Conditioned medium from cultured human keratinocytes has growth stimulatory properties on different human cell types

Evidence for growth-stimulatory properties of keratinocyte-conditioned medium (KCM) on human fibroblasts, endothelial cells, keratinocytes, smooth muscle cells, and a mouse fibroblast cell line (3T3 cells) is presented. On human fibroblasts KCM caused an increase of over 400% in DNA synthesis as revealed by 3H-thymidine incorporation and autoradiography. The proliferative effect was comparable to that of platelet-derived growth factor (PDGF), but was not inhibited by PDGF antibodies and exceeded that of transforming growth factor-alpha (TGF-alpha), epidermal growth factor (EGF), insulin-like growth factor-I (IGF-I), and basic fibroblast growth factor (bFGF). Furthermore, KCM was found to stimulate smooth muscle cells, keratinocytes, and endothelial cells more potently than PDGF, EGF/TGF-alpha, and bFGF, respectively. KCM was also potent in stimulating thymidine incorporation in 3T3 cells, whereas EGF showed a twenty-fold weaker stimulatory effect. Because keratinocytes have been shown to secrete TGF-alpha, which binds to the EGF receptor, binding of factors in KCM to the EGF receptor was assayed. The displacement of radiolabeled EGF by KCM corresponded to a low concentration of EGF (0.5 ng/ml), implying that the growth-stimulatory effect of KCM was not mediated via activation of EGF receptors. Taken together, these results suggest the presence of hitherto unidentified growth-stimulatory factor(s), expressed and secreted by cultured human keratinocytes.