Normal human melanocytes that express a bFGF transgene still require exogenous bFGF for growth in vitro

The expression of basic fibroblast growth factor (bFGF) has been implicated as an important factor in the development of malignant melanoma. The timing of this expression suggests that bFGF plays a role early in melanoma tumor progression. Benign nevi produce bFGF, and cells cultured from these lesions show a loss of dependence on exogenous bFGF for growth. We have examined the effects of constitutive bFGF expression on the in vitro growth requirements of normal human melanocytes. bFGF was overexpressed in normal human epidermal melanocytes through genomic insertion of a human bFGF cDNA in a retroviral vector. These melanocytes produced the 18 kDa bFGF isoform as well as the higher molecular weight isoforms. The bFGF was not released into the culture medium, but it was present in the cell nucleus. The bFGF produced by these cells was mitogenic for 3T3 fibroblasts and therefore possessed functional activity; however, melanocytes producing bFGF had the same appearance and growth patterns as those infected with control virus or uninfected melanocytes. Expression of bFGF did not confer independence from the exogenous mitogen, nor would these cells form colonies in a soft-agar medium. These results indicate that expression of bFGF alone is not enough to cause aberrant growth of normal human melanocytes.