News and Publications

Regulation of the Response of Cells to Growth Factors and Oxidative Stress

P.A. Maher, J.F. Reilly, S. Mickey

We have 2 main areas of research: growth factor signaling in neuronal and nonneuronal cells and oxidative stress in neuronal cells. The growth factor we study, fibroblast growth factor 2 (FGF-2), is a member of a family of polypeptides that have a role in a wide array of biological processes, including cell growth, differentiation, angiogenesis, tissue repair, and transformation. FGFs interact with a family of cell-surface receptors that mediate many of the cellular responses to these growth factors. We are interested in how FGF-2 induces distinct cellular responses in different cell types that express the same cell-surface receptors for FGF.

We hypothesized that cell type--specific FGF receptor--dependent signaling pathways exist. We are taking 2 different approaches to determine these pathways. In the first approach, we used the intracellular part of FGF receptor-1 as the bait to isolate cellular proteins with which the receptor interacts. Using this approach, we obtained evidence for the specific interaction of 2 proteins with the receptor. Although these are known proteins, they have not been implicated previously in FGF signaling.

One of the proteins, Grb14, appears to function as a linker and can connect FGF receptor-1 to multiple, downstream signaling pathways. Grb14 is a complex protein with multiple, distinct domains involved in protein-protein interactions. Determination of the proteins that interact with these domains should enable us to characterize further FGF signaling pathways. The other protein we detected is an enzyme that catalyzes the synthesis of an isoprenoid involved in many different cellular functions. The precise role of this protein in FGF signaling is not clear but is under investigation.

In the second approach, we used a combination of specific inhibitors and assays for pathway activation to characterize the role of known signaling pathways in FGF signaling. With this approach, we detected pathways that are required for FGF-dependent proliferation but not differentiation and vice versa. For example, we found that a member of the mitogen-activated protein kinase family that was previously thought to be activated only by stress is also strongly activated by FGF in cells that undergo proliferation but only weakly activated in cells that differentiate. Inhibitors of this enzyme block the ability of FGF to stimulate proliferation but have no effect on its ability to induce differentiation. These and other data indicate that a single FGF receptor can activate multiple signaling pathways in a cell type--dependent manner. Thus, it should be possible to inhibit the action of FGF on specific cell types while leaving other cells untouched.

The other project in the laboratory focuses on understanding how neuronal cells respond to oxidative stress. Oxidative stress has been implicated in neuronal cell death associated with both acute neurologic injuries and a number of neurodegenerative diseases, including Alzheimer's disease and Parkinson's disease. For a model system, we study oxidative glutamate toxicity in a neuronal cell line. During the past year, we characterized in detail this pathway of cell death.

Early in the process leading to cell death, levels of the intracellular antioxidant glutathione decrease. In addition, both protein synthesis and protease activation are required. Once glutathione levels decrease to less than 20%, a tremendous increase in the level of intracellular peroxides occurs. This increase somehow triggers a massive influx of calcium into the cells that is followed closely by cell death. Morphologically, this death resembles that seen during the development of the brain. Thus, death by oxidative stress proceeds by a series of well-defined steps. Surprisingly, we found that blockage of any one of these steps can abrogate the entire process.

PUBLICATIONS

Li, Y., Maher, P., Schubert, D. A role for 12-lipoxygenase in nerve cell death caused by glutathione depletion. Neuron 19:453, 1997.

Li, Y., Maher, P., Schubert, D. Requirement for cGMP in nerve cell death caused by glutathione depletion. J. Cell Biol. 139:1317, 1997.

Reilly, J., Maher, P.A., Kumari, V.G. Regulation of astrocyte GFAP expression by TGF-ß and FGF-2. Glia 22:202, 1998.

Sagara, Y., Dargusch, R., Chambers, D., Davis, J., Schubert, D., Maher, P. Cellular mechanisms of resistance to chronic oxidative stress. Free Radic. Biol. Med. 24:1375, 1998.

Stachowiak, E.K., Maher, P.A., Tucholski, J., Mordechai, E., Joy, A., Moffett, J., Coons, S., Stachowiak, M.K. Nuclear accumulation of fibroblast growth factor receptors in human glioma cells: Association with proliferation. Oncogene 14:2201, 1997.

Stachowiak, M.K., Moffett, J., Maher, P.A., Tucholski, J., Stachowiak, E. Growth factor regulation of cell growth and proliferation in the nervous system: A new intracrine nuclear mechanism. Mol. Neurobiol. 15:257, 1997.

Tan, S., Maher, P.A., Schubert, D. The role of protein phosphorylation in amyloid beta protein toxicity. Brain Res. 765:159, 1997.

Tan, S., Wood, M., Maher, P.A. Oxidative stress induces a form of programmed cell death with the characteristics of both apoptosis and necrosis in neuronal cells. J. Neurochem. 71:95, 1998.

Tetzke, T.A., Caton, M.C., Maher, P.A., Parandoosh, Z. Fibroblast growth factor saporin mitotoxins on human bladder cell lines. Clin. Exp. Metastasis 15:620, 1997.