Metastasis is a primary cause of mortality due to cancer. Early metastatic growth involves both a remodeling of the extracellular matrix surrounding tumors and invasion of tumors across the basement membrane. Up-regulation of extracellular matrix degrading proteases such as urokinase plasminogen activator (uPA) and matrix metalloproteinases has been reported to facilitate tumor cell invasion. Autocrine transforming growth factor- (TGF-) signaling may play an important role in cancer cell invasion and metastasis; however, the underlying mechanisms remain unclear. In the present study, we report that autocrine TGF- supports cancer cell invasion by maintaining uPA levels through protein secretion. Interestingly, treatment of paracrine/exogenous TGF- at higher concentrations than autocrine TGF- further enhanced uPA expression and cell invasion. The enhanced uPA expression by exogenous TGF- is a result of increased uPA mRNA expression due to RNA stabilization. We observed that both autocrine and paracrine TGF--mediated regulation of uPA levels was lost upon depletion of Smad4 protein by RNA interference. Thus, through the Smad pathway, autocrine TGF- maintains uPA expression through facilitated protein secretion, thereby supporting tumor cell invasiveness, whereas exogenous TGF- further enhances uPA expression through mRNA stabilization leading to even greater invasiveness of the cancer cells.Malignant tumors are characterized by their ability to metastasize to distant organs. The initial steps of metastasis involve invasive growth of tumors across the basement membrane and migration through the extracellular matrix (ECM).2 Because the enzymatic degradation of both the basement membrane and ECM barriers requires a number of ECM-degrading proteases (1, 2) and is a critical early event in metastasis, invasiveness may be modulated by the expression of ECM-degrading proteases in tumor cells in response to autonomous and microenvironmental signals. Among the increasing number of ECMdegrading proteases implicated in metastasis, considerable attention has been focused on the family of matrix metalloproteinases (MMPs) and the plasminogen activator system. One of the regulators of these ECM-degrading proteases is transforming growth factor- (TGF-).TGF- is a multifunctional cytokine that regulates cell proliferation, differentiation, plasticity, and migration in a contextdependent manner (reviewed in Refs. 3 and 4). TGF- transduces signaling through a transmembrane type II receptor (TRII), a constitutively active serine/threonine kinase receptor (5). Upon ligand binding, the TRII recruits and transphosphorylates intracellular TGF- type I receptor (TRI), thereby stimulating TRI serine/threonine kinase activity (6). The TRI then activates the downstream effectors, Smad2 and Smad3, by phosphorylation. The activated Smad proteins form complexes with the common Smad mediator, Smad4, and then translocate to the nucleus, where the Smad complexes regulate transcription of TGF- target genes in conjunction with vario...