Cathepsin D protein levels in colorectal tumors: Divergent expression patterns suggest complex regulation and function

Cathepsin D protein patterns were analyzed in 59 colorectal tumors by Western blotting, glycosylation and immunohistochemical assays. Measurement of protein content by laser densitometry of tumor/normal pairs on Western blots revealed loss of cathepsin D protein in more than 50% of colorectal tumors. Independent loading controls and statistical estimates of reproducibility on duplicate assays confirmed frequent decreases in cathepsin D. For cases having a tumor/normal ratio (T/N) <1, the average T/N was 0.50±0.19, equivalent to the loss of one cathepsin D allele. However, 2-fold increases in cathepsin D protein levels were also observed in approximately 1/3 of tumors, supporting the concept that colorectal cancers develop via divergent molecular pathways and that cathepsin D may function differently in different cancers. Although normal cathepsin D expression was detected in some earlier stage tumors, protein levels became increasingly bimodal with progression such that cathepsin D levels were increased in 1/3 but decreased in 2/3 of stage III and IV cancers. Other laboratories have reported both significant loss and gain of chromosome 11 (site of the cathepsin D gene) in different colorectal tumors, providing a possible mechanism for our observations on cathepsin D. However, differential regulation of cathepsin D expression by mutant versus wild-type p53 may also contribute to variable cathepsin D levels in colorectal cancers. Immunohistochemical studies demonstrated a shift from a predominantly punctate distribution of cathepsin D protein in normal mucosa to a more diffuse cytoplasmic distribution in tumor tissues. Mutant forms of cathepsin D were not detected in tumors either as changes in electrophoretic mobility or altered glycosylation but minor changes in protein sequence could not be ruled out. Loss of cathepsin D protein may provide an advantage to colorectal tumors related to a loss of cathepsin D function in proapototic or antiangiogenic pathways while increased cathepsin D may promote cancer cell proliferation or invasion.