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 investigate 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 these cells to produce a growth factor they normally do not express. Modified 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 IGF-I or insulin for their sustained growth in vitro under serum-free conditions. The growth of these cells did require added epidermal growth factor (EGF) and bovine pituitary extract. The addition of an antibody that neutralizes IGF-I inhibited cell growth, suggesting that IGF-I must be secreted by the cells to promote cell proliferation. To investigate the role of IGF-I in vivo, we grafted modified keratinocytes expressing IGF-I onto athymic mice. Grafts of epithelial sheets of modified cells formed a stratified epithelium comparable to control grafts of unmodified cells. When analyzed for keratin 16 expression and by quantitative staining for the nuclear proliferation antigen Ki-67, however, modified epithelia showed an increase in these markers of proliferation when compared with grafts of unmodified cells. This study demonstrates that genetic modification can be used to modify the autocrine control of keratinocyte proliferation. The de novo synthesis of IGF-I by keratinocytes could sustain keratinocytes growth in vitro and stimulate proliferation in vivo without significantly altering epidermal differentiation. These data further support the role of IGF-I as a paracrine mediator of epidermal proliferation and as a potential signal of mesenchymal-epithelial interactions.