Androgen Resistance, Part 3

Reprinted from PCRI Insights May 2003 v 6.2
By Charles E. (Snuffy) Myers, M.D., Founder and Medical Director, The American Institute for Diseases of the Prostate, Charlottesville, VA, and Member of the PCRI Medical Advisory Board

Part 1 of this article                               Part 2 of this article

Importance of Genetic Damage in Prostate Cancer Progression
As with many other cancers, treating prostate cancer is made difficult by the cell’s ability to develop resistance to various treatments. The cancer cells have the capacity to spread throughout the body, invade normal tissues, and grow in other tissues beyond the prostate gland. Cancer cells are this adaptable because their genetic makeup can change over time.

A protein called p53 is the keystone in the system that detects and repairs gene damage. Protein p53 is aptly named the "guardian of the genome," because when gene damage occurs, p53 coordinates events that cause the cell to stop its growth and repair the damage. If the damage is too great, p53 becomes the catalyst directing the damaged cell to commit suicide. It is estimated that p53 doesn’t function normally in approximately half of all human cancers. In essentially every case, cancers lacking a normally functioning p53 are more likely to spread widely and lead to the patient's death.

Prostate Cancer Progression and p53
While there is a growing list of genes that suppress prostate cancer development and progression, the evidence supporting p53's role is the most extensive. At the time of diagnosis, p53 is abnormal in less than 10% of prostate cancers with Gleason scores less than 6. In contrast, this protein can be abnormal in over 80% of men with Gleason scores of 8-10. We have known since 1992 that an abnormal p53 at the time of radical prostatectomy predicts a rapid relapse following surgery and a short survival rate. In one study, p53 was abnormal in 16 out of 17 cases (94%) of hormone resistant prostate cancer. Conversely, close to 90% of men with normal p53 levels at the time of radical prostatectomy are in remission up to 15 years following surgery.

Why does an abnormal p53 serve so well to identify life-threatening prostate cancer? There are two hypotheses that seem reasonable. Cells that do not have a functional p53 protein are relatively resistant to many forms of treatment. Additionally, the lack of a functional p53 protein may allow the cancer cell to change more rapidly over time, enhancing the likelihood it will emerge resistant to treatment.

There are some difficulties with incorporating a cancer's p53 measurement into a viable treatment plan. The best measure is obtained at the time of radical prostatectomy when the entire tumor specimen can be tested. Prostate biopsies can be tested in men who do not have a radical prostatectomy, but, unfortunately, it has been shown that the biopsy samples only a small part of the cancer and can miss it altogether. Therefore, biopsies cannot definitely determine the p53 status of men undergoing radiation therapy, radioactive seed implantation, hormonal therapy, or watchful waiting. I am particularly concerned with those men who choose watchful waiting, because a cancer that has an abnormal p53 is a dangerous malignancy that can quickly progress beyond a cure. The problem here is that a man who has chosen watchful waiting can't be sure of his p53 status, because his only information comes from the prostate biopsy, which can easily miss large areas of the cancer with abnormal p53 levels.

An alternative approach to determining patient p53 status has recently emerged in the form of a blood test that, while still experimental, looks quite promising. (See Figure 1.) The immune system can recognize an abnormal p53 protein as something that doesn't belong. The formation of antibodies to combat the abnormal p53 protein is a common immune system response. In patients with abnormal p53, anti-p53 antibodies in the blood are frequent. In a wide range of cancers, including prostate, the presence of anti-p53 antibodies in the blood correlates with the presence of abnormal p53 in the patient’s cancer and with a poor response to treatment. Pending further investigation, this blood test may prove to be useful in identifying men with prostate cancers likely to have an abnormal p53 status.

Figure 1: Immunoreactivity for p53. (A) prostatic adenocarcinoma showed strong immunoreactivity for p53 in almost all of neoplastic cells. (B) in contrast, another tumor completely lacked immunostaining. Immunoperoxidase technique, reduced from x 250.

While it is generally conceded that the presence of an abnormal p53 protein indicates a more dangerous cancer, there is no consensus on how best to treat these patients. A number of approaches are undergoing clinical testing. One method uses viruses to carry the normal p53 gene into the cancer cells. Another approach finds drugs that can restore nearly normal function to an abnormal p53 gene.
Some drugs manage to kill cancer cells despite the absence of a normal p53 gene. None of these approaches are sufficiently developed to warrant use outside of a clinical trial.

Taxol® does appear to be active against cancer cells that lack a functioning p53 and may even selectively enhance radiation therapy’s efficacy against such cells. This drug is a component of several drug combinations that have proven active against advanced hormone-refractory prostate cancer. The controversy over whether or not radiation therapy's ability to kill cancer cells is independent of p53 is a strong one; investigators report opposing conclusions. Anecdotally, I have dramatic responses to radiation therapy in individual patients whose tumor biopsies stained heavily for P53. In summary, loss of normal p53 function is one of the changes that can dramatically increase the cancer's capacity to change over time and foster the development of a disease resistant to androgen withdrawal and other forms of treatment. Successfully treating men with high risk cancer may require a way of killing cancer cells lacking functional p53.
Continued

References

K.M. Ryan, et al. "Regulation and function of the p53, tumor suppressor protein" Current Opinion in Cell Biology 13: 332, 2001.

Y. Liu and M. Kulesz-Martin ”p53 protein at the hub of cellular DNA damage response pathways through sequence-specific and non-sequence-specific DNA binding” Carcinogenesis 22: 851,2001.

T.Visakorpi, et al. “Small subgroup of aggressive, highly proliferative prostatic carcinomas defined by p53 accumulation” Journal of the National Cancer Institute 84: 883, 1992.

R.B. Myers, et al. “Accumulation of the p53 protein occurs more frequently in metastatic than in localized prostatic adenocarcinomas” Prostate 25: 243, 1994.

J.J. Bauer, et al. “p53 nuclear protein expression is an independent prognostic marker in clinically localized prostate cancer patients undergoing radical prostatectomy” Clinical Cancer Research 1: 1295, 1995.

D. Theodorescu, et al. “p53, bcl-2 and retinoblastoma proteins as long-term prognostic markers in localized carcinoma of the prostate” Journal of Urology 158: 131, 1997.

F.J. Meyers, et al. “Very frequent p53 mutations in metastatic prostate carcinoma and in matched primary tumors” Cancer 83: 2534, 1998.

H.B. Heidenberg, et al. “Alteration of the tumor suppressor gene p53 in a high fraction of hormone refractory prostate cancer” Journal of Urology 154: 414, 1995.

M. Borre, et al. “p53 accumulation associated with bcl-2, the proliferation marker MIB-1 and survival in patients with prostate cancer subjected to watchful waiting” Journal of Urology 164:716, 2000.

S.S. Bacus, et al. “Taxol®-induced apoptosis depends on MAP kinase pathways (ERK and p38) and is independent of p53” Oncogene 20: 147, 2001.

C.J. Li, et al. “Induction of apoptosis by beta-lapachone in human prostate cancer cells” Cancer Research 55: 3712, 1995.

M.A. Carducci, et al. “Phenylbutyrate induces apoptosis in human prostate cancer and is more potent than phenylacetate” Clinical Cancer Research 2: 379, 1996.

A. Gotoh, et al. “Cytotoxic effects of recombinant adenovirus p53 and cell cycle regulator genes (p21 WAF1/CIP1 and p16CDKN4) in human prostate cancer” Journal of Urology 158: 636, 1997.

K. Davidson, et al. “Mitoguazone induces apoptosis via a p53-independent mechanism” Anticancer Drugs 9: 635,1998.

J. L. Herrmann, et al. “Prostate carcinoma cell death resulting from inhibition of proteosome activity is independent of functional Bcl-2 and p53” Oncogene 17: 2889, 1998.

J.R. Chapman, Jr., et al. “Brefeldin A-induced apoptosis in prostatic cancer DU-145 cells: a possible p53-independent death pathway” British Journal of Urology International 83: 703, 1999.

Rb Protein
A second protein, Rb, plays an important role in the evolution of hormone-resistant prostate cancer. Normal and cancerous cells grow by dividing in two. Before dividing, the cell must double its DNA and other components, and this process has a series of four discrete steps labeled G1, S, G2, and M. During G1, the cell starts to ramp up the machinery of replication and assemble needed materials. During S phase, the DNA of the cell is doubled. During G2, the machinery that will pull the two cells apart is assembled. During M, the cell divides into two daughter cells. In the sequence, G1 is an important step: this is the point when a cell must commit to the process of doubling. The process requires enormous resources and cells are very vulnerable to injury during their DNA doubling period. The Rb protein acts to control whether cells enter G1 or stay in a safer resting state. (See Figure 2.)

Figure 2: The Rb Cell Cycle. It develops from a resting cell all the way to the point at which it is a proliferating cell.

How? This paragraph explains how. The Rb protein plays an important role in sensing whether appropriate growth factors and nutrients are present to allow for cell growth and division. The Rb protein works by binding together the key components of the machinery needed for the cell to proceed from G1 to S. Once conditions for cell growth are in place, the Rb protein is inactivated by our old friend, protein phosphorylation, which releases the components needed for S to proceed.

More recently, Rb’s role has been expanded. When DNA is damaged during S, it is very important that this damage be repaired before the DNA duplication is finished: otherwise the information encoded by the DNA may change. Since DNA damage during S causes phosphate removal from Rb, the dephosphorylated Rb now binds the machinery involved in cell growth. This results in an arrest of the S phase and allows DNA repair to proceed prior to completion of cell duplication. (See Figure 3.)

Figure 3: Immunoreactivity for Rb Protein. (A) prostatic adenocarcinoma showed strong immunoreactivity for Rb protein in most of neoplastic cells. (B) another tumor had only few cells immunopositive. Immunoperoxidase technique, reduced from x 250.

As recent research has shown, Rb binds to the androgen receptor, as well as the machinery needed for cells to pass from G1 to S. Furthermore, the binding of Rb to the androgen receptor increases receptor effectiveness, making Rb an androgen receptor co-activator. The impact of Rb is subtler than a simple increase in the androgen receptor's effectiveness. Following exposure to elevated levels of androgen and estrogen, normal prostate tissue lacking Rb rapidly undergoes conversion to prostate cancer. This suggests that normal Rb protects prostate cells from the cancer-causing effects of these sex hormones. There is additional evidence that Rb plays a role in the interaction between androgens and prostate tissue. A study by Kaltx-Wittmer, et al examined the presence of Rb in patient cancer specimens before and after hormonal therapy. The study found that Rb was absent in 6% of cases before androgen withdrawal, compared with 22% after hormonal therapy had failed.

In addition to its role in the action of androgens, the presence of Rb appears to play an important role in prostate cancer cell death following exposure to radiation therapy. As you might expect, the presence or absence of Rb at the time of radical prostatectomy can have important implications for a patient's long-term survival. Daniel Theodorescu, et al. from the University of Virginia looked at the presence or absence of Rb in radical prostatectomy specimens in a group of men who had been followed for up to twenty-five years after surgery. The presence of Rb in this sample was associated with a 78% reduction in the death risk when compared with cases in which the protein was absent. In this study, which also examined p53 status, both Rb and p53 were far superior to Gleason grade scores or the presence of capsular penetration in predicting survival after radical prostatectomy.

Traditional explanations for the role of Rb deletion in the development and progression of cancer maintain that Rb deletion allows the cancer cell to become independent of the growth factors and nutrients that control cell growth. The recent evidence for Rb playing a role in cellular response to DNA damage suggests that the absence of Rb may also promote a genomic instability fostering cancer progression. Certainly, the increased sensitivity of normal prostate cells to hormonally stimulated carcinogenesis supports this inference.

The evidence is clear: loss of Rb fosters the evolution of hormone-resistant disease and may impair the response to radiation therapy. Unfortunately, I can find no clearly articulated therapeutic strategy designed to attack prostate cancer cells lacking Rb.
Continued

References

C. Kaltz-Wittmer, et al. “FISH analysis of gene aberrations in advanced prostatic carcinomas before and after androgen deprivation therapy” Lab Investigations 80: 1455, 2000.

D. Theodorescu, et al. “p53, bcl-2 and retinoblastoma proteins as long-term prognostic markers in localized carcinoma of the prostate” Journal of Urology 158: 131, 1997.

K. E. Knudsen, et al. “Multiple G1 regulatory elements control the androgen-dependent proliferation of prostatic carcinoma cells” Journal Biologic Chemistry 273: 20213, 1998.

A. F. Freibourg, et al. “Differential requirements for ras and the retinoblastoma tumor suppressor protein in the androgen dependence of prostatic adenocarcinoma cells” Cell Growth Differentiation 11: 361, 2000.

S. Yeh, et al. “Retinoblastoma, a tumor suppressor, is a coactivator for the androgen receptor in human prostate cancer DU145 cells” Biochemistry Biophysics Research Communications 248: 361, 1998.

C. Bowen, et al. “Radiation-induced apoptosis mediated by retinoblastoma protein” Cancer Research 58: 3275, 1998.

Y.Wang, et al. “Sex hormone-induced carcinogenesis in Rb-deficient prostate tissue” Cancer Research 60: 6008, 2000.

R. Bookstein, et al. “Suppression of tumorigenicity of human prostate carcinoma cells by replacing a mutated Rb gene” Science 247: 712, 1990.

K. Hoffman, et al. “E2F activity is biphasically regulated by androgens in LNCaP cells” Biochemistry Biophysics Research Communications 183: 97, 2001.

Gene Damage In Prostate Cancer
The role of a normal p53 is to allow tissues to respond effectively to gene damage. Loss of Rb may also foster the emergence of hormone resistant prostate cancer. The impact of a nonfunctional p53 or Rb will be enhanced by factors that increase the rate of gene damage. In the prostate cancer field, we now know of at least two factors that may accelerate gene damage. George Wilding, M.D., and his colleagues at University of Wisconsin have shown that exposing prostate cells to androgen triggers the production of hydrogen peroxide and other oxidants in the laboratory. Furthermore, Dr. Wilding and his coworkers have shown that antioxidants such as vitamin E, selenium, and vitamin C, lessen the impact of these oxidants. Oxidants generated from hydrogen peroxide have been known to damage DNA and to foster the development of cancer. Until recently, we have lacked direct evidence that such a process occurs in the human prostate.

In the August 2001 issue of Cancer Research, D.C. Malins, et al supplied this missing piece of the puzzle. These investigators measured oxidant damage to genes in prostate tissues as a function of age, showing a steady increase in the amount of gene damage with increasing age. Furthermore, there was a strong correlation between the amount of damage and the risk of prostate cancer.

If oxidant gene damage plays a role in the development of prostate cancer, you would anticipate that the intake of antioxidants would reduce the risk of prostate cancer. And that anticipation has been shown to be correct. Oral ingestion of the antioxidants selenium, vitamin E, and lycopene are all associated with a decrease in the risk of developing prostate cancer or in the risk of dying of this disease.

The reason for this may be related to the protein glutathione S-transferase, which plays an important role in inactivating cancer-causing chemicals. William Nelson, M.D., and his colleagues at Johns Hopkins have shown that this protein is deactivated very early in the development of prostate cancer. This means that the cancer cells have lost an important defense against chemicals that are able to cause gene damage and promote genetic instability. The net result is that human prostate cancer cells are more susceptible to gene damage from cancer-causing chemicals present in the diet or environment. For example, C. P. Nelson, et al have found that glutathione S-transferase deactivates chemicals found in well-cooked meats in normal tissues, but not prostate cancer cells.

One approach decreases the damage caused by oxidants and cancer-causing chemicals. The implications of these findings amount to this: Defects in p53 and glutathione S-transferase function can combine with oxidant damage to foster the development of gene alterations. As these gene alterations accumulate, the risk increases that hormone resistant cancer cells will arise. For this reason, the intake of antioxidants and cancer preventative agents might slow or block the accumulation of gene alterations and slow cancer progression.
Continued

References

M. O. Ripple, et al. “Pro oxidant-antioxidant shift induced by androgen treatment of human prostate carcinoma cells” Journal National Cancer Institute 89:40, 1997.

M. O. Ripple, et al. “Effect of antioxidants on androgen-induced AP-1 and NF-kappaB DNA- binding activity in prostate carcinoma cells” Journal National Cancer Institute 91: 1227, 1999.

D. Malins, et al. “Age-related Radical-induced DNA Damage Is Linked to Prostate Cancer” Cancer Research 61: 6025, 2001.

J.D. Brooks, et al., “CG island methylation changes near the GSTP1 gene in prostatic intraepithelial neoplasia” Cancer Epidemiol Biomarkers Previews 7: 531, 1998.

J.D. Brooks, et al., “CG island methylation changes near the GSTP1 gene in prostatic intraepithelial neoplasia” Cancer Epidemiol Biomarkers Previews 7: 531, 1998.

W.H. Lee, et al., “CG island methylation changes near the GSTP1 gene in prostatic carcinoma cells detected using the polymerase chain reaction: a new prostate cancer biomarker” Cancer Epidemiol Biomarkers Previews 6: 443, 1997.

C . P. Nelson, et al., “Protection against 2-hydroxyamino-1-methyl-6-phenylimidazo (4,5- b) pyridine cytotoxicity and DNA adduct formation in human prostate by glutathione S-transferase P1” Cancer Research 61: 103, 2001.

Summary
In this and the previous issue of Insights, I have covered what we know about how prostate cancer cells manage to grow despite surgical or medical castration. From this review, it should now be clear to you that there are many different methods prostate cancer can use to survive and grow in the face of androgen withdrawal. It is not appropriate to regard all hormone-refractory prostate cancers as similar in origin, nor are they all likely to respond optimally to the same treatment. Increasingly, we have the means available to determine the basis for hormone resistance in individual patients.

In this issue, I have mentioned selected drugs and natural products that block major pathways that support the survival and growth of prostate cancer cells in the face of androgen withdrawal. One obvious approach to this problem is to use one or more of these agents in combination with androgen withdrawal. For example, the combined inhibition of bcl-2 and akt might both speed the death of prostate cancer cells following androgen withdrawal and reduce the likelihood that hormone resistant cancer might emerge. The research I have reviewed also provides a strong rationale for the addition of Taxol® or Taxotere® to androgen withdrawal because these agents inactivate bcl-2 and may be active against cancer cells with inactive P53.

I think that successful prevention and treatment of hormone-resistant prostate cancer will only result from an approach that seeks to block, in a comprehensive fashion, each of the major pathways supporting cancer cell survival. If we leave a major pathway to cancer cell survival untouched, we will find that pathway will be active in the cancers that progress through the treatment. I think this is the explanation for why “complete androgen blockade” and the combination of hormonal therapy with Taxol® or Taxotere® have now proven less impressive than anticipated. Each of these steps is reasonable, but the therapy fails because it leaves too many alternative escape routes for the cancer.

At the American Institute for Disease of the Prostate, one of our major research interests is to find the best way to combine androgen withdrawal with agents that block the known pathways to hormone resistance. One of the major problems with androgen withdrawal is that tumor cell death is slow and typically spans nine months or more. Our initial goal is to increase the speed of tumor cell kill dramatically so that it is complete within three months. The rationale for this is that in every setting where drugs cure cancer, complete remission is attained within three months. This is true for acute leukemia, Hodgkins and non-Hodgkins lymphoma as well as for cancer of the testes. Our second goal is to develop a nontoxic combination of oral agents that effectively prevents recurrence once hormonal therapy is discontinued. We believe that achieving these goals will be useful steps in successfully treating hormone-resistant prostate cancer.

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