Our research objective is to understand the molecular and genetic mechanisms responsible for cell growth, differentiation and neoplastic transformation. We study the oncogenes, tumor-suppressor genes and signal-transducing proteins involved in BALB/c mouse plasmacytomas, B-cell lymphomas and other mouse and human experimental tumor systems. These are valuable experimental models, because they have many biological and molecular genetic features in common with human multiple myeloma, non-Hodgkin's lymphomas, and other human malignancies that are in need of mechanistic understanding in order to devise more specific therapy and preventive measures. BALB/c plasmacytomas, like rat immunocytomas and human Burkitt lymphomas, are characterized by constitutive expression of messenger RNA and protein from the master oncogene, c-Myc. It is still not clear which additional mutations are required for complete transformation. One mechanism that may be important consequence of altered Myc expression is genomic instability, in the form of extra- and intra-chromosomal amplification, and the subsequent overexpression of several genes that are crucial for DNA synthesis and cell proliferation, including cyclin D2. We are actively engaged in learning how many such genes can be amplified by this mechanism and what determines their expression or lack thereof. In the study of signal transduction, we are investigating protein kinase C (PKC), a multigene family of 11 structurally related isoenzymes that are important mediators of many forms of signal transduction. Using a variety of expression vectors, we have overexpressed many of the PKCs in fibroblasts, lymphocytic, myeloid and smooth muscle cell lines. This has made possible the identification of specific functions and intracellular targets for the individual PKC isoenzymes. We have been focusing on the delta and epsilon isoenzymes, which have opposite effects on cell proliferation. We have shown that PKC-delta is responsible for myeloid differentiation and growth inhibition, while overexpressed PKC-epsilon stimulates cell growth and transforms fibroblasts into tumor cells. We are dissecting the structure of these isoenzymes to determine which protein domains control these functions. We have shown that most of the isoenzyme-specific determinants are located in the catalytic half (the carboxyl-terminal domain) of these PKCs by creating reciprocal chimeric cDNAs that encode molecules that are half PKC-delta and half PKC-epsilon. Chimeric molecules that have carboxyl-terminal PKC-delta sequences are able to cause macrophage differentiation much like the parent all-PKC-delta protein. Similarly, a PKC chimera with a PKC-epsilon carboxyl-terminus, retains the neoplastic transformation potential of the all-PKC-epsilon protein. We are further dissecting the structure of the catalytic domain to determine which sub-domains determine PKC isoform-specific functions. We are also studying the nature of PKC's involvement in apoptosis, in cytoskeleton-related changes in cell shape, and in metastasis of these and other types of tumors, including human prostate cancer. We have shown that phorbol ester-activation of overexpressed PKC-delta disrupts the actin cytoskeleton in human and mouse lymphocytes, leading to the loss of membrane ruffling, a surface alteration needed for cell movement, and the loss of the typical elongated shape of these cells. We think that this effect is due to PKC-mediated changes in the phosphorylation of the adaptor molecule, paxillin.
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