TARGETED EXPRESSION OF INSULIN-LIKE GROWTH-FACTOR TO HUMAN KERATINOCYTES - MODIFICATION OF THE AUTOCRINE CONTROL OF KERATINOCYTE PROLIFERATION

Somatomedin C/insulin-like growth factor-I (IGF-I) is required for the proliferation of keratinocytes in vitro. In skin, the cells known to synthesize IGF-I are melanocytes and fibroblasts of the dermis, To inv estigate the role of IGF-I as a mediator of keratinocyte proliferation , we have used retroviral-mediated gene transfer to introduce the gene encoding human IGF-I into diploid human keratinocytes, thus causing t hese cells to produce a growth factor they normally do not express, Mo dified cells synthesized and secreted significant levels of IGF-I (560 ng/10(7) cells/24 h) in vitro. Cells expressing IGF-I were no longer dependent on exogenously added EGF-I or insulin for their sustained gr owth in vitro under serum-free conditions. The growth of these cells d id require added epidermal growth factor (EGF) and bovine pituitary ex tract. The addition of an antibody that neutralizes IGF-I inhibited ce ll growth, suggesting that IGF-I must be secreted by the cells to prom ote cell proliferation, To investigate the role of IGF-I in vivo, we g rafted modified keratinocytes expressing IGF-I onto athymic mice. Graf ts of epithelial sheets of modified cells formed a stratified epitheli um comparable to control grafts of unmodified cells, When analyzed for keratin 16 expression and by quantitative staining for the nuclear pr oliferation antigen Ki-67, however, modified epithelia showed an incre ase in these markers of proliferation when compared with grafts of unm odified cells, This study demonstrates that genetic modification can b e used to modify the autocrine control of keratinocyte proliferation, The de novo synthesis of IGF-I by keratinocytes could sustain keratino cytes growth in vitro and stimulate proliferation in vivo without sign ificantly altering epidermal differentiation, These data further suppo rt the role of IGF-I as a paracrine mediator of epidermal proliferatio n and as a potential signal of mesenchymal-epithelial interactions.