Received March 12, 2007
Accepted on September 19, 2007

BASIC RESEARCH

tPA Protects Renal Interstitial Fibroblasts and Myofibroblasts from Apoptosis

Kebin Hu ^*, Ling Lin , Xiaoyue Tan ^*, Junwei Yang , Guojun Bu , Wendy M. Mars ^*, and Youhua Liu ^*1

Departments of *Pathology and Medicine, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania; Department of Medicine, First Affiliated Hospital, Nanjing Medical University, Nanjing, China; and Departments of Pediatrics, Cell Biology and Physiology, Washington University School of Medicine, St. Louis, Missouri

¹ To whom correspondence should be addressed. E-mail: liuy{at}upmc.edu.

	Abstract

Activation and expansion of interstitial fibroblasts and myofibroblasts play an essential role in the evolution of renal fibrosis. After obstructive injury, mice lacking tissue-type plasminogen activator (tPA) have fewer myofibroblasts and less interstitial fibrosis than wild-type controls. This suggests that tPA controls the size of the fibroblast/myofibroblast population in vivo, and this study sought to determine the underlying mechanism. In vitro, tPA inhibited staurosporine or H₂O₂-induced caspase-3 activation, prevented cellular DNA fragmentation, and suppressed the release of cytochrome C from mitochondria into the cytosol in a rat interstitial fibroblast cell line (NRK-49F). tPA also protected TGF-1–activated myofibroblasts from apoptosis. This antiapoptotic effect of tPA was independent of its protease activity but required its membrane receptor, the LDL receptor–related protein 1 (LRP-1). Deletion or knockdown of LRP-1 abolished tPA-mediated cell survival, whereas re-introduction of an LRP-1 minigene in a mouse LRP-1–deficient fibroblast cell line (PEA-13) restored the cytoprotective ability of tPA. tPA triggered a cascade of survival signaling involving extracellular signal–regulated kinase 1/2 (Erk1/2), p90RSK, and phosphorylation of Bad. Blockade of Erk1/2 activation abrogated the antiapoptotic effect of tPA, whereas expression of constitutively active MEK1 promoted cell survival similar to tPA. In vivo, compared with wild-type controls, apoptosis of interstitial myofibroblasts was increased in tPA^-/- mice after obstructive injury, and myofibroblasts were completely depleted 4 wk after relief of the obstruction. Together, these findings illustrate that tPA is a survival factor that prevents apoptosis of renal interstitial fibroblasts and myofibroblasts through an LRP-1–, Erk1/2-, p90RSK-, and Bad-dependent mechanism.