mTORC1 drives HIF-1α and VEGF-A signalling via multiple mechanisms involving 4E-BP1, S6K1 and STAT3

Area of Science:

• Biochemistry
• Molecular Biology
• Oncology

Background:

• Tuberous Sclerosis Complex (TSC) is associated with vascularized tumors like angiomyolipomas.
• Rapalogues show promise in TSC tumor regression by targeting mechanistic/mammalian target of rapamycin complex 1 (mTORC1) signaling.
• mTORC1 inhibition is hypothesized to suppress hypoxia-inducible factors (HIFs) and vascular endothelial growth factors (VEGFs).

Purpose of the Study:

• To investigate the molecular mechanisms by which mTORC1 inhibition affects HIF-1α and VEGF-A levels in TSC-associated tumors.
• To elucidate the role of signal transducer and activation of transcription 3 (STAT3) in mTORC1-mediated HIF-1α regulation.
• To understand the differential regulation of HIF-1α and VEGF-A by mTORC1 signaling pathways.

Main Methods:

• Utilized a Tsc2+/- mouse model with renal cystadenoma cells.
• Administered rapamycin to assess its effects on HIF-1α and VEGF-A protein and mRNA levels.
• Investigated the impact of STAT3 inhibition/knockdown and phosphorylation on HIF-1α transcription.
• Examined the roles of 4E-binding protein 1 (4E-BP1) and ribosomal protein S6 kinase-1 (S6K1) in HIF-1α translation and VEGF-A expression.

Main Results:

• Rapamycin treatment reduced HIF-1α protein and, to a lesser extent, VEGF-A levels in renal cystadenoma cells.
• mTORC1 promotes HIF-1α mRNA transcription via STAT3, which is directly phosphorylated by mTORC1 during hypoxia.
• mTORC1 regulates HIF-1α translation through 4E-BP1 and S6K1, but not by affecting HIF-1α degradation.
• VEGF-A expression is primarily controlled by 4E-BP1/eIF4E, suggesting both HIF-1α-dependent and -independent mTORC1-mediated mechanisms for VEGF-A regulation.

Conclusions:

• mTORC1 is a central mediator in TSC tumor pathogenesis, driving HIF-1α synthesis through STAT3, 4E-BP1, and S6K1.
• mTORC1 influences VEGF-A expression via both HIF-1α-dependent and -independent pathways.
• These findings provide critical insights into the molecular basis of rapalogue efficacy in treating vascularized tumors in TSC and have implications for anti-angiogenic therapies.