IDENTIFICATION OF INSULIN-LIKE GROWTH-FACTOR BINDING-PROTEINS FROM CULTURED HUMAN EPIDERMAL-KERATINOCYTES

The role and mechanisms of action of insulin-like growth factors (IGFs ) in skin remain unclear. Epidermal keratinocytes possess IGF-I recept ors and are responsive to IGF-I, which is primarily derived from under lying dermal fibroblasts. IGF binding proteins (IGFBPs), also synthesi zed by fibroblasts, may be involved in paracrine targeting of IGF-I to its receptors. We therefore examined whether human keratinocytes synt hesize IGFBPs and their mRNAs. Following culture in complete medium (c ontaining bovine pituitary extract and epidermal growth factor) Wester n ligand blotting (WLB) of cell conditioned medium revealed a major ba nd of 32 kD, a less abundant IGFBP of 24 kD at all passages, and a 37- 42 kD IGFBP which increased in abundance in late passage. Immunoprecip itation followed by WLB confirmed that the predominant 32 kD band was IGFBP-2. Radioimmunoassay of IGFBP-1, -3, and -6 revealed detectable l evels of IGFBP-3 and significant levels of IGFBP-6, but not IGFBP-1. N orthern analysis following culture in complete medium revealed that at early passage IGFBP-1, -2, -4, and -6 mRNAs were detectable. IGFBP-3 and -5 mRNAs were not detectable. Following culture in growth factor-f ree medium a 37-42 kD band, consistent with IGFBP-3, was predominant a nd a 24 kD band consistent with IGFBP-4 was also present. These data d emonstrate the expression of a distinct pattern of IGFBPs by cultured human keratinocytes dependent on culture conditions. Keratinocyte-deri ved IGFBPs are likely to play a role in the transport and targeting of IGF-I from dermally derived fibroblasts to the epidermis. (C) 1995 Wi ley-Liss, Inc.