Cathepsin L is a lysosomal enzyme capable of catalyzing the cleavage of a variety of functionally important substrates in the cytoplasm, the extracellular matrix and even the nucleus. Cathepsin L deficiency has been documented to result in proliferation arrest and increased cellular susceptibility to death suggesting a survival function of this enzyme. We have shown recently that both chemical and siRNA mediated inhibition of cathepsin L resulted in increased susceptibility of drug resistant cancer cells to senescence and subsequently to apoptosis, thus reversing their drug resistance phenotype. Here, we report that cathepsin L inhibition affects chromatin remodeling by controlling the pool of acetylated histones in the nucleus through their cleavage. In vitro experiments demonstrated that recombinant cathepsin L readily cleaves purified histones. In addition, both chemical and siRNA mediated inhibition of cathepsin L led to accumulation of the acetylated form of histone H3. Furthermore, cathepsin L inhibitor synergized with histone deacetylase inhibitor, Trichostatin A, to induce cellular proliferation arrest and death. Chromatin immunoprecipitation experiments indicated that the promoter region of p21/WAF1 was hyperacetylated in response to cathepsin L inhibition leading to enhanced expression of p21/WAF1 and cellular senescence. This was followed by a cell death that was independent of apoptosis but characterized by increased cytoplasmic vacuolization and expression of the mannose 6 phosphate receptor both markers of autophagy. All together, these findings suggest that cathepsin L may exert its survival function by maintaining the balance of active/repressed chromatin configurations and preventing cancer cell from undergoing senescence and autophagic death. Further understanding of these pathways is expected to facilitate the discovery of novel therapeutic targets for cancer treatment.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]