EXPRESSION OF PRO FORM OF PROSTATE-SPECIFIC ANTIGEN BY MAMMALIAN-CELLS AND ITS CONVERSION TO MATURE, ACTIVE FORM BY HUMAN KALLIKREIN-2

To study the expression, biosynthesis, and processing of prostate-spec ific antigen (PSA) in mammalian cells, recombinant PSA was expressed i n Syrian hamster tumor cell line AV12-664 (AV12-PSA), Expression of PS A was monitored by the Tandem-hip PSA assay, PSA was secreted into the medium during the logarithmic phase of cell growth at >9 mu g/ml and was stable, The PSA purified from spent medium of AV12-PSA cells did n ot exhibit any enzymatic activity and did not complex with the proteas e inhibitor, alpha-1-antichymotrypsin. These findings indicated that a n inactive form of PSA was expressed by AV12-PSA cells. NH2-terminal s equencing confirmed the identity of the PSA purified from the spent me dium of AV12-PSA cells to be pro-PSA. This demonstrates that PSA is ex pressed as pro-PSA by mammalian cells and suggests that pro-PSA may be present in biological fluids, Human kallikrein 2 (hK2), another membe r of the hK family, is also expressed predominantly in prostate epithe lium. Although hK2 has been shown to exhibit trypsin-like activity, li ttle is known about its natural substrates. Using purified proteins, w e show that hK2 can convert pro-PSA to mature, enzymatically active PS A, thus establishing a physiological connection between hK2 and PSA, T hese findings imply that hK2 may be regulating PSA activity in vivo.