This is an interesting article by William Faloon reviewed and critiqued
by Stephen B. Strum.

http://www.lef.org/magazine/mag2005/dec2005_supp_psa_01.htm

It says, in summary:

Accumulating data suggest that PSA is no longer merely a laboratory
test of prostate gland activity. Instead, PSA is recognized as a
functional protein: an enzyme that may facilitate prostate cancer cell
proliferation, invasion, and metastasis. Taking steps to suppress PSA
may reduce prostate cancer risk and progression. Meaningful reductions
in PSA, as demonstrated in many of the studies cited in this article,
appear achievable by using natural supplements like lycopene, soy,
green tea, and boron, as well as through prescription drugs such as
Avodart® or Proscar®, which normally reduce serum PSA levels by
40-50%.