Regulation of P-glycoprotein Expression and Function by CD44 in Breast Cancer

PUBLIC ABSTRACT

Women with breast cancer respond to chemotherapy about 40%-70% of the time when they present with advanced breast cancer. Although there is an initial response to therapy, only about 20% of the women are cured. Moreover, the duration of the response to therapy is usually 9 months. Thus, most responses are of limited duration, and nearly all patients with Stage IV disease die of disease progression. This is in part related to the development of cross-resistance to anticancer drugs. Actually, the major causes of treatment failure in breast cancer are the presence or development of metastatic disease and drug resistance.

There is indirect evidence that the metastatic and drug resistance characteristics of the tumor may be functionally linked. We recently uncovered a functional relationship between two proteins involved in these tumor characteristics: P-gp (confers drug resistance to tumors) and CD44 (gives tumors the potential to spread or metastasize). We found that these two proteins are present in the cancer cell membranes and that they bind tightly to each other. When we introduced the gene for the CD44 protein into breast cancer cells, unexpectedly the gene for P-gp also increased and the cancer cells became resistant to anticancer drugs. We then designed a unique yeast 2-hybrid assay with which we are able to express these two proteins and detect their interaction. We have identified parts of the CD44 protein that may be responsible for the increase in anticancer drug resistance. The tail of the molecule that is inside the cell (intracytoplasmic) gets chemically modified after the cell is exposed to certain chemicals by moving a phosphorus molecule from one part of the tail to another. We are proposing to test the hypothesis that when the phosphorus moves, the cell becomes drug resistant. Additionally, CD44 seems to increase drug resistance when its tail gets cleaved and then moves into the nucleus of the cell. In the nucleus, the cell has all the machinery to make new proteins. We are going to test the hypothesis that the CD44 tail moves into the nucleus and stimulates the production of P-gp. This possibility is very exciting because there are only a few known proteins that control expression of other proteins by this mechanism. One is a protein known to cause Alzheimer's disease, which has opened the door to drug discoveries for that disease. Therefore, this would be a novel way to look and study breast cancer and, if true, we possibly would be able to target the tail of CD44 to treat breast cancer.

We also expect that by understanding how these two proteins bind to each other and how CD44 induces the production of P-gp, we may be able to customize drugs to specifically avoid the binding of the two proteins and therefore will allow the anticancer drugs to kill the cancer cells. This work is particularly relevant to breast cancer because breast cancer stem cells are those cells in a tumor that are CD44 (+). Breast cancer stem cells are a small population of cells within the tumor that are responsible for the generation and perpetuation of breast cancer. Therefore, our findings will contribute also to the understanding of stem cells, which are probably the cells that need to be targeted by drugs.

The timeline for these discoveries is such that in 3 years we will know whether the tail of CD44 is a good target for treatment of breast cancer and we will also know what aspects of the CD44 tail should be targeted. This would allow for the synthesis of new drugs that could begin to be tested probably 5 years later.