S961, a biosynthetic insulin receptor antagonist, downregulates insulin receptor expression & suppresses the growth of breast cancer cells.

Background & objectives: Insulin resistance associated with hyperinsulinaemia and overexpression of insulin receptors (IRs) have been intricately linked to the pathogenesis and treatment outcomes of the breast carcinoma. Studies have revealed that upregulated expression of IRs in breast cancer pathogenesis regulates several aspects of the malignant phenotype, including cell proliferation and metastasis. This study was aimed to investigate the pivotal role of an IR antagonist S961 on IR signalling and other biological parameters in MCF-7, MDA-MB-231 and T47D cell lines. Methods: The effect of human insulin and S961 on growth, proliferation rate and clonogenic potential of breast cancer cells was evaluated by MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazoliumbromide] assay and clonogenic assay. The mRNA expression of IR isoforms (IR-A and IR-B) was measured in the breast carcinoma cells using quantitative PCR. Results: The study revealed that breast cancer cells predominantly expressed IR-A isoform and showed extensive growth and proliferation owing to IR overexpression. It was found that S961 downregulated the IRs (IR-A and IR-B) with nanomolar dose and efficiently blocked expression of IRs even in the presence of insulin. IR mRNA expression levels were significantly downregulated in the continued presence of S961. S961 also inhibited cellular proliferation and colony formation in breast tumour cells. Interpretation & conclusions: IR antagonist, S961 showed distinct antagonism in vitro and appeared to be a powerful therapeutic modality that might provide insight into the pathogenesis of impaired IR signalling.