Human kallikrein 2 (hK2), but not prostate‐specific antigen (PSA), rapidly complexes with protease inhibitor 6 (PI‐6) released from prostate carcinoma cells

Human kallikrein 2 (hK2) is a secreted, trypsin‐like protease that shares 80% amino acid sequence identity with prostate‐specific antigen (PSA). hK2 has been shown to be a serum marker for prostate cancer and may also play a role in cancer progression and metastasis. We have previously identified a novel complex between human kallikrein 2 (hK2) and protease inhibitor 6 (PI‐6) in prostate cancer tissue. PI‐6 is an intracellular serine protease inhibitor with both antitrypsin and antichymotrypsin activity. In the current study we have shown that PI‐6 forms a rapid in vitro complex with hK2 but does not complex with PSA. Recombinant mammalian cells expressing both hK2 and PI‐6 showed hK2‐PI‐6 complex in the spent media only after cell death and lysis. Similarly, LNCaP cells expressing endogenous hK2 and PI‐6 showed extracellular hK2‐PI‐6 complex formation concurrently with cell death. Immunostaining of prostate cancer tissues with PI‐6 monoclonal antibodies showed a marked preferential staining pattern in cancerous epithelial cells compared with noncancerous tissue. These results indicate that the hK2‐PI‐6 complex may be a naturally occurring marker of tissue damage and necrosis associated with neoplasia. Both hK2 and PI‐6 were shed into the lumen of prostate cancer glands as granular material that appeared to be cellular necrotic debris. The differential staining pattern of PI6 in tissues suggests a complex regulation of PI‐6 expression that may play a role in other aspects of neoplastic progression. © 2001 Wiley‐Liss, Inc.