Mitochondrial ribosomal small subunit (MRPS) MRPS23 protein-protein interaction reveals phosphorylation by CDK11-p58 affecting cell proliferation and knockdown of MRPS23 sensitizes breast cancer cells to CDK1 inhibitors

BACKGROUND

Post-translational modification of some mitoribosomal proteins has been found to regulate their functions. MRPS23 has been reported to be overexpressed in various cancers and has been predicted to be involved in increased cell proliferation. Furthermore, MRPS23 is a driver of luminal subtype breast cancer. However, its exact role and function in cancer remains unknown. METHODS AND RESULTS: Our previous study identified protein-protein interactions involving MRPS23 and CDK11A. In this study, we confirmed the interaction of MRPS23 with the p110 and p58 isoforms of CDK11A. Phosphoprotein enrichment studies and in vitro kinase assay using CDK11A/cyclin D3 followed by MALDI-ToF/ToF analysis confirmed the phosphorylation of MRPS23 at N-terminal serine 11 residue. Breast cancer cells expressing the MRPS23 (S11G) mutant showed increased cell proliferation, increased expression of PI3-AKT pathway proteins [p-AKT (Ser47), p-AKT (Thr308), p-PDK (Ser241) and p-GSK-3β (Ser9)] and increased antiapoptotic pathway protein expression [Bcl-2, Bcl-xL, p-Bcl2 (Ser70) and MCL-1] when compared with the MRPS23 (S11A) mutant-overexpressing cells. This finding indicated the role of MRPS23 phosphorylation in the proliferation and survival of breast cancer cells. The correlation of inconsistent MRPS23 phosphoserine 11 protein expression with CDK11A in the breast cancer cells suggested phosphorylation by other kinases. In vitro kinase assay showed that CDK1 kinase also phosphorylated MRPS23 and that inhibition using CDK1 inhibitors lowered phospho-MRPS23 (Ser11) levels. Additionally, modulating the expression of MRPS23 altered the sensitivity of the cells to CDK1 inhibitors.

CONCLUSION

In conclusion, phosphorylation of MRPS23 by mitotic kinases might potentially be involved in the proliferation of breast cancer cells. Furthermore, MRPS23 can be targeted for sensitizing the breast cancer cells to CDK1 inhibitors.

Production and characterization of monoclonal antibodies against recombinant extracellular domain of CD99

BACKGROUND AND OBJECTIVE

CD99/MIC2 gene product is a heavily glycosylated transmembrane protein which plays a major role in homotypic cell adhesion, apoptosis of double positive T cells and vesicular protein trafficking. It is over expressed in various cancers and has been considered as an ideal therapeutic target. The present study focused at developing monoclonal antibodies against the extracellular domain (ECD) of CD99 using hybridoma technology.

MATERIALS AND METHODS

In order to generate monoclonal antibodies, the recombinant ECD of CD99 was used for immunizing the mice. Resulting hybridomas were screened through indirect ELISA. Clones which gave high absorbance values were sub cloned by limiting dilution followed by isotype determination, IP, WB and FACS. The monoclonal antibody 547F2 4F12 was purified from culture supernatant using FPLC and further screened using IF. Finally, the antibodies were validated for specificity using siRNA knock-down.

RESULTS

We were able to establish stable hybridoma clones secreting CD99 antibodies. The antibodies reacted with both the recombinant ECD as well as the wild type CD99 and their isotype's were determined as IgM.

CONCLUSION

Based on these results, we propose that the purified monoclonal antibody 547F2 4F12 could be possibly used for targeting tumors which over express CD99.

A polypeptide from the junction region sequence of EWS-FLI1 inhibits Ewing's sarcoma cells, interacts with the EWS-FLI1 and partner proteins

The EWS-FLI1 chimeric protein uniquely expressed in Ewing’s sarcoma has an obligate role in its aetiology. In our previous report we showed that ectopic expression of the DNA sequences form the junction region (a.a 251–280) can inhibit Ewing’s sarcoma cell growth. In the present report, we introduced a peptide (TAT/NLS/EWS-PEP) comprising of thirty amino acids spanning the junction in conjunction with HIV-1-trans-activating (TAT) and nuclear localization signal sequence (NLS). Peptide uptake and localization studies revealed presence of peptide in ~99% of transduced cells and in the nucleus. Peptide transfection induced cytotoxicity relative to untreated and TAT-NLS peptide treated Ewing’s sarcoma cells. The peptide inhibited clonogenicity, cell cycle, bromo-deoxy uridine (BrdU) uptake and invasion capacity of treated cells. The treatment also affected epithelial to mesenchymal transition (EMT) markers and EWS-FLI1 target gene expression levels. Co-immunoprecipitation experiments involving ectopically expressed full-length EWS-FLI1 protein and the peptide revealed an interaction. Additionally, we found that peptide interaction also occurs with the protein-GGAA microsatellite sequences complex known to contain EWS-FLI1. Further, in the pull-down assay, the peptide was found to interact with proteins known to potentially interact with EWS-FLI1. Based on these results we conclude that peptide could be applied in targeting EWS-FLI1 protein.