Cathepsin B, a cysteine protease implicated in metastatic progression, is also expressed during regression of the rat prostate and mammary glands.

We have developed a novel library-to-library cross-screening technology to clone unique mRNAs that are expressed during tissue regression. We have cloned a number of regression selected genes (RSG) that are expressed during the regression of the mammary gland and ventral prostate of the rat after the removal of the respective trophic hormone. In this investigation, we have characterized one of these genes, RSG-2, that is homologous to cathepsin B, a thiol protease that has been previously identified as one of the extracellular proteases which is activated in metastatic cells. The steady-state levels of RSG-2 mRNA in the normal prostate are low but detectable. In the regressing prostate, RSG-2 mRNA levels peak at 3-4 days after castration, at the time that tissue regression is maximal. The gene is induced in a similar fashion in the regressing mammary gland. Using in situ hybridization, we have established that RSG-2 mRNA is expressed in the luminal epithelial cells of the prostate and mammary gland that are known to undergo active cell death, suggesting that it may be a general marker for secretory epithelial cell death. Analysis of the distribution of the cathepsin B protein by immunofluorescence microscopy demonstrates that there is diffuse, but punctate, expression of the protein in all of the luminal epithelial cells of the normal prostate and mammary gland. However, at early times after hormone ablation in both glands, the majority of the increase in cathepsin B protein appears to result from redistribution to the basal aspect of the cells. At later time points, there appears to be increased amounts of the protein which is localized to the apoptotic bodies. These results suggest that RSG-2, or cathepsin B, is required for the local degradation of the basement membrane, which is one of the earliest morphologically recognizable events of active cell death.