Called alpha tocopherol associated protein, or TAP, the binding protein is known to perform important cellular functions, such as regulating the synthesis of cholesterol in the liver.
But for the first time, a team at the University of Rochester has found it also disrupts an important signaling pathway in prostate cancer cells and suppresses growth of the cancer.
Further, the protein appears to regulate retention of vitamin E in prostate cancer cells and increases the effect of vitamin E in limiting the proliferation of cancer cells, according to the team led by ShuYuan Yeh, assistant professor of urology and pathology.
The new research, reported in the 1 November issue of Cancer Research (vol 65, issue 21, pp9807-16), shows that expression levels of TAP are significantly lower in prostate cancer than in a normal prostate.
Compared with the high levels of TAP in normal prostate tissue, the protein was significantly reduced in human prostate cancer samples and in several prostate cancer cell lines tested by the Rochester team.
Their findings suggest that TAP facilitates the transport of vitamin E into prostate tissue and helps retain high concentrations of the vitamin in the cells.
Yeh also found that TAP increases vitamin E's capacity to control the rapid growth of the cancer cells.
"Vitamin E and TAP have distinct pathways to modulate prostate cancer cell growth and we hypothesize that vitamin E and TAP can work together to elicit better tumour suppression effects," the researchers write.
Yeh has previously investigated vitamin E's impact on prostate cancer.In an article in the Proceedings of the National Academy of Sciences in 2002, Yeh and her research team showed that vitamin E interferes with two proteins that play a central role in the development of prostate cancer.
The researchers found then that vitamin E disrupts the ability of prostate cancer cells to make both prostate-specific antigen (PSA) and the androgen receptor, a key player in the development and progression of the disease.
"No one really knows how vitamin E works," commented co-author on the new study, Edward Messing. "TAP may give us insights into how vitamin E works anywhere in the body and why it is a required vitamin."
Messing heads the Rochester part of the Selenium and Vitamin E Cancer Prevention Trial, designed to test whether either one, or a combination, prevents prostate cancer. Participants will be tracked for at least seven years, until about 2011.
After skin cancer, prostate cancer is the most common type of cancer in US men. This year alone, there will be an estimated 232,000 new cases of prostate cancer and more than 30,000 US deaths.