A keratinocyte cell line synthesizes a predominant insulin-like growth factor-binding protein (IGFBP-3) that modulates insulin-like growth factor-I action.

Insulin-like growth factor-I (IGF-I) is an important regulator of epidermal proliferation and has been shown in vitro to be a powerful stimulator of keratinocyte growth. It is synthesized by fibroblasts in the dermis, along with several IGF-binding proteins (IGFBPs), which are known to modulate IGF-I responsiveness of virtually all tissues studied. Because it was not known how or in what form IGF-I produced in the dermis acts on epidermal keratinocytes in vivo, we investigated the possible role of IGFBPs in modulating the response of epidermal keratinocytes to IGF-1. We show here that tIGF-I, a non-IGFBP-binding analogue of IGF-1, is a more potent mitogenic stimulator of the keratinocyte cell line HaCaT than IGF-I, suggesting that keratinocytes produce IGFBPs that modulate their response to IGF-I. To confirm this and to identify which IGFBPs were produced, we analyzed HaCaT cell-conditioned medium and mRNA, with the following findings: HaCaT cells produce a major IGFBP, identified as IGFBP-3, and a minor 24-kD IGFBP, likely to be IGFBP-4. Northern analysis revealed a 2.6-kb IGFBP-3 mRNA; however, IGFBP-4 mRNA was not detectable. We conclude that production of predominantly IGFBP-3 by the HaCaT cell line modulates its sensitivity to IGF-I stimulation. Epidermal IGFBPs thus have a potential role in vivo in the interaction of dermis derived IGF-I with epidermal keratinocytes.