Autophagy is a major degradation pathway that utilizes lysosome hydrolases to degrade cellular constituents and is often induced under cellular stress conditions to restore cell homeostasis. Another prime degradation pathway in the cells is ubiquitin-proteasome system (UPS), in which proteins tagged by certain types of polyubiquitin chains are selectively recognized and removed by proteasome. Although the two degradation pathways are operated independently with different sets of players, recent studies have revealed reciprocal cross talks between UPS and autophagy at multiple layers. In this review, we summarize the roles of protein ubiquitination and deubiquitination in controlling the initiation, execution, and termination of bulk autophagy as well as the role of ubiquitination in signaling certain types of selective autophagy. We also highlight how dysregulation of ubiquitin-mediated autophagy pathways is associated with a number of human diseases and the potential of targeting these pathways for disease intervention.

Transforming growth factor-β (TGF-β) affects cellular proliferation, differentiation, and interaction with the extracellular matrix primarily through interaction with the type I and type II TGF-β receptors. The type II receptors for TGF-β and activin contain putative serine-threonine kinase domains. A murine serine-threonine kinase receptor, Tsk 7L, was cloned that shared a conserved extracellular domain with the type II TGF-β receptor. Overexpression of Tsk 7L alone did not increase cell surface binding of TGF-β, but coexpression with the type II TGF-β receptor caused TGF-β to bind to Tsk 7L, which had the size of the type I TGF-β receptor. Overexpression of Tsk 7L inhibited binding of TGF-β to the type II receptor in a dominant negative fashion. Combinatorial interactions and stoichiometric ratios between the type I and II receptors may therefore determine the extent of TGF-β binding and the resulting biological activities.

Transforming growth factor-β (TGF-β) is a multifunctional protein that regulates cell proliferation and differentiation and extracellular matrix production. Although two receptor types, the type I and type II receptors, have been implicated in TGF-β-induced signaling, it is unclear how the many activities of TGF-β are mediated through these receptors. With the use of cells overexpressing truncated type II receptors as dominant negative mutants to selectively block type II receptor signaling, the existence of two receptor pathways was shown. The type II receptors, possibly in conjunction with type I receptors, mediate the induction of growth inhibition and hypophosphorylation of the retinoblastoma gene product pRB. The type I receptors are responsible for effects on extracellular matrix, such as the induction of fibronectin and plasminogen activator inhibitor I, and for increased JunB expression. Selective inactivation of the type II receptors alters the TGF-β response in a similar manner to the functional inactivation of pRB, suggesting a role for pRB in the type II, but not the type I, receptor pathway.

Metastasis is the major cause of poor prognosis in colorectal cancer (CRC), and increasing evidence supports the contribution of miRNAs to cancer progression. Here, we found that high expression of miR-103 and miR-107 (miR-103/107) was associated with metastasis potential of CRC cell lines and poor prognosis in patients with CRC. We showed that miR-103/107 targeted the known metastasis suppressors death-associated protein kinase (DAPK) and Krüppel-like factor 4 (KLF4) in CRC cells, resulting in increased cell motility and cell–matrix adhesion and decreased cell–cell adhesion and epithelial marker expression. miR-103/107 expression was increased in the presence of hypoxia, thereby potentiating DAPK and KLF4 downregulation and hypoxia-induced motility and invasiveness. In mouse models of CRC, miR-103/107 overexpression potentiated local invasion and liver metastasis effects, which were suppressed by reexpression of DAPK or KLF4. miR-103/107–mediated downregulation of DAPK and KLF4 also enabled the colonization of CRC cells at a metastatic site. Clinically, the signature of a miR-103/107 high, DAPK low, and KLF4 low expression profile correlated with the extent of lymph node and distant metastasis in patients with CRC and served as a prognostic marker for metastasis recurrence and poor survival. Our findings therefore indicate that miR-103/107–mediated repression of DAPK and KLF4 promotes metastasis in CRC, and this regulatory circuit may contribute in part to hypoxia-stimulated tumor metastasis. Strategies that disrupt this regulation might be developed to block CRC metastasis. Cancer Res; 72(14); 3631–41. ©2012 AACR.