tPA Is a Potent Mitogen for Renal Interstitial Fibroblasts: Role of β1 Integrin/Focal Adhesion Kinase Signaling

Proliferation and expansion of interstitial fibroblasts are predominant features of progressive chronic kidney diseases. However, how interstitial fibroblast proliferation is controlled remains ambiguous. Here we show that tissue-type plasminogen activator (tPA) is a potent mitogen that promotes interstitial fibroblast proliferation through a cascade of signaling events. In vitro, tPA promoted cell proliferation of rat kidney fibroblasts (NRK-49F), as assessed by cell counting, cell proliferation assay, and bromodeoxyuridine labeling. tPA also accelerated NRK-49F cell cycle progression. Fibroblast proliferation induced by tPA was associated with an increased expression of numerous proliferation-related genes, including c-fos, c-myc, proliferating cell nuclear antigen, and cyclin D1. The mitogenic effect of tPA was independent of its protease activity, but required LDL receptor-related protein 1. Interestingly, inhibition of β1 integrin signaling prevented tPA-mediated fibroblast proliferation. tPA rapidly induced tyrosine phosphorylation of focal adhesion kinase (FAK), which led to activation of its downstream mitogen-activated protein kinase signaling. Blockade of FAK, but not integrin-linked kinase, abolished the tPA-triggered extracellular signal-regulated protein kinase 1/2 activation, proliferation-related gene induction, and fibroblast proliferation. In vivo, proliferation of interstitial myofibroblasts in tPA null mice was attenuated after obstructive injury, compared with the wild-type controls. These studies illustrate that tPA is a potent mitogen that promotes renal interstitial fibroblast proliferation through LDL receptor-related protein 1-mediated β1 integrin and FAK signaling.