Controlled celullar proliferation requires the integration of both stimulatory and inhibitory signals. Transforming growth factor beta-1 (TGF-beta 1) is unique in that it is capable of conveying either a growth stimulatory or growth inhibitory signal. It is of interest to consider that the loss of a growth inhibitory response may be just as significant in the development of cancer as the acquisition of a growth stimulatory response. Thus, identification of the regulatory substrates for normal epithelial cell growth inhibition by TGF-beta-1 is of fundamental importance. Our recent data demonstrate that the inhibition of epithelial cell proliferation by TGF-beat-1 occurs late in the G1 phase of the cell cycle, approximately 1-2 hours from the onset of DNA synthesis. Associated with this inhibitory effect on cellular proliferation is a TGF-beta-1- induced decrease in the phosphorylation and histone H1 kinase activity of p34cdc2. The protein kinase p34c6c2, a ubiquitous inducer of mitosis in eukaryotic cells, has also been implicated in the regulation of the G1/S transition. During progression through the cell cycle p34cdc2 undergoes a series of posttranslational modifications, subunit rearrangements, and oscillations in its synthetic rate, which are thought to be associated with its activation. Despite rapid progress in the characterization of p34cdc2 at the induction of mitosis in mammalian cells, little is known about the mechanisms regulating its activation in G1 and S phase. The experiments proposed in this application are designed to test the hypothesis that the inhibitory action of TGF-beta 1 on epithelial cell proliferation is mediated through its effects on p34cdc2 activity at the Gl/S transition. Specifically, the effects of TGF-beta-1 on the epithelial cycle-dependent post-translational regulation (i.e. phosphorylation) and biosynthetic rate of p34cdc2 will be investigated. These objectives will be achieved by the following specific aims: 1) Determine the effect of TGF-beta-1 on the phosphoamino acid profile and kinase activity of p34cdc2 throughout the cell cycle of epithelial cells. 2) Determine the mechanism by which TGF-beta-1 decreases p34cdc2 phosphorylation at the G1/S phase border. 3) Determine the effect of TGF-beta-1 on the cell cycle-dependent rate of synthesis of p34cdc2. 4) Determine the effect of TGF-beta-1 on p34cdc2 regulation in lung epithehal cells transfected with activated oncogenes. These studies should increase our understanding of normal growth control and provide mechanistic information as to potential regulatory targets for carcinoma development.
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