Tissue plasminogen activator increases canine endothelial cell proliferation rate through a plasmin-independent, receptor-mediated mechanism.
BACKGROUND
Tissue plasminogen activator (tPA) is elevated in cancer patients and is thought to promote tumor angiogenesis by facilitating endothelial cell migration through plasmin-mediated degradation of extracellular matrix. Due to the presence of an epidermal growth factor (EGF)-finger domain in the tPA A-chain and the existence of an endothelial cell (EC) receptor that binds this domain, it was hypothesized that tPA has a direct receptor-mediated effect on EC proliferation, independent of plasmin.


RESULTS
Using cultured canine ECs, tPA (7.25 microg/ml, approximately 107 nM) increased proliferation as much as 50 and 170% in the absence and presence of growth factors, respectively. tPA-induced increases in EC proliferation occurred independent of plasmin generation, as the plasmin inhibitor, aprotinin (10 microg/ml) did not inhibit tPA-induced proliferation. However, tPA-induced proliferation was inhibited dose-dependently to a maximum of 78% using a monoclonal antibody against the tPA EGF-finger domain. This antibody, known to inhibit tPA binding to its receptor, did not inhibit tPA-induced plasmin generation. To investigate the role of potential signal transduction pathways, ECs were exposed to lavendustin A, a tyrosine kinase inhibitor, at 33.5 microM (IC50 for basic fibroblast growth factor). Lavendustin A did not inhibit tPA-induced EC proliferation. However, Rp-cAMP, an inhibitor of cAMP-dependent kinases, specifically inhibited tPA-induced EC proliferation in a dose-dependent manner (IC50 = 50.5 microM). Pertussis toxin at maximal concentrations for this system (0.5 ng/ml) did not inhibit tPA-induced EC proliferation.


CONCLUSIONS
These results lend support to the hypothesis that tPA may have a direct receptor-mediated effect on EC proliferation and that this effect occurs independent of plasmin and may be dependent upon protein kinase A activity.
Welling
T H
TH
Jobst Vascular Research Laboratories, The University of Michigan Medical School, Ann Arbor, Michigan, 48109, USA.
Huber
T S
TS
Messina
L M
LM
Stanley
J C
JC
D000818:Animals; D002455:Cell Division / Q000187:drug effects; D002478:Cells, Cultured; D017868:Cyclic AMP-Dependent Protein Kinases / Q000502:physiology; D004285:Dogs; D004730:Endothelium, Vascular / Q000166:cytology*; D005341:Fibrinolysin / Q000502:physiology*; D011956:Receptors, Cell Surface / Q000502:physiology*; D015398:Signal Transduction; D010959:Tissue Plasminogen Activator / Q000494:pharmacology*
D011956:Receptors, Cell Surface; D017868:Cyclic AMP-Dependent Protein Kinases; C067262:PLAT protein, human; D010959:Tissue Plasminogen Activator; D005341:Fibrinolysin