Lower Levels of IGF Binding Protein Seen in High-Risk Black Men

PHILADELPHIA—Black men have reduced levels of insulin-like growth factor binding protein 3 (IGF-BP3), compared to white men, and this may be a factor in the higher prostate cancer rates seen in this population, researchers reported at the 90th annual meeting of the American Association for Cancer Research (AACR) in Philadelphia.

A number of studies have provided evidence for an association between alterations in the plasma levels of insulin-like growth factor-1 (IGF-1) and IGF-BP3 in prostate cancer risk. IGF-1, for example, has been shown to stimulate prostate cells to grow. IGF-BP3 binds to IGF-1 in the blood, and lower levels of the binding protein could result in more unbound (free) IGF-1. It is thought that only free IGF-1 is available (or “free”) to exert its growth effects on cells.

“Our goals were to determine the plasma levels of these proteins in a group of men at increased risk for developing prostate cancer and to investigate the relationship between these levels and their race, age, and smoking status,” said James V. Tricoli, PhD, of the Fox Chase Cancer Center.

The study group consisted of 105 men age 35 to 69 with no personal history of prostate cancer but at least one first-degree relative diagnosed with the disease. An enzyme-linked immunoadsorbent assay (ELISA) was used to quantitate plasma levels of IGF-1 and IGF-BP3.

Significant Decrease in IGF-BP3

The study showed that the mean plasma level of IGF-1 was not significantly different between black men (162.3 ng/mL) and white men (172.1 ng/mL) (P = .415). However, the mean plasma level of IGF-BP3 was lower in black men (2,789 ng/mL) than in white men (3,216 ng/ml), and this decrease was highly significant (P = .0045)

This supports a hypothesis that lower IGF-BP3 levels, resulting in greater IGF-1 bioavailability, may be partially responsible for the increased risk of prostate cancer in black men.

“Exactly how increased levels of ‘free’ IGF-1 might contribute to the development of prostate cancer and the clinical implications of lower IGF-BP3 levels will require further study,” Dr. Tricoli said. The trial, which has since been published in Urology (July 1999), also showed significantly lower IGF-1 and IGF-BP3 plasma levels in smokers vs nonsmokers.

“For reasons that we do not completely understand, black men have a roughly twofold greater incidence of prostate cancer than white men,” Dr. Tricoli said at a press conference. He cautioned that, at this point, “we have no solid evidence that the plasma level of IGF-BP3 is an indicator for the presence of prostate cancer, or a predictor for the occurrence of prostate cancer, in blacks or in any other population of men.”

Dr. Tricoli noted that the Fox Chase Cancer Center researchers are currently conducting studies aimed at determining whether lower levels of IGF-BP3 can predict the earlier onset of prostate cancer in high-risk men and examining how this protein is involved in the basic biology of the disease.