TGF-beta isoforms differentially attenuate EGF mitogenicity and receptor activity in fetal lung mesenchymal cells

To evaluate signaling interactions, combinations of epidermal growth factor (EGF) and transforming growth factor-beta (TGF-beta) isoforms were applied to primary fetal mouse lung mesenchymal cells isolated at 16 days of gestation. The three isoforms of TGF-beta had similar mitogenic potentials, as assessed by thymidine incorporation (half-maximal effective concentration similar to 2 ng/ml). However, combined exposure to EGF and TGF-beta yielded an isoform-dependent attenuation of EGF-induced mitogenesis. Combinations of 20 ng/ml EGF and 2 ng TGF-beta 1, TGF-beta 2, or TGF-beta 3 resulted in thymidine incorporation values 0.76, 0.74, and 0.86 times that of EGF alone, respectively; attenuation of EGF mitogenicity, interactions between EGF and TGF-beta isoforms, and differences between isoforms were all statistically significant by analysis of variance. Treatment with TGF-beta isoforms significantly reduced EGF- induced receptor angiotensin II substrate phosphorylation. TGF-beta isoform-specific signaling also significantly attenuated EGF-induced phosphorylation of the mitogen-activated protein (MAP) kinase extracellular signal-regulated kinase 2. These results suggest that isoform-specific TGF-beta signaling modulates the EGF signal transduction pathway upstream of MAP kinase.