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Transforming growth factor beta activation of p44mapk in proliferating cultures of epithelial cells
1Department of Pharmacology, Pennsylvania State University College of Medicine, Hershey 17033, USA.
Abstract:
Transforming growth factor-beta (TGF-beta) 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-beta is still unknown. We report here that both TGF-beta 1 and TGF-beta 2 resulted in a rapid activation of mitogen-activated protein kinase (MAPK) p44mapk, 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-beta 2 induced a sustained activation of p44mapk under these conditions, lasting for at least 90 min after initial growth factor treatment. Another TGF-beta-sensitive epithelial cell line (CCL 64) displayed a similar rapid increase in p44mapk activity when treated with TGF-beta 1. In contrast, in IEC 4-6 cells that are resistant to TGF-beta effects on growth and DNA synthesis, TGF-beta 2 treatment did not result in an activation of p44mapk. In contrast to the results in proliferating cultures, treatment of quiescent cultures of IEC 4-1 cells with TGF-beta 2 resulted in no significant change in either DNA synthesis or p44mapk activity within 15 min of TGF-beta 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 p44mapk. Together, our results suggest an association between activation of p44mapk and both TGF-beta-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-beta specifically, we have proposed a model to explain how activation of p44mapk may be associated with a growth-inhibitory response.
Insights
Transforming growth factor-beta (TGF-beta) rapidly activates mitogen-activated protein kinase (MAPK) p44mapk in growth-inhibited epithelial cells. This suggests MAPK activation is linked to both TGF-beta growth inhibition and growth factor stimulation.
Area of Science:
- Cell Biology
- Molecular Biology
- Signal Transduction
Background:
- Transforming growth factor-beta (TGF-beta) is a key regulator of epithelial cell proliferation, acting as a potent inhibitor.
- The precise molecular mechanisms underlying TGF-beta's diverse cellular effects, particularly its growth-inhibitory actions, remain incompletely understood.
- Mitogen-activated protein kinases (MAPKs) are critical signaling molecules involved in various cellular processes, including growth regulation.
Purpose of the Study:
- To investigate the role of mitogen-activated protein kinase (MAPK) activation in mediating the cellular responses to TGF-beta.
- To determine if MAPK activation is associated with TGF-beta-induced growth inhibition in epithelial cells.
- To explore the relationship between MAPK activation and growth factor-stimulated proliferation.
Main Methods:
- Treatment of intestinal epithelial cells (IEC 4-1, IEC 4-6) and CCL 64 cells with TGF-beta 1 and TGF-beta 2.
- Assessment of DNA synthesis inhibition and activation of p44mapk.
- Comparison of responses in proliferating versus quiescent cell cultures.
- Stimulation of cells with epidermal growth factor, insulin, and transferrin (EIT) to assess growth stimulation.
Main Results:
- Both TGF-beta 1 and TGF-beta 2 rapidly activated p44mapk in sensitive epithelial cells (IEC 4-1, CCL 64) within 5-10 minutes, correlating with significant DNA synthesis inhibition.
- TGF-beta 2 induced sustained p44mapk activation in IEC 4-1 cells for at least 90 minutes under growth-inhibitory conditions.
- TGF-beta treatment did not activate p44mapk in TGF-beta-resistant IEC 4-6 cells.
- Growth-stimulatory EIT treatment led to sustained p44mapk activation in IEC 4-1 cells.
- Quiescent IEC 4-1 cells showed no significant change in DNA synthesis or p44mapk activity upon TGF-beta 2 treatment within 15 minutes.
Conclusions:
- Activation of p44mapk is associated with both TGF-beta-mediated growth inhibition and EIT-mediated growth stimulation in epithelial cells.
- MAPK activation itself may not determine the specific cellular outcome (inhibition vs. stimulation), suggesting downstream events dictate the response.
- A model is proposed where p44mapk activation contributes to the growth-inhibitory effects of TGF-beta through downstream signaling pathways.