Dampened VEPH1 activates mTORC1 signaling by weakening the TSC1/TSC2 association in hepatocellular carcinoma

•Loss of VEPH1 correlates with poor prognosis in HCC. •VEPH1 acts as a tumor suppressor in HCC. •VEPH1 antagonizes mTORC1 signaling by enhancing TSC1/TSC2 binding ability. •Rapamycin inhibits VEPH1 deficiency-induced mTORC1 activation and hepatocellular carcinogenesis. Background & Aims Abnormal activation of mTORC1 signaling occurs at high frequency in hepatocellular carcinoma (HCC). However, the underlying causes of this aberrant activation remain elusive. In this study, we identified ventricular zone expressed pleckstrin homology domain-containing 1 (VEPH1) as a novel tumor suppressor that acts via the mTORC1 axis. Methods We performed quantitative reverse-transcription PCR (92 pairs), western blot (30 pairs), and immunostaining (225 cases) assays in HCC tissue samples to evaluate VEPH1 expression. We explored the functional effects of VEPH1 on tumor growth and metastasis. Molecular and biochemical strategies were used to gain insight into mechanisms underlying the tumor-suppressive function of VEPH1. Results VEPH1 is frequently silenced in HCC tissues, primarily resulting from let-7d upregulation. Decreased VEPH1 expression is associated with poor prognosis and aggressive tumor phenotypes in patients with HCC. VEPH1 mediates its tumor-suppressing activity through regulation of cell proliferation, migration and invasion in vitro and in vivo. The VEPH1 fragments 580–625aa and 447–579 aa bind directly to TSC1 (719–1,164aa) and TSC2 (1–420 aa), respectively, enhancing TSC1/TCS2 binding and promoting translocation of TSC2 to the membrane, which leads to increased TSC2 Ser1387 phosphorylation. Subsequently, Rheb is inactivated by the GTPase activity of TSC2, inhibiting mTORC1 signaling and contributing to changes in HCC carcinogenesis and metastasis. Rapamycin, the mTOR inhibitor, can inhibit the pro-tumorigenic effect of VEPH1 knockdown. Loss of VEPH1 correlates with decreased TSC2 Ser1387 phosphorylation and increased mTOR activity in HCC specimens. Conclusions The loss of VEPH1 leads to aberrantly activated mTORC1 signaling in HCC; rapamycin (or rapalogs) may serve as an effective treatment option for patients with HCC and dampened VEPH1 expression. Lay summary Abnormally activated mammalian target of rapamycin (mTOR) signaling is associated with poor tumor differentiation, early tumor recurrence and worse overall survival in patients with hepatocellular carcinoma. Herein, we identify low VEPH1 expression as a potential cause of abnormally activated mTOR signaling in hepatocellular carcinoma tissues. mTOR inhibitors could thus be an effective treatment option for patients with HCC and low VEPH1 expression. Abnormal activation of mTORC1 signaling occurs at high frequency in hepatocellular carcinoma (HCC). However, the underlying causes of this aberrant activation remain elusive. In this study, we identified ventricular zone expressed pleckstrin homology domain-containing 1 (VEPH1) as a novel tumor suppressor that acts via the mTORC1 axis. We performed quantitative reverse-transcription PCR (92 pairs), western blot (30 pairs), and immunostaining (225 cases) assays in HCC tissue samples to evaluate VEPH1 expression. We explored the functional effects of VEPH1 on tumor growth and metastasis. Molecular and biochemical strategies were used to gain insight into mechanisms underlying the tumor-suppressive function of VEPH1. VEPH1 is frequently silenced in HCC tissues, primarily resulting from let-7d upregulation. Decreased VEPH1 expression is associated with poor prognosis and aggressive tumor phenotypes in patients with HCC. VEPH1 mediates its tumor-suppressing activity through regulation of cell proliferation, migration and invasion in vitro and in vivo. The VEPH1 fragments 580–625aa and 447–579 aa bind directly to TSC1 (719–1,164aa) and TSC2 (1–420 aa), respectively, enhancing TSC1/TCS2 binding and promoting translocation of TSC2 to the membrane, which leads to increased TSC2 Ser1387 phosphorylation. Subsequently, Rheb is inactivated by the GTPase activity of TSC2, inhibiting mTORC1 signaling and contributing to changes in HCC carcinogenesis and metastasis. Rapamycin, the mTOR inhibitor, can inhibit the pro-tumorigenic effect of VEPH1 knockdown. Loss of VEPH1 correlates with decreased TSC2 Ser1387 phosphorylation and increased mTOR activity in HCC specimens. The loss of VEPH1 leads to aberrantly activated mTORC1 signaling in HCC; rapamycin (or rapalogs) may serve as an effective treatment option for patients with HCC and dampened VEPH1 expression.