Studies on Thiol Protease Inhibitor Isolated from Human Breast Cancer Tissue

Protease activity is considered to be involved in malignant tumor growth, invasion and metastasis. Although, protease activity is thought to be controlled by protease inhibitors, there are a few reports concerning the relationship between cancer and protease inhibitor derived from cancer tissue.
In the present study, thiol protease inhibitors (TPIs) were isolated from human breast cancer tissue, normal mammary gland tissue and the human breast cancer cell line (YMB-1). Their biochemical properties were investigated, giving the following results.
1) TPIs in human breast cancer extracts were significantly higher than in normal mammary gland extracts.
2) TPI was purified from both human breast cancer and human normal mammary gland extracts by papain-Sepharose affinity chromatography and Sephacryl S-200 gel filtration. Two kinds of TPIs (]ow-molecular weight TPI and high-molecular weight TPI) were purified from both tissues. Their molecular weights were 14,000 and 90,000, respectively as determined by gel filtration.
3) Low-molecular weight (LMW-) TPI had higher specific activity than high-molecular weight (HMW-) TPI. In breast cancer tissue extracts, LMW-TPI was dominant. Contrarily, HMW-TPI was dominant in normal gland tissue extracts.
4) Only HMW-TPI reacted with anti-urinary thiol protease inhibitor (UTPI) rabbit IgG by double immunodiffusion and immunoelectrophoresis.
5) LMW-TPI inhibited papain competitively using S-2302 as substrate. Its Km and Ki were 1.3 x 1Q·3M and 6.1 x 10·8M, respectively.
6) LMW-TPI was found to be stable to heat and pH variation.
7) TPIs were also purified from the human breast cancer cell line (YMB-1). Both TPIs which were extracted from cultured cells and released into the medium, were confirmed to be LMW-TPI.
Breast cancer cells may have lower ability to produce HMW-TPI than normal mammary gland cells. The difference of antigenicity to anti-UTPI IgG between HMW-TPI and LMW-TPI may be useful for diagnosis in near future.