Research Projects

G-protein coupled receptor kinase-5 (GRK5) is increasingly recognized for its non-canonical roles beyond GPCR signalling. It is known to phosphorylate several histone deacetylases (HDACs), including HDAC5 and HDAC6 (Zhang et al., 2011), indicating its role in transcriptional control and chromatin remodelling. SIRT1, a class III HDAC, has a critical role in numerous cellular processes, including histone deacetylation, cell cycle progression, and centriole duplication, partly through its deacetylation of proteins such as PLK2 (Ling et al., 2018). GRK5 is also known to localize to centrosomes and regulate cell cycle (Michal et al., 2012), particularly through phosphorylation and activation of nucleophosmin (NPM-2) and promoting microtubule nucleation in the centrosomes (So et al., 2012). Interestingly, phosphosite plus analysis reveals that SIRT1 has a perfect substrate motif (-D/ESEIEE-) for GRK5-mediated phosphorylation at the S-693 position, thereby suggesting it might be a novel substrate of GRK5. We hypothesise that GRK5 may be a part of the SIRT1-PLK2 axis, where it could potentially phosphorylate and activate SIRT1, which could in turn deacetylate and hence promote the degradation of PLK2 in the early G1 phase. The inactivated PLK2 is known to prevent precocious centriole reduplication during mitosis/early G1 (Ling et al., 2018). We have previously used the PolST antigen as a tool for kinase library screening. PolST antigen induces severe mitotic arrest followed by apoptosis, which is dependent upon its ability to bind serine/threonine phosphatase, Protein Phosphatase 2A (PP2A) (Gillani et al., 2022). PP2A is a master regulator of the cell cycle, and our preliminary findings show that GRK5 is also regulated by PP2A (unpublished data). Given that SIRT1 also undergoes phosphorylation dependent regulation by mitotic proteins (Ling et al., 2018), it is likely that PP2A contributes to the dynamic regulation of both GRK5 and SIRT1 during the cell cycle. Additionally, our prior studies revealed that GRK5 is increased in various cancer cell lines, including SW480, SW460, and H1299. Our recent data confirmed that GRK5 expression is increased in colorectal tumors, both at mRNA and protein levels (manuscript submitted). Bioinformatic analyses from TCGA and cBioPortal also showed that GRK5 is highly altered in various cancers like CRC and ovarian cancer. Very interestingly, PLK2 and SIRT1 are other prominent candidates that are also reported to be perturbed in CRC (Ou et al., 2016; Wang et al., 2023), highlighting the clinical relevance of this axis. Building on these and some other published reports, we therefore want to dissect the regulatory interplay between GRK5-SIRT1-PLK2 in controlling unwarranted centriole duplication, to confirm PP2A’s role in the regulation of this axis during mitosis, and to explore its contribution to tumorigenesis and drug resistance, particularly in colorectal cancer.