|
|
|
|
|
Recent work has established that the degradation of TS is carried out by the 26S proteasome. However, the enzyme is not ubiquitinylated, nor is the ubiquitinylation pathway required for its degradation. Thus, TS is targeted to the proteasome in a ubiquitin-independent manner. Mutagenesis studies indicate that the N-terminal region of the TS polypeptide, particular the first three residues, directs the enzyme to the proteasome. Current efforts are focused on the mechanism by which these residues control the susceptibility of TS to intracellular degradation.
The expression of nuclear factor kappa B (NF-kB), which is a transcription factor that regulates the expression of a multitude of genes involved in cell growth, differentiation, and apoptosis, is up-regulated in many types of cancer. We have demonstrated that reduced NF-kB levels leads to resistance to apoptosis by TS inhibitors in a human colon tumor cell line. Thus, NF-kB is required for maximal drug response. Indeed, this also appears to be the case for apoptosis induced by the death ligands TNF-alpha and TRAIL. These findings are being followed up in cultured cells and in animal models.
Animal models of drug response and toxicity. Much of our current work involves use of animal models to examine the role of TS as a chemotherapeutic target in vivo. As noted above, numerous studies of cultured cell lines have indicated that the structure and concentration of TS are important determinants of response to both fluoropyrimidine and folate-based TS inhibitors. A fundamental question remains: How much of the information obtained from /in vitro/ studies of cultured cell lines is applicable to naturally existing tumors in an intact animal host? We are examining the impact of alterations in TS structure and expression on tumor response to TS inhibitors in animal models of intestinal cancer. For example, recent work with MIN mice, which carry a mutant /Apc/ gene and are predisposed to intestinal tumorigenesis, shows that tumor development in these animals is reduced by 70-80% following systemic treatment with 5-fluorouracil, a powerful TS inhibitor that has clinical utility in the control of colorectal cancer. Folate analogs, such as raltitrexed, appear to stimulate tumorigenesis in the MIN mouse, while the combination of 5-fluorouracil and raltitrexed elicits a powerful (i.e., >90%) inhibition of tumor development. The molecular and biochemical basis of the synergism between these two drugs, both of which are TS inhibitors, are under investigation.
Tumors are infiltrated by a heterogeneous population of non-neoplastic, host-derived cells that make up the stroma. Most cells of the tumor stroma are products of hematopoiesis. Though their function within tumors is not fully defined, considerable evidence points to the idea that they regulate tumor growth and progression, as well as response to therapeutic agents. We have proposed that these infiltrating stromal cells regulate tumor response to therapeutic agents. Using bone marrow transplantation, we have generated chimeric MIN mice in which the neoplastic cells and stromal cells within tumors are distinct with regard to sensitivity to TS inhibitors. Preliminary evidence indicates that the drug response of tumors in these animals reflects the chemosensitivity of the exogenously introduced stromal cells. This important finding could dramatically change the way we think about targeting of anti-cancer drugs.
Inflammation and cancer. In collaboration with Dr. Heinz Baumann of Roswell Park Cancer Institute, we are conducting a study of the role of inflammation in tumorigenesis. By taking advantage of animal models such as the MIN mouse, we are addressing the hypothesis that inflammation supports tumor cell proliferation by providing growth-promoting molecules (cytokines, growth factors, etc.) and by assisting in remodeling of the tumor microenvironment. Focus is on the physiological role(s) of several inflammation-related proteins, including haptoglobin, alpha-1-acid glycoprotein, serine proteinase inhibitors, interleukin-6, and matrix metalloproteinases. We have shown that genetic misregulation of these proteins results in alteration in intestinal tumorigenesis. The mechanisms underlying these effects are under investigation.
For Recent Publications, please go to Dr. Bergers Lab Page: http://www.biol.sc.edu/~bergerlab