Transforming Growth Factor Activation of p44 in Proliferating Cultures of Epithelial Cells (*)

Abstract

Transforming growth factor-β (TGF-β) is a potent growth inhibitor of a variety of epithelial cell types. The primary signaling mechanism involved in mediating this and other cellular effects of TGF-β is still unknown. We report here that both TGF-β₁ and TGF-β₂ resulted in a rapid activation of mitogen-activated protein kinase (MAPK) p44, occurring within 5-10 min of growth factor addition. This effect occurred in exponentially proliferating cultures of intestinal epithelial (IEC) 4-1 cells under conditions in which DNA synthesis was inhibited by 95% to 98%. Furthermore, TGF-β₂ induced a sustained activation of p44 under these conditions, lasting for at least 90 min after initial growth factor treatment. Another TGF-β-sensitive epithelial cell line (CCL 64) displayed a similar rapid increase in p44 activity when treated with TGF-β₁. In contrast, in IEC 4-6 cells that are resistant to TGF-β effects on growth and DNA synthesis, TGF-β₂ treatment did not result in an activation of p44. In contrast to the results in proliferating cultures, treatment of quiescent cultures of IEC 4-1 cells with TGF-β₂ resulted in no significant change in either DNA synthesis or p44activity within 15 min of TGF-β addition. In contrast, addition of the growth-stimulatory combination of factors (epidermal growth factor + insulin + transferrin = EIT) to quiescent and proliferating IEC 4-1 cells stimulated DNA synthesis and resulted in a sustained activation of p44. Together, our results suggest an association between activation of p44 and both TGF-β-mediated growth inhibition and EIT-mediated growth stimulation. This suggests that the specificity for the cellular effects of growth factors may not occur at the level of MAPK activation per se, but rather at downstream events that include phosphorylation of distinct transcriptional complexes and activation of a select assortment of genes. With regard to TGF-β specifically, we have proposed a model to explain how activation of p44 may be associated with a growth-inhibitory response.