Insulin-like growth factors are mitogenic for human keratinocytes and a squamous cell carcinoma.
 Normal adult human keratinocytes in monolayer culture and SCL-1, a skin-derived squamous-cell carcinoma cell line, were investigated for the expression of receptors for insulin-like growth factors (IGF) and insulin.
 As demonstrated by affinity crosslinking, radiolabeled IGF-1, IGF-2, and insulin bound specifically to both cell types.
 Each cell expressed type I IGF receptors, with affinity for IGF-1 greater than IGF-2 much greater than insulin.
 Insulin receptors, with highest affinity for insulin, were also present on both cells.
 However, keratinocytes and SCL-1 cells differed in 125I-IGF-2 binding.
 125I-IGF-2-bound to both type I and type II IGF receptors in normal keratinocytes, but bound predominantly to membrane-associated IGF binding proteins in SCL-1.
 IGF-1 was slightly more potent than IGF-2 in stimulating growth of both keratinocytes and SCL-1 cells.
 In keratinocytes, concentrations of IGF-1 ranging from 5-100 ng/ml, and of IGF-2 from 50-100 ng/ml, resulted in a significant increase in cell number.
 At the maximum dose of 100 ng/ml, either IGF-1 or IGF-2 caused a 2.3-times increase in cell number.
 In SCL-1 cells, IGF-1 was more potent than IGF-2 or insulin at lower concentrations, but either IGF-1 or IGF-2 at the maximal concentration of 333 ng/ml stimulated a 4.7-times increase in thymidine incorporation.
 The stimulatory effect of insulin in SCL-1 was 10-50 times less potent than that of the IGF.
 The effect of either IGF on SCL-1 was completely inhibited by the type I IGF receptor antibody alpha IR-3, suggesting that both IGFs are mitogenic through the type I IGF receptor.
 Insulin action was partially blocked by alpha IR-3, suggesting that insulin can act through both the insulin and type I IGF receptors.
 It thus appears that IGF-1 and IGF-2 are mitogens for normal and transformed human keratinocytes and that their actions are primarily mediated through the type I IGF receptor, whereas insulin is a mitogen through both the IGF-1 receptor and the insulin receptor.
