TGF-β regulates differentially the proliferation of fetal and adult human skin fibroblasts via the activation of PKA and the autocrine action of FGF-2

Transforming growth factor-β (TGF-β) is a potent regulator of cell proliferation; interestingly its action is clearly cell type-dependent. In particular, it inhibits epithelial and endothelial cells' proliferation, while its action on many mesenchymal cells has been reported to be stimulatory. In this direction, we have recently shown that TGF-β regulates the proliferation of normal human skin fibroblasts according to their developmental origin: i.e. it inhibits fetal fibroblasts, while it stimulates the proliferation of adult ones. Here, we present evidence on the mechanisms underlying this differential action. Concerning fetal fibroblasts, we have found that TGF-β activates Protein Kinase A (PKA) and induces the expression of the cyclin-dependent kinase inhibitors (CKIs) p21CIP1/WAF1 and p15INK4B. Moreover, the specific PKA inhibitor H-89 blocks the induction of both CKIs and annuls the TGF-β-mediated inhibitory effect, indicating the central role of PKA in this process. In contrast, in adult cells no PKA activation is observed. Moreover, TGF-β stimulates cell proliferation by activating the MEK-ERK pathway, as the MEK inhibitor PD98059 blocks this effect. A specific neutralizing antibody against Fibroblast Growth Factor-2 (FGF-2) inhibits both ERK activation and the mitogenic activity of TGF-β, indicating that the latter establishes an autocrine loop, via FGF-2, leading to cell proliferation. This loop requires FGF receptor-1 (FGFR-1), as its down-regulation by siRNA approach prevents TGF-β from stimulating ERK-1/2 activation and DNA synthesis. In conclusion, the differential proliferative response of fetal and adult normal human skin fibroblasts to TGF-β is regulated by distinct signaling pathways and furthermore it may provide information on the bimodal effect of this factor on cell proliferation, in general.