GROWTH-FACTOR INDUCED MODULATION OF ENDOTHELIN-1 BINDING TO HUMAN SMOOTH-MUSCLE CELLS

Endothelin-1 (ET-1) has been shown to cooperate with other growth factors to enhance mitogenesis of fibroblasts and vascular smooth-muscle cells (SMCs) in vitro. One possible mechanism underlying such enhancement is the comodulation of receptor density/affinity for one factor by the other. In previous work, we showed that pretreatment of Swiss 3T3 fibroblasts with such growth factors as epidermal growth factor (EGF), platelet-derived growth factor (PDGF), or basic fibroblast growth factor (bFGF) resulted in increased binding of I-125-ET-1 to these cells by two-, four-, and fivefold, respectively. To determine whether similar effects occur in human cells, I-125-ET-1 binding to early-passage human aortic SMCs was examined in untreated cells and in cells pre-treated for 16 h with 1.0 nM of EGF, PDGF, or bFGF. In untreated cells, Scatchard analysis confirmed 26,500 +/- 2,000 (n = 4) binding sites with an apparent K(d) of 105 +/- 53 pM. Pretreatment with EGF increased the number of binding sites to 36,500 +/- 4,950 (n = 3) with no significant change in K(d) (128 +/- 38 pM). Similarly, pretreatment with 1.0 nM bFGF also increased the number of I-125-ET-1 binding sites to 34,000 +/- 1,700 (n = 3) with no significant change in K(d) (94 +/- 13 pM). Unlike EGF and bFGF, pre-treatment with PDGF-BB resulted in a decrease of I-125-ET-1 binding sites (14,600 +/- 2,300 sites/cell; n = 3) with no significant change in K(d) (95 +/- 23 pM). These results extend our previous observations with murine fibroblasts and may provide an important insight into the regulation of ET activity, particularly under pathological conditions.