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 technolog y to clone unique mRNAs that are expressed during tissue regression. W e have cloned a number of regression selected genes (RSG) that are exp ressed during the regression of the mammary gland and ventral prostate of the rat after the removal of the respective trophic hormone. In th is investigation, we have characterized one of these genes, RSG-2, tha t is homologous to cathepsin B, a thiol protease that has been previou sly identified as one of the extracellular proteases which is activate d in metastatic cells. The steady-state levels of RSG-2 mRNA in the no rmal prostate are low but detectable. In the regressing prostate, RSG- 2 mRNA levels peak at 3-4 days after castration, at the time that tiss ue regression is maximal. The gene is induced in a similar fashion in the regressing mammary gland. Using in situ hybridization, we have est ablished that RSG-2 mRNA is expressed in the luminal epithelial cells of the prostate and mammary gland that are known to undergo active cel l death, suggesting that it may be a general marker for secretory epit helial cell death. Analysis of the distribution of the cathepsin B pro tein by immunofluorescence microscopy demonstrates that there is diffu se, but punctate, expression of the protein in all of the luminal epit helial cells of the normal prostate and mammary gland. However, at ear ly times after hormone ablation in both glands, the majority of the in crease 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 apoptot ic bodies. These results suggest that RSG-2, or cathepsin B, is requir ed for the local degradation of the basement membrane, which is one of the earliest morphologically recognizable events of active cell death .