Interaction of the human prostacyclin receptor and the NHERF4 family member intestinal and kidney enriched PDZ protein (IKEPP).

Prostacyclin and its I prostanoid receptor, the IP, play central roles in hemostasis and in re-endothelialization in response to vascular injury. Herein, intestinal and kidney enriched PDZ protein (IKEPP) was identified as an interactant of the human (h) IP mediated through binding of PDZ domain 1 (PDZ(D1)) and, to a lesser extent, PDZ(D2) of IKEPP to a carboxyl-terminal Class I 'PDZ ligand' within the hIP. While the interaction is constitutive, agonist-activation of the hIP leads to cAMP-dependent protein kinase (PK) A and PKC-phosphorylation of IKEPP, coinciding with its increased interaction with the hIP. Ectopic expression of IKEPP increases functional expression of the hIP, enhancing its ligand binding and agonist-induced cAMP generation. Originally thought to be restricted to renal and gastrointestinal tissues, herein, IKEPP was also found to be expressed in vascular endothelial cells where it co-localizes and complexes with the hIP. Furthermore, siRNA-disruption of IKEPP expression impaired hIP-induced endothelial cell migration and in vitro angiogenesis, revealing the functional importance of the IKEPP:IP interaction within the vascular endothelium. Identification of IKEPP as a functional interactant of the IP reveals novel mechanistic insights into the role of these proteins within the vasculature and, potentially, in other systems where they are co-expressed.

Protein kinase C-related kinase 1 and 2 play an essential role in thromboxane-mediated neoplastic responses in prostate cancer.

The prostanoid thromboxane (TX) A2 is increasingly implicated in neoplastic progression, including prostate cancer (PCa). Mechanistically, we recently identified protein kinase C-related kinase (PRK) 1 as a functional interactant of both the TPα and TPß isoforms of the human T prostanoid receptor (TP). The interaction with PRK1 was not only essential for TPα/TPß-induced PCa cell migration but also enabled the TXA2-TP axis to induce phosphorylation of histone H3 at Thr11 (H3Thr11), an epigenetic marker both essential for and previously exclusively associated with androgen-induced chromatin remodelling and transcriptional activation. PRK1 is a member of a subfamily of three structurally related kinases comprising PRK1/PKNα, PRK2/PKNγ and PRK3/PKNß that are widely yet differentially implicated in various cancers. Hence, focusing on the setting of prostate cancer, this study investigated whether TPα and/or TPß might also complex with PRK2 and PRK3 to regulate their activity and neoplastic responses. While TPα and TPß were found in immune complexes with PRK1, PRK2 and PRK3 to regulate their activation and signalling, they do so differentially and in a TP agonist-regulated manner dependent on the T-loop activation status of the PRKs but independent of their kinase activity. Furthermore, TXA2-mediated neoplastic responses in prostate adenocarcinoma PC-3 cells, including histone H3Thr11 phosphorylation, was found to occur through a PRK1- and PRK2-, but not PRK3-, dependent mechanism. Collectively, these data suggest that TXA2 acts as both a neoplastic and epigenetic regulator and provides a mechanistic explanation, at least in part, for the prophylactic benefits of Aspirin in reducing the risk of certain cancers.