HEPARIN INDUCES DIMERIZATION AND CONFERS PROLIFERATIVE ACTIVITY ONTO THE HEPATOCYTE GROWTH-FACTOR ANTAGONISTS NK1 AND NK2

Hepatocyte growth factor (HGF) is a potent epithelial mitogen whose ac tions are mediated through its receptor, the proto-oncogene c-Met. Two truncated variants of HGF known as NK1 and NK2 have been reported to be competitive inhibitors of HGF binding to c-Met, and to function as HGF antagonists (Lokker, N.A., and P.J. Godowski. 1993. J. Biol. Chem. 268: 17145-17150; Chan, A,M., J.S. Rubin, D.P. Bottaro, D.W. Hirschfi eld, M. Chedid, and S.A. Aaronson. 1991. Science (Wash. DC). 254:1382- 1387). We show here, however, that NK1 acts as a partial agonist in mi nk lung cells, Interestingly, NK1, which is an HGF antagonist in hepat ocytes in normal conditions, was converted to a partial agonist by add ing heparin to the culture medium. The interaction of NK1 and heparin was further studied in BaF3 cells, which express little or no cell sur face heparan sulfate proteoglycans. In BaF3 cells transfected with a p lasmid encoding human c-Met, heparin and NK1 synergized to stimulate D NA synthesis and cell proliferation. There was no effect of heparin on the IL-3 sensitivity of BaF3-hMet cells, and no effect of NK1 plus he parin in control BaF3 cells, indicating that the response was specific and mediated through c-Met. The naturally occurring HGF splice varian t NK2 also stimulated DNA synthesis in mink lung cells and exerted a h eparin-dependent effect on BaF3-hMet cells, but not on BaF3-neo cells. The activating effect of heparin was mimicked by a variety of sulfate d glycosaminoglycans. Mechanistic studies revealed that heparin increa sed the binding of NK1 to BaF3-hMet cells, stabilized NK1, and induced dimerization of NK1. Based on these studies, we propose that the norm al agonist activity of NK1 and NK2 in mink lung cells is due to an act ivating interaction with an endogenous glycosaminoglycan. Consistent w ith that model, a large portion of the NK1 binding to mink lung cells could be blocked by heparin. Moreover, a preparation of glycosaminogly cans from the surface of mink lung cells induced dimerization of NK1. These data show that the activity of NK1 and NK2 can be modulated by h eparin and other related glycosaminoglycans to induce proliferation in cells expressing c-Met.