Platelet-derived
growth factors (PDGFs) are critical for development; their over-expression is associated with fibrogenesis. Full-length
PDGF-C is secreted as an inactive dimer, requiring cleavage to allow receptor binding. Previous studies indicate that
tissue-type plasminogen activator (tPA) is the specific
protease that performs this cleavage; in vivo confirmation is lacking. We demonstrate that primary hepatocytes from tpa KO mice produce less cleaved active
PDGF-CC than do wild type hepatocytes, suggesting that tPA is critical for in vitro activation of this
growth factor. We developed mice that over-express full-length human
PDGF-C in the liver; these mice develop progressive
liver fibrosis. To test whether tPA is important for cleavage and activation of
PDGF-C in vivo, we intercrossed
PDGF-C transgenic (Tg) and tpa knock-out (KO) mice, anticipating that lack of tPA would result in decreased
fibrosis due to lack of hPDGF-C cleavage. To measure levels of cleaved, dimerized
PDGF-CC in sera, we developed an ELISA that specifically detects cleaved
PDGF-CC. We report that the absence of tpa does not affect the phenotype of `
PDGF-C Tg mice.
PDGF-C Tg mice lacking tPA have high serum levels of cleaved
growth factor, significant
liver fibrosis, and gene expression alterations similar to those of
PDGF-C Tg mice with intact tPA. Furthermore,
urokinase plasminogen activator and
plasminogen activator inhibitor-1 expression are increased in
PDGF-C Tg; tpa KO mice. Our ELISA data suggest a difference between in vitro and in vivo activation of this
growth factor, and our mouse model confirms that multiple
proteases cleave and activate
PDGF-C in vivo.