Iroquois-Homeobox 4 (IRX4) is recently implicated to be associated with prostate cancer (PCa) risk through Genome-Wide Association Studies and follow-up functional studies (1-3). However, the mechanism of IRX4 action in PCa aetiology and regulation of its expression is poorly understood.
Expression of IRX4 was upregulated in prostate tumour samples compared to adjacent non-malignant tissues. IRX4 knockdown reduced LNCaP cell proliferation and migration. Pathway analysis of the transcriptome from IRX4 knockdown samples in LNCaP cells revealed that the Androgen Receptor (AR) pathway was inhibited, suggesting that IRX4 may mediate effective androgen signaling. This was further strengthened by our observation that IRX4 directly interact with an important AR co-factor, FOXA1, in IRX4 immunoprecipitation assays.
Interestingly, IRX4 expression was up-regulated in VCaP cells with androgen treatment, while down-regulated in LNCaP cells. In-silico analysis identified binding of ERG and AR , upstream of IRX4 only in VCaP cells. Knockdown of ERG in VCaP cells up-regulated the androgen mediated expression of IRX4, while ERG overexpression in LNCaP cells down-regulated IRX4 expression, suggesting ERG and AR coordinate the expression of IRX4. Sequencing of AR/ERG binding region identified a Multiple-Nucleotide Length polymorphism (MNLP-rs386684493) where a stretch of 47bp sequence is replaced by a novel 21bp sequence. VCaP cells have an intact AR binding site (47bp/47bp) whereas LNCaP cells have disrupted AR binding site (21bp/21bp), thus showing opposite directionality during androgen stimulation. The androgen responsiveness of the 47bp allele of MNLP was further confirmed by reporter vector assay. Moreover, 21bp/21bp genotype was correlated with poor overall survival in the PCa patients who underwent androgen deprivation therapy.
Herein, we have demonstrated that IRX4 is regulated by androgens in an MNLP allele-specific manner and IRX4 may mediate effective androgen signaling. Further studies on IRX4 structure and its interaction with AR-cofactors may provide insights for developing novel drugs for PCa treatment.