Plasminogen Activator Inhibitor-1 Regulates Integrin αvβ3 Expression and Autocrine Transforming Growth Factor β Signaling*

Fibrosis is characterized by elevated transforming growth factor β (TGFβ) signaling, resulting in extracellular matrix accumulation and increased PAI-1 (plasminogen activator inhibitor) expression. PAI-1 induces the internalization of urokinase plasminogen activator/receptor and integrin αvβ3 from the cell surface. Since increased αvβ3 expression correlates with increased TGFβ signaling, we hypothesized that aberrant PAI-1-mediated αvβ3 endocytosis could initiate an autocrine loop of TGFβ activity. We found that in PAI-1 knock-out (KO) mouse embryonic fibroblasts), αvβ3 endocytosis was reduced by ∼75%, leaving αvβ3 in enlarged focal adhesions, similar to wild type cells transfected with PAI-1 small interfering RNA. TGFβ signaling was significantly enhanced in PAI-1 KO cells, as demonstrated by a 3-fold increase in SMAD2/3-containing nuclei and a 2.9-fold increase in TGFβ activity that correlated with an increase in αvβ3 and TGFβ receptor II expression. As expected, PAI-1 KO cells had unregulated plasmin activity, which was only partially responsible for TGFβ activation, as evidenced by a mere 25% reduction in TGFβ activity when plasmin was inhibited. Treatment of cells with an αvβ3-specific cyclic RGD peptide (GpenGRGD) led to a more profound (59%) TGFβ inhibition; a nonspecific RGD peptide (GRGDNP) inhibited TGFβ by only 23%. Human primary fibroblasts were used to confirm that PAI-1 inhibition and β3 overexpression led to an increase in TGFβ activity. Consistent with a fibrotic phenotype, PAI-1 KO cells were constitutively myofibroblasts that had a 1.6-fold increase in collagen deposition over wild type cells. These data suggest that PAI-1-mediated regulation of αvβ3 integrin is critical for the control of TGFβ signaling and the prevention of fibrotic disease

Palygorskite in the Late Miocene red clay sediment from the Chinese Loess Plateau and its paleoclimatic implications

Palygorskite is the predominant component of the Red Clay sediment on the Chinese Loess Plateau. The morphological characteristics and microstructures of palygorskite in four sections of late Miocene Red Clay were investigated using power X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). The XRD results suggest that the clay minerals in the Red Clay are mainly illite, kaolinite, chlorite, and palygorskite with smectite. Two types of palygorskite microtexture were observed in the Red Clay sediment: (1) relatively straight, fibrous crystals in bundles or intertwined aggregates or in matted, felted masses on other minerals and silky aggregates radiating from platy minerals; and (2) single crystals scattering among or coating detrital particles. Based on SEM investigations, the first aggregate is considered to be the major type of microtexture. The occurrence of this type of microtexture supports the hypothesis that palygorskite developed through pedogenesis of the red clay deposits. The authigenic palygorskites are thought to be formed through the transformation of existing aeolian mineral (e.g., smectite) and direct chemical deposition in soil pore solution. Consequently, the formation of palygorskite was largely controlled by the strength of the in situ pedogenesis process induced by the East Asian summer, and palygorskite can serve as an index mineral of arid and semiarid environment since the late Miocene for the Chinese Loess Plateau. These findings add to existing fundamental mineral data related to the climatic evolution of the northwest China arid region and of arid and semiarid zones in the Northern Hemisphere