Proteasome-dependent degradation of Smad7 is critical for lung cancer metastasis

Lung cancer is one of the cancers with highest morbidity and mortality rates and the metastasis of lung cancer is a leading cause of death. Mechanisms of lung cancer metastasis are yet to be fully understood. Herein, we demonstrate that mice deficient for REG gamma, a proteasome activator, exhibited a significant reduction in tumor size, numbers, and metastatic rate with prolonged survival in a conditional Kras/p53 mutant lung cancer model. REG gamma enhanced the TGF beta-Smad signaling pathway by ubiquitin-ATP-independent degradation of Smad7, an inhibitor of the TGF beta pathway. Activated TGF beta signaling in REG gamma-positive lung cancer cells led to diminished expression of E-cadherin, a biomarker of epithelial-mesenchymal transitions (EMT), and elevated mesenchymal markers compared with REG gamma-deficient lung cancer cells. REG gamma overexpression was found in lung cancer patients with metastasis, correlating with the reduction of E-Cadherin/Smad7 and a poor prognosis. Overall, our study indicates that REG gamma promotes lung cancer metastasis by activating TGF-beta signaling via degradation of Smad7. Thus, REG gamma may serve as a novel therapeutic target for lung cancers with poor prognosis.