The contribution of pericellular proteolysis to
tumor progression is well documented. To better understand
protease biology and facilitate clinical translation, specific proteolytic systems need to be better defined. In particular, the precise role of endogenous
protease inhibitors still needs to be deciphered. We reported previously that
cystatin M, a potent endogenous inhibitor of lysosomal
cysteine proteases, significantly suppressed in vitro cell proliferation, migration, and
Matrigel invasion. Here, we show that scid mice orthotopically implanted with
breast cancer cells expressing
cystatin M show significantly delayed primary
tumor growth and lower metastatic burden in the lungs and liver when compared with mice implanted with mock controls. The incidence of
metastasis, however, appeared to be unaltered between the
cystatin M group and the control group. Experimental
metastasis assays suggest that
cystatin M suppressed
tumor cell proliferation at the secondary site. By using
laser capture microdissection and quantitative reverse transcription-polymerase chain reaction, we found consistent expression of
cystatin M in normal human breast epithelial cells, whereas expression was decreased by 86% in invasive
ductal carcinoma (IDC) cells of stage I to IV patients. Complete loss of expression of
cystatin M was observed in two of three IDCs from stage IV patients. Immunohistochemical studies confirmed that expression of
cystatin M in IDCs was partially or completely lost. We propose
cystatin M as a novel candidate tumor suppressor gene for
breast cancer.