expression with the control PC3Neo cells. These cell lines were created and maintained in DMEM with 10%FBS , 1% PS and 5µg/ml puromycin. This was followed by RNA extraction in triplicates for each of the two cell lines i.e. PC3TBX2DN and the PC3Neo control followed by comparative gene expression from the data obtained from RNA-Seq analysis.