Modulation of Drug Resistance by Protein Kinase C Inhibitors

Institution: Stanford University
Investigator(s): Yuefeng Lu, Ph.D. -
Award Cycle: 1997 (Cycle III) Grant #: 3FB-0056 Award: $24,182
Award Type: Postdoctoral Fellowship
Research Priorities
Detection, Prognosis and Treatment>Innovative Treatment Modalities: search for a cure



Initial Award Abstract (1997)
Chemotherapy, in association with surgery and radiation therapy, is very important for the treatment of breast cancer. In chemotherapeutically drug-sensitive breast cancer cells, the drug molecules are able to enter and accumulate inside the cell. Upon reaching a certain concentration, the drugs cause cellular damage that usually turns on the intrinsic pathways that lead to cell death, a process called apoptosis or programmed cell death. Unfortunately, the effectiveness of chemotherapy can be diminished by the development of drug resistance. Drug resistance can occur via two important mechanisms: by decreased drug accumulation inside the cells (or increased drug exclusion), and by the failure of the drugs to induce the programmed cell death. Increased drug exclusion has been correlated to the presence of higher amounts of a protein called P-glycoprotein, or P-gp, on the cell surface. This protein works like a pump that excludes drugs from cells and prevents drug accumulation inside. Programmed cell death is a highly regulated cellular event. Drug-resistant cells may have defects in the regulation of this pathway or they may have mutations in one or more of its key components. Consequently, even when drug accumulation inside the cell is high, the cell may still be able to survive.

A protein that has been shown to play important roles in both drug exclusion and programmed cell death is Protein kinase C, or PKC. PKC adds phosphorous groups onto other proteins, which usually results in dramatic changes in the activities of these proteins. PKC has more than twelve different forms, called isozymes, at least six of which are present in breast cancer cells. Though they have similar structures and requirements for activation, each isozyme may regulate different cellular events, or have different or even reverse effects on the same event. Drug resistant breast cancer cells have an elevated PKC activity, however the role(s) of individual PKC isozymes in drug resistance are largely unknown.

This laboratory has developed several isozyme-selective PKC inhibitors. Application of these inhibitors, which are not available in other laboratories, would allow us to examine the impact of each inhibitor on drug exclusion and the programmed cell death, and therefore the function of individual PKC isozymes in drug resistance. PKC isozymes that promote drug exclusion and/or suppress drug-induced cancer cell death would be identified. Isozyme-selective inhibitors of these PKC isozymes may represent leading compounds for the development of novel agents that circumvent drug-resistance and potentiate the effects of breast cancer chemotherapy. Drugs developed based on these inhibitors should have fewer side effects and greater effectiveness because they are isozyme-selective.


Final Report (1998)
Chemotherapy is very important for the treatment of breast cancer. In drug-sensitive cancer cells, the drug molecules are able to enter and accumulate inside the cell. Upon reach certain concentration, the drugs cause cellular damage that usually turns on the intrinsic pathways that lead to cell death, a process called programmed cell death or apoptosis. Unfortunately, the effectiveness of chemotherapy can be diminished by cellular development of drug resistance. Drug resistance can occur by two important mechanisms: by decreased drug accumulation inside the cells (or increased drug exclusion) and by the failure of the drugs to induce the programmed cell death.

A protein that has been shown to play important roles in both drug exclusion and programmed cell death is protein kinase C, or PKC. PKC adds phosphorous groups onto other proteins, which usually turns on or off the activities of those proteins. PKC has at least twelve different forms, called isozymes, at least six of which are present in breast cancer cells. Though they have similar structures and requirements for activation, each isozyme may regulate different cellular events, or have different or even reverse effects on the same event. Drug resistant cells have elevated PKC activity, however, the roles of individual PKC isozymes in drug resistance are largely unknown.

I have developed two methods toS manipulate PKC activity in the breast cancer cells involving several isozyme-selective PKC inhibitors developed in our laboratory. In the first method, I have introduced the DNA fragments of these inhibitors into the cancer cells. I found that the cells that expressed b -PKC inhibitor showed characteristics of programmed cell death. These cells also had much higher death rate than the cells that expressed no inhibitor or the inhibitor of d -PKC or e -PKC. In the second method, I decreased the PKC level in these cells by a method called PKC down regulation. The cells with down regulated PKC were more sensitive to vitamin E and ceramide, agents that have previously been found to induce programmed cell death in breast cancer cells.

These results showed that PKC may play an important roles in the programmed death of the breast cancer cells. The data also indicated that application of the isozyme-selective PKC inhibitors may lead to the identification of PKC isozymes that suppress programmed cell death. Isozyme-selective inhibitors of these PKC isozymes may represent leading compounds for the development of novel agents that potentiate chemotherapy effects.