In vitro paracrine regulation of human keratinocyte growth by fibroblast‐derived insulin‐like growth factors

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Abstract
Human keratinocytes isolated from a skin biopsy and cultured in vitro on a feeder‐layer of irradiated fibroblasts reconstitute a stratified squamous epithelium suitable for grafting onto patients suffering from large burn wounds. Since conditioned medium from 3T3‐J2 cells can partially substitute for the intact feederlayer, we studied the possible involvement of insulin‐like growth factors acting in a paracrine fashion. IGFs were measured (after Sephadex G‐50 gel‐chromatography in acid conditions) in media conditioned by a feeder‐layer of lethally irradiated 3T3‐J2 fibroblasts on which keratinocytes were grown. Immunoreactive (IR) IGF‐I, IGF‐II, and IGF binding activity were present in the medium conditioned by the feeder‐layer. The medium conditioned by keratinocytes showed nearly undetectable amounts of IR IGF‐I and IGF‐II, suggesting that keratinocytes are unable to synthesize IGFs peptides. Recombinant IGF‐I and IGF‐II, and conditioned medium from 3T3‐J2 cells, caused a dose‐dependent increase of 3H‐thymydine incorporation in cultured keratinocytes. The stimulatory effect of IGF and of 3T3‐J2 conditioned medium was inhibited by the MoAb Sm 1.2, which recognizes both IGF‐I and IGF‐II but not insulin, and by the MoAb αIR‐3, which is a specific antagonist of type‐1 IGF receptor. Fetal mouse‐derived 3T3‐J2 cells and adult human skin fibroblasts were equally able to sustain keratinocyte growth and in both cases addition of Sm 1.2 MoAb causes a 50% decrease in the keratinocyte number. When the non‐IGF‐producing BALB/c 3T3 cells were used as a feederlayer, the keratinocytes number was similar to that observed with 3T3‐J2 and with human fibroblasts plus the Sm 1.2 MoAb. IGF‐I and IGF‐II restored the BALB/c 3T3 growth promoting activity to the level of 3T3‐J2 and of normal human fibroblasts. Our results suggest that fetal mouse 3T3‐J2 and human fibroblasts synthesize IGF peptides, while keratinocytes do not. Fibroblast‐derived IGFs stimulate keratinocyte growth in a paracrine fashion, suggesting their role in the regulation of keratinocyte proliferation in skin growth and in wound healing.