Prostate cancer is the second leading cause of cancer death among American men and will be responsible for approximately 29,000 deaths in 2003. Localized prostate cancer can be treated by surgical removal of the prostate or radiotherapy. However, for many men, the disease has spread beyond the prostate by the time of diagnosis. For these individuals, treatment with compounds that block the action of male hormones (androgens) is generally effective, but rarely curative. Unfortunately, in almost all cases, patients eventually become resistant to hormonal therapies, usually within a few years. Many chemotherapy agents have been tested in prostate cancer clinical trials, but none have produced any significant increase in survival rate. Therefore, new therapeutic agents are urgently needed for the treatment of hormone-insensitive prostate cancer.
We have discovered that certain compounds called G-Rich Oligonucleotides (or "GROs") can inhibit proliferation and induce cell death in hormone-insensitive prostate cancer cell lines. The GROs are also active against other types of cancer cells, but have minimal effects on normal cells, indicating that their activity is "tumor-selective." We have recently demonstrated that one of these GROs can completely inhibit tumor growth in a mouse model of prostate cancer. Currently, this GRO is in the final stages of preclinical testing, and human trials are anticipated in the near future. Based on previous clinical trials of compounds with a similar chemical composition to GROs (antisense oligonucleotides), we expect that GROs will lack many of the toxic side effects usually associated with chemotherapy.
The mechanism of GRO activity is not yet fully understood, but it is clear that they work in a novel way that is different from chemotherapy agents or other types of oligonucleotides (e.g., antisense). The activity of GROs is related to their ability to form very stable structures known as quadruplexes. These quadruplexes can bind to specific target proteins that recognize their unusual three-dimensional shape, which is called an "aptamer" effect. Because they are bound to GROs, these target proteins cannot interact with other cellular proteins that they would normally bind to. This may result in the GROs effectively blocking the function of the target proteins.
Prostate cancer, like all cancers, develops when cells grow without the normal control mechanisms. This is usually a result of inappropriate or increased expression of proteins involved in cell proliferation, or because of inactivation of proteins that would normally ensure regulation of cell growth. Ideally, a therapeutic agent would target only these deregulated proteins in order to inhibit the growth of cancer cells while leaving healthy cells undamaged. We believe that the GROs exhibit tumor-selective activity because they target some of these deregulated proteins. We have identified two GRO-binding proteins that are likely to mediate tumor-selective GRO effects. One of these proteins (called NEMO) is critical for activation of a cellular pathway that is thought to promote the survival of prostate cancer cells and help them avoid cell death induced by chemotherapy. The second protein (called nucleolin) is present at much higher levels in malignant cells compared to normal cells and is also involved in many of the cellular processes that are dysfunctional in prostate cancer cells.
The purpose of this proposal is to further investigate the mechanism of GROs and to examine the molecular basis for their tumor-selective activity. This will be achieved by investigating specific GRO-induced changes in the pathway controlled by NEMO using molecular biology techniques. Because nucleolin is a multifunctional protein that binds to many other proteins, it will be necessary to use a different technique ("proteomics") to explore the role of nucleolin in GRO effects. The GROs have a subtle effect on nucleolin, probably blocking its binding to a few other proteins but leaving most interactions unaffected. Using the proteomics approach will allow us to simultaneously examine many proteins and identify which of the nucleolin-binding proteins are specifically blocked by GRO treatment.
GROs are promising experimental therapeutic agents in the fight against prostate cancer. They are highly active against hormone-insensitive cells and are expected to be well tolerated without unpleasant side effects. Perhaps most importantly, they work by an entirely new mechanism that may give them some advantages over current therapies. The proposed experiments will give us a better understanding of the GRO mechanism and should identify the protein interactions that must be blocked to achieve cancer-specific activity. This may ultimately lead to the development of therapies that are even more active and specific than GROs. The likelihood of a successful outcome in this project is also favored by the experience of the Principal Investigator in this field. She was part of a collaborative effort that led to the discovery of the GROs and has a strong track record of investigating their mechanism and activity in prostate cancer.