429 mTORC1 feedback to AKT modulates MiT/TFE-driven lysosomal biogenesis and EGFR degradation

Abstract

The Microphthalmia family of transcriptional factors (MiT/TFE) controls lysosomal biogenesis and is negatively regulated by the nutrient sensor mTORC1. Some tumors bypass this regulatory circuit via genetic alterations that drive MiT/TFE expression and activity; however, the mechanisms by which cells with intact/constitutive mTORC1 signaling maintain lysosomal catabolism remain undefined. Using the murine epidermis as a model system, we find that epidermal Tsc1 deletion results in a phenotype characterised by wavy hair and curly whiskers, due to increased EGFR degradation in epidermis and primary murine keratinocytes. Unexpectedly, mTORC1 activation with Tsc1 loss increases lysosomal content via up-regulated expression and transcriptional activity of MiT/TFEs, while genetic deletion of epidermal Rheb/ Rptor, or prolonged pharmacologic mTORC1 inactivation has the reverse effect. GSEA of transcriptome changes showed that the CLEAR (Co-ordinated Lysosomal Expression and Regulation) network of lysosomal genes was significantly downregulated in Rptor cKO epidermis. This paradoxical up-regulation of lysosomal biogenesis by mTORC1 is mediated by feedback inhibition of AKT, and a resulting suppression of AKT-induced MiT/TFE proteasomal degradation. Thus, inhibition of hyperactive AKT in the context of mTORC1 loss fully restores MiT/TFE expression and transcriptional activity. These data suggest that oncogenic feedback loops work to restrain or maintain cellular lysosomal content during chronically inhibited or constitutively activated mTORC1 signaling respectively, and reveal a mechanism by which mTORC1 regulates upstream receptor tyrosine kinase signaling.