Differential expression of the TPα and TPβ isoforms of the human T Prostanoid receptor during chronic inflammation of the prostate: Role for FOXP1 in the transcriptional regulation of TPβ during monocyte-macrophage differentiation

Inflammation is linked to prostate cancer (PCa) and to other diseases of the prostate. The prostanoid thromboxane (TX)A₂ is a pro-inflammatory mediator implicated in several prostatic diseases, including PCa. TXA₂ signals through the TPα and TPβ isoforms of the T Prostanoid receptor (TP) which exhibit several functional differences and transcriptionally regulated by distinct promoters Prm1 and Prm3, respectively, within the TBXA2R gene. This study examined the expression of TPα and TPβ in inflammatory infiltrates within human prostate tissue. Strikingly, TPβ expression was detected in 94% of infiltrates, including in B- and T-lymphocytes and macrophages. In contrast, TPα was more variably expressed and, where present, expression was mainly confined to macrophages. To gain molecular insight into these findings, expression of TPα and TPβ was evaluated as a function of monocyte-to-macrophage differentiation in THP-1 cells. Expression of both TPα and TPβ was upregulated following phorbol-12-myristate-13-acetate (PMA)-induced differentiation of monocytic THP-1 to their macrophage lineage. Furthermore, FOXP1, an essential transcriptional regulator down-regulated during monocyte-to-macrophage differentiation, was identified as a key trans-acting factor regulating TPβ expression through Prm3 in THP-1 cells. Knockdown of FOXP1 increased TPβ, but not TPα, expression in THP-1 cells, while genetic reporter and chromatin immunoprecipitation (ChIP) analyses established that FOXP1 exerts its repressive effect on TPβ through binding to four cis-elements within Prm3. Collectively, FOXP1 functions as a transcriptional repressor of TPβ in monocytes. This repression is lifted in differentiated macrophages, allowing for upregulation of TPβ expression and possibly accounting for the prominent expression of TPβ in prostate tissue-resident macrophages.

Regulated expression of the TPβ isoform of the human T prostanoid receptor by the tumour suppressors FOXP1 and NKX3.1: Implications for the role of thromboxane in prostate cancer

The prostanoid thromboxane (TX)A₂ signals through the TPα and TPβ isoforms of T Prostanoid receptor (TP) that are transcriptionally regulated by distinct promoters termed Prm1 and Prm3, respectively, within the TBXA2R gene. We recently demonstrated that expression of TPα and TPβ is increased in PCa, differentially correlating with Gleason grade and with altered CpG methylation of the individual Prm1/Prm3 regions within the TBXA2R. The current study sought to localise the sites of CpG methylation within Prm1 and Prm3, and to identify the main transcription factors regulating TPβ expression through Prm3 in the prostate adenocarcinoma PC-3 and LNCaP cell lines. Bisulfite sequencing revealed extensive differences in the pattern and status of CpG methylation of the individual Prm1 and Prm3 regions that regulate TPα and TPβ expression, respectively, within the TBXA2R. More specifically, Prm1 is predominantly hypomethylated while Prm3 is hypermethylated across its entire sequence in PC-3 and LNCaP cells. Furthermore, the tumour suppressors FOXP1 and NKX3.1, strongly implicated in PCa development, were identified as key transcription factors regulating TPβ expression through Prm3 in both PCa cell lines. Specific siRNA-disruption of FOXP1 and NKX3.1 each coincided with up-regulated TPβ protein and mRNA expression, while genetic-reporter and chromatin immunoprecipitation (ChIP) analyses confirmed that both FOXP1 and NKX3.1 bind to cis‑elements within Prm3 to transcriptionally repress TPβ in the PCa lines. Collectively these data identify Prm3/TPβ as a bona fide target of FOXP1 and NKX3.1 regulation, providing a mechanistic basis, at least in part, for the highly significant upregulation of TPβ expression in PCa.