mTOR-dependent upregulation of xCT blocks melanin synthesis and promotes tumorigenesis.

Loss of either TSC1 or TSC2 causes tuberous sclerosis complex (TSC) via activation of mTOR signaling pathway. The two prominent features of TSC are skin lesions including hypomelanic macules and benign tumors in multiple organs, whose molecular alterations are largely unknown. We report here that Xc- cystine/glutamate antiporter (xCT) was elevated in Tsc2-/- or Pten-/- cells, Tsc1 knockout mouse tissues and TSC2-deficient human kidney tumor. xCT was transcriptionally boosted by mTOR-mediated Oct1 signaling cascade. Augmented xCT led to reduction of eumelanin and elevation of pheomelanin in Tsc1 skin knockout mice through mTOR signaling pathway. Disruption of xCT suppressed the proliferation and tumorigenesis of Pten-null cells and Tsc2-null cells. mTOR hyperactive cells were more sensitive to inhibitors of mTOR or xCT. Combined inhibition of mTOR and xCT synergistically blocked the propagation and oncogenesis of mTOR hyperactive cells. Therefore, oncogenic mTOR activation of xCT is a key connection between aberrant melanin synthesis and tumorigenesis. We suggest that xCT is a novel therapeutic target for TSC and other aberrant mTOR-related diseases.

TSC1/TSC2 inactivation inhibits AKT through mTORC1-dependent up-regulation of STAT3-PTEN cascade.

Aberrant activation of mammalian target of rapamycin complex 1 (mTORC1), caused by loss or inactivation of TSC1/TSC2 protein complex, leads to negative feedback inhibition of Akt. The exact mechanisms of this process are still not fully understood. Here we present evidence for the involvement of STAT3, a known mTORC1 regulated transcription factor, in this process. We demonstrate that STAT3 promotes the transcription of PTEN by directly binding on the PTEN promoter. Elevated PTEN then inhibits the proliferation of Tsc1(-/-) or Tsc2(-/-) cells through down-regulation of Akt signaling. Activation of PTEN in this pathway may thus serve as a protective mechanism against hyper-activated mTORC1 mediated tumorigenesis and contribute to the benign nature of tumors caused by loss of either TSC1 or TSC2.

Lactate dehydrogenase B is critical for hyperactive mTOR-mediated tumorigenesis.

Mammalian target of rapamycin (mTOR) is a major downstream effector of the receptor tyrosine kinase (RTK)-phosphoinositide 3-kinase (PI3K)-v-akt murine thymoma viral oncogene homologue 1 (AKT) signaling pathway. Although this signaling network is frequently altered in cancer, the underlying mechanisms that cause tumorigenesis as a result of activated mTOR remain largely unknown. We report here that expression of lactate dehydrogenase B (LDHB), a critical enzymatic activator of glycolysis, was upregulated in an mTOR-dependent manner in TSC1(-/-), TSC2(-/-), PTEN(-/-), or activated AKT1-expressing mouse embryonic fibroblasts (MEF). LDHB gene expression was transactivated by signal transducer and activator of transcription 3 (STAT3), a key tumorigenic driver in many cancers, acting as a downstream mTOR effector in both mouse MEFs and human cancer cells. LDHB attenuation blunted the tumorigenic potential of oncogenic TSC2-null cells in nude mice. We concluded that LDHB is a downstream target of mTOR that is critical for oncogenic mTOR-mediated tumorigenesis. Our findings offer proof of concept for targeting LDHB as a therapeutic strategy in cancers driven by aberrant activation of the RTK-PI3K-AKT-mTOR signaling cascade.