Problems with proposed prostate-cancer treatment

A new study by Dr. Norman Greenberg challenges the idea that prostate cancer can be treated successfully by blocking the activity of a protein called insulin-like growth factor, or IGF-1.

IGF-1 is a polypeptide hormone that can influence growth, differentiation and survival of cells expressing the type 1 receptor (IGF-1R). Past clinical, epidemiological and experimental studies have strongly implicated IGF-1 as a contributing factor in the natural history of prostate cancer. However, very little has been done to prove absolutely that the expression or activation of the IGF-1 signaling pathway at physiologically relevant levels is sufficient to cause a healthy prostate cell to become a cancer cell.

Greenberg, an investigator in the Clinical Research Division, and colleagues conducted a pair of experiments by manipulating gene expression directly in the epithelial compartment of the mouse prostate gland to better understand the role of IGF-1R.
 
In contrast to studies that correlated elevated levels of IGF-1 with the risk of developing prostate cancer, Greenberg’s research showed that eliminating IGF-1R expression in an otherwise normal mouse prostate caused the cells to proliferate and become hyperplastic. Although persistent loss of IGF-1R expression ultimately induced cell stasis and death, both of these processes are regulated by the tumor suppressor gene p53 that is commonly mutated in human prostate cancers. Hence the researchers hypothesized that tumors with compromised p53 might not respond predictably to therapies targeting IGF1 signaling.

To test their reasoning they conducted a second experiment by crossing mice carrying the prostate-specific IGF-1R knockout alleles with transgenic mice that develop spontaneous prostate cancer when p53 and select other genes are compromised. The results were as predicted: Prostate epithelial-specific deletion of IGF-1R facilitated the emergence of aggressive prostate cancer in the genetically engineered tumor-prone mice.

Published in the May 1 edition of Cancer Research, the study supports a critical role for IGF-1R signaling in prostate-tumor development and identifies an important IGF-1R-dependent growth control mechanism, according to the authors. The title of the paper is “Conditional Deletion of Insulin-Like Growth Factor-1 Receptor in Prostate Epithelium.”

“If our predictions hold true, tumor cells with intact p53 may show the best response to therapy targeting the IGF-1R signal; however, when p53 is not functioning normally, response to this therapy may not be as expected,” said Greenberg, the study’s corresponding author.

Greenberg’s message to clinicians who administer IGF-R1 therapy: “We’re all hoping for good results, but let’s proceed with caution.”

A search of the database for clinical trials registered with the National Cancer Institute found 18 trials in process that use therapies to inhibit IGF-R1. None of them include a tumor’s p53 status as a criterion for recruiting research participants, Greenberg said.

In addition to lead author Dr. Brent Sutherland, of the Hutchinson Center, contributing research also came from scientists at Baylor College of Medicine in Houston, Texas; the Center for Cancer and Stem Cell Biology at Texas A&M University; and the Institut National de la Sante et de la Recherche Medicale in Paris, France.

The National Cancer Institute, the Prostate Cancer Foundation and Phi Beta Psi funded the study.