miR-125b-5p/STAT3 Pathway Regulated by mTORC1 Plays a Critical Role in Promoting Cell Proliferation and Tumor Growth

Aberrant activation of the mammalian target of rapamycin complex 1 (mTORC1) plays a critical role in tumorigenesis. However, the precise underlying mechanism is still not fully understood. Although accumulating evidence suggests that mTORC1 signaling is regulated by microRNAs (miRNAs), whether miRNAs are involved in the tumorigenesis mediated by mTORC1 dysregulation remains largely unclear. In our study, the comparison between tuberous sclerosis complex 1 (Tsc1) -/- or Tsc2-/- mouse embryonic fibroblasts (MEFs) and the control cells revealed the involvement of microRNA-125b-5p (miR-125b-5p) in the tumorigenesis driven by mTORC1 activation. Our study also showed that loss of TSC1 or TSC2 led to significant downregulation of miR-125b-5p and upregulation of signal transducer and activator of transcription 3 (STAT3) via mTORC1 activation. Overexpression of miR-125b-5p inhibited the proliferation of the cells with hyperactivated mTORC1 both in vitro and in vivo. Furthermore, we demonstrated that STAT3 is a direct target of miR-125b-5p. Depletion of STAT3 mimicked the effect of ectopic expression of miR-125b-5p, and reintroduction of STAT3 rescued the compromised cell proliferation driven by miR-125b-5p overexpression in Tsc1-/- or Tsc2-/- MEFs. We conclude that the miR-125b-5p/STAT3 pathway plays a crucial role in hyperactivated mTORCI-mediated tumorigenesis and miR-125b-5p is a potential therapeutic target.