Post-transcriptional regulation of the tumor suppressor gene TSC1 and its relevance in Oral Squamous Cell Carcinoma

Abstract

Oral cancer or oral squamous cell carcinoma (OSCC) is a subset of head and neck cancer (excluding non-melanoma skin cancer), with an estimated 354,864 new cases and 177,384 deaths reported annually. India accounts for 34% of the world's oral cancer burden with an incidence of 119,992 and mortality of 72,616 cases annually, second only to breast cancer. Despite advancements in the therapeutic approaches, the prognosis of OSCC fares poorly, with an overall 5-year survival rate ranging from 34% to 62.9%. The PI3K/AKT/mTOR pathway, which regulates fundamental cellular processes like proliferation, cell cycle, metabolism, development, and differentiation, is most commonly deregulated in cancers, including OSCC. TSC1 (Tuberous sclerosis complex 1), by forming a complex with TSC2 and TBC1D7, negatively regulates this pathway by inhibiting the activation of mTORC1. Downregulation of TSC1 is reported in oral as well as several other cancers and is associated with an unfavorable clinical outcome in patients. The expression and function of TSC1 are regulated by various mechanisms like promoter methylation, loss-of-heterozygosity (LOH), loss of function mutations, and phosphorylation. In recent years, increasing evidence suggests the involvement of microRNAs (miRNAs) in oral carcinogenesis by acting as tumor suppressors or oncogenes. In this study, we worked on the post-transcriptional regulation of TSC1 by miR-130a and its role in the pathogenesis of OSCC. The current study clearly illustrates that miR-130a represses the expression of TSC1 at both the transcript and protein levels. The dual-luciferase reporter assay confirms the direct interaction between miR-130a and TSC1 3'UTR. Overexpression of miR-130a activates the PI3K/AKT/mTOR pathway, while its inhibition via antagomiR-130a repressed the pathway. Expression analysis of miR-130a and TSC1 in matched normal and oral tumor tissues through RT-qPCR analysis shows an inverse correlation in a majority (19/36) of OSCC samples, suggesting the physiological significance of their interaction. Further, miR-130a increases the cell proliferation, invasion, and anchorage-independent growth of OSCC cells by targeting the 3'UTR of TSC1. The mice injected with antagomiR-130a treated OSCC cells showed reduced tumor weight and volume compared to those injected with mock treated cells, which confirms the oncogenic role of miR-130a. Taken together, these results suggest that miR-130a exerts its oncogenic function in OSCC, in part, by regulating TSC1, whereby the inhibition of miR-130a holds potential in OSCC therapeutics and perhaps in other cancers as well.