Role of mitochondrial ribosomal protein S18-2 in cancerogenesis and in regulation of stemness and differentiation

Abstract: Mitochondria carry their own ribosomes (mitoribosomes) for the translation of mRNA encoded by mitochondrial DNA. The architecture of mitoribosomes is mainly composed of mitochondrial ribosomal proteins (MRPs), which are encoded by nuclear genomic DNA. Emerging experimental evidences reveal that several MRPs are multifunctional and they exhibit important extra-mitochondrial functions, such as involvement in apoptosis, protein biosynthesis and signal transduction. Dysregulations of the MRPs are associated with severe pathological conditions, including cancer. Cancer cells are immortal, i.e. they proliferate in limitless mode, avoiding cell cycle control which is a characteristic of normal cells. Retinoblastoma susceptibility (RB) protein is the major regulator of cell cycle and it operates by inhibiting the activity of E2F1 transcription factor through direct binding. This doctoral thesis is devoted to the extra-mitochondrial role of a RB interacting MRP, the MRP-S18-2 (or S18-2), in different aspects of cancer. S18-2 is localized to the small subunit of mitoribosome and it belongs to S18 family of MRPs, a family consisting of three proteins. Previously, it was shown that S18-2 binds to RB, inhibiting the association of the latter with a transcription factor E2F1 that regulates G1/S transition of cell cycle. Overexpression of S18-2 in rat embryonic fibroblasts led to their immortalization with induction of stem cells markers. In this thesis, I describe that overexpression of S18-2 can also immortalize terminally differentiated rat skin fibroblasts. The immortalized cells displayed characteristics of transformed cells with severe chromosomal instability, disrupted cell cycle, enhanced telomerase activity and the ability to produce tumors in experimental animals. We also showed that rodent cells immortalized by S18-2 overexpression were targeted by NK cells mediated cytotoxicity. Phylogenetic analysis of S18 proteins revealed specific gene duplication events that resulted in three S18 homologs in metazoan. In addition, a Gly132 polymorphism in S18-2 was observed in colon adenocarcinoma that was confirmed by PCR analysis and direct DNA sequencing. The S18-2 protein level was increased, along with free E2F1, in endometrial cancer (EC). Moreover, high S18-2 levels may be associated with epithelial to mesenchymal transition in EC cells. Elevated levels of S18-2 were also found in prostate cancer (PCa). S18-2 could induce the CXCR4 mediated migration of PCa cells in vitro and in a zebrafish model. We also demonstrated new functional consequence of RB and S18-2 interaction in maintenance of a stem cell phenotype, using RB1 knockout mouse embryonic fibroblasts. A cytoplasmic protein complex between S18-2, RB and the Ring finger protein 2 (RNF2) was detected. This enhanced the E3 ligase activity of RNF2, thus, maintaining cell stemness. Our data supports and provide evidence to suggest that S18-2, a RB interacting protein, plays important roles in the development of cancer as a potent oncoprotein.