LAUSR

Simple Summary In this study, we employed a cell-based assay with a luciferase gene controlled by the RET promoter to identify small molecules that inhibit the transcription of the RET gene. The RET gene plays a role in the development of medullary thyroid carcinoma (MTC) as part of multiple endocrine neoplasia type 2 (MEN2) syndrome. Adefovir dipivoxil was found to inhibit the transcription of the RET gene, reducing the expression of endogenous RET protein, inhibiting RET-dependent cell proliferation, and increasing apoptosis in MTC cells. Additionally, adefovir dipivoxil was found to interfere with Stat3 activity both in silico and in a biochemical assay, as well as in MTC cell lines. These results suggest that the cell-based assay is useful in identifying transcriptional inhibitors for oncogenes and has potential as a strategy to control cancer growth and development. Abstract Aberrant gene expression is often linked to the progression of various cancers, making the targeting of oncogene transcriptional activation a potential strategy to control tumor growth and development. The RET proto-oncogene’s gain-of-function mutation is a major cause of medullary thyroid carcinoma (MTC), which is part of multiple endocrine neoplasia type 2 (MEN2) syndrome. In this study, we used a cell-based bioluminescence reporter system driven by the RET promoter to screen for small molecules that potentially suppress the RET gene transcription. We identified adefovir dipivoxil as a transcriptional inhibitor of the RET gene, which suppressed endogenous RET protein expression in MTC TT cells. Adefovir dipivoxil also interfered with STAT3 phosphorylation and showed high affinity to bind to STAT3. Additionally, it inhibited RET-dependent TT cell proliferation and increased apoptosis. These results demonstrate the potential of cell-based screening assays in identifying transcriptional inhibitors for other oncogenes.