TSC2 acting as a transcription factor for miR-514b-3p and regulating PI3K-AKT-MTOR pathway via nucleus

The PI3K-AKT-MTOR signalling axis is pivotal in regulating cell survival, proliferation, and growth. TSC2 (tuberous sclerosis complex subunit 2) is a well-established negative regulator of this pathway, which primarily acts by suppressing the MTORC1 activity. While the cytoplasmic role of TSC2 is well characterized, emerging evidence suggests its additional nuclear functions. Previous work from our laboratory identified TSC2 as a transcriptional repressor of the EREG (Epiregulin) gene. Building on this foundation, the present study investigates the transcriptional role of TSC2 in miRNA (microRNA) gene regulation. A genome-wide miRNA microarray profiling of TSC2-depleted cells from an oral squamous cell carcinoma (OSCC) cell line, SCC131, identified 19 upregulated and 24 downregulated miRNAs. Of them, miR-514b-3p emerged as one of the most significantly upregulated miRNAs. TSC2 knockdown resulted in robust miR-514b-3p upregulation, whereas TSC2 overexpression suppressed its expression. Moreover, TSC2 negatively regulates MIR514B promoter activity in an NLS-dependent manner. The chromatin immunoprecipitation analysis showed direct binding between TSC2 and MIR514B promoter, establishing miR-514b-3p as a transcriptional target of TSC2. We further identified TSPAN9 (Tetraspanin 9) as a direct downstream target of miR-514b-3p. The dual-luciferase reporter assay and Western blot analysis confirmed direct interaction between miR-514b-3p and TSPAN9 3UTR. Furthermore, TSC2 positively regulates TSPAN9 levels by repressing miR-514b-3p, thereby establishing a novel TSC2-miR-514b-3p-TSPAN9 regulatory axis. Additionally, we uncovered crosstalk between TSC2-miR-514b-3p-TSPAN9 axis and the canonical PI3K-AKT-MTOR signalling, where miR-514b-3p positively, and TSPAN9 negatively regulates the PI3K-AKT-MTOR pathway. Interestingly, AKT functions as an upstream regulator of this axis by modulating TSC2 nuclear localization. Collectively, this study provides new insights into the non-canonical, nucleus-dependent transcriptional functions of TSC2, thus expanding its role beyond cytoplasmic signalling regulation and underscoring its significance in the cellular signalling networks.