Our overall goal is to characterize the cell signaling pathways that inhibit the uncontrolled growth of epithelial-derived tumor cells, the cell type of most human cancers. Molecular, genetic and cell biological experimental strategies are being utilized to explore the mechanisms by which extracellular signals (such as steroid hormones, growth factors and certain dietary compounds) coordinately regulate the proliferation and cell-cell interactions of reproductive tumor cells.
Glucocorticoids have potent anti-proliferative effects on steroid-responsive mammary tumor cells by regulating the expression and activity of cell signaling molecules. We isolated a novel serum and glucocorticoid-inducible Ser/Thr protein kinase, Sgk, that is unique in its transcriptional control by several distinct signal transduction pathways, including glucocorticoids, serum, growth factors, environmental stress, and the p53 tumor suppressor protein. The tissue distribution and developmental profile of Sgk is consistent with a physiological role for Sgk in cellular stress responses. We are investigating the osmotic shock and radiation-induced signaling pathways that target the Sgk gene promoter and the Sgk protein. Our current evidence has documented that Sgk plays a key role in the cell survival to extracellular stress signals. In addition to its transcriptional control, Sgk activity and localization can be regulated in a stimulus dependent manner. We are exploring the mechanism controlling the nuclear localization of Sgk in proliferating cells and its cytoplasmic localization in stressed or glucocorticoid treated cells. Furthermore, characterization of the substrate specificity of Sgk, in combination with the isolation of Sgk-interacting proteins through a yeast two-hybrid screen, has implicated Sgk as a unique convergence point for proliferative and stress signaling pathways. Our goals are to define the Sgk-mediated cell signaling events in normal and transformed cells as well as to determine the cellular functions and in vivo role of this unique kinase.
In a complementary area of research, we are investigating the opposing action of glucocorticoids and growth factors on the control of cell-cell interactions in nontransformed and transformed mammary cells. We have discovered that glucocorticoids induce tight junction formation by a multistep process under conditions in which the cells undergo a stringent growth arrest. The early events include the steroid-stimulated expression of the Id-1 transcriptional regulator, an inhibition of RhoA production and the regulation of cellular factors that interact with certain junctional complex components. Glucocorticoids induce a reorganization of the apical junctional complexes thatcauses a redistribution of the major cytoplasmic and membrane associated tight junction proteins leading to tight junction assembly. We have recently discovered that the steroid regulation of membrane organization and tight junction functionality are distinct cellular steps that can be distinguished by their dependence on Ras and RhoA signaling pathways as well as by their targeting by growth factors. We plan to define the glucocorticoid-regulated events and regulatory components that control tight junction formation in order to dissect the signaling pathways that mediate cell-cell interactions and to test their potential roles in epithelial cell tumorigenesis.
In a third major area of research, we are investigating, in collaboration with Prof. Bjeldanes' laboratory (Dept. Nutritional Sciences), the mechanism by which indole-3-carbinol (I3C), a compound produced in Brassica plants such as broccoli, suppresses the proliferation of human breast cancer and prostate cancer cells. I3C induces a G1 block in cell cycle progression through a process that selectively inhibits CDK6 (cyclin dependent kinase-6) gene expression and which is independent of the effects of estrogen. Moreover, I3C acts synergistically with tamoxifen, an anti-estrogen currently used in breast cancer therapies, to inhibit the growth of estrogen-dependent breast cancer cells. One of our key goals is to define the indole signaling pathway that leads to the transcriptional control of cell cycle genes. We have recently shown that both the CDK6 promoter and CDK2 kinase activity is down regulated by I3C, and efforts are underway to define the cellular targets of I3C that mediate these anti-proliferative effects. We are also attempting to develop more potent synthetic derivatives of I3C and investigate the in vivo tumor effects of I3C to design novel classes of I3C-based anti-breast cancer and anti-prostate cancer agents. We are also investigating the anti-cancer properties of other natural compounds.