The study, published in Cancer Research, also found the key protein -- identified as Stat5 -- works together with hormone receptors in recurrent prostate cancer cells in what they describe as a synergistic relationship in which each helps the other maintain itself.

"These findings validate Stat5 as a potential drug target in prostate cancer, and in particular in a form of prostate cancer for which there are no effective therapies," lead researcher Dr. Marja Nevalainen of Jefferson Medical College of Thomas Jefferson University, in Philadelphia, said in a statement.

Nevalainen's team examined human prostate cancer cells of 198 patients with prostate cancer recurrence and found that Stat5 was active in 74 percent of all recurrent prostate cancers. Of these patients, 127 had been treated with androgen deprivation therapy.

Androgen deprivation therapy reduces levels of the male hormones -- androgens, or testosterone and dihydrotestosterone -- in the body. Lowering androgen levels often makes prostate cancers shrink or grow more slowly, however, hormone therapy does not cure prostate cancer.