Abstract A33: Small molecule cathepsin L inhibitor KGP94 suppresses hypoxia and acidosis potentiated metastatic capacity of prostate and breast cancer cells

Tumor metastasis is the primary cause of treatment failure and high mortality rates in prostate and breast cancer patients. The development of novel therapeutic strategies that impair the metastatic process therefore is critical to improving prostate and breast cancer patient survival. Elevated cathepsin L (CTSL) secretion by tumor cells has been correlated with metastatic aggressiveness and with poor patient prognosis. Metastatic occurrence can be exacerbated by hypoxic and acidic tumor microenvironments. The goals of the present studies were to explore the role of CTSL in tumor microenvironment-triggered elevation of invasiveness and to elucidate whether the administration of the CTSL specific small molecule inhibitor 3-bromophenyl-3-hydroxyphenyl-ketone thiosemicarbazone (KGP94) could inhibit the enhanced metastatic phenotype in human prostate and breast cancer cells. Comparison of CTSL secretion levels in a number of prostate and breast cancer cells of varying invasive capacities revealed a positive correlation between CTSL secretion and invasive potential. Highly metastatic prostate and breast cancer cell lines PC-3ML and MDA-MB-231 respectively, were used for further studies. In response to treatment with 10μM and 25μM of KGP94 under normoxic conditions, PC-3ML and MDA-MB-231 showed a marked decrease in migratory and invasive potential. These results occurred in the absence of cytotoxicity as determined using a clonogenic cell survival assay. Aberrant microenvironmental conditions (hypoxia; 1%O2 or acidosis; pH, 6.8) enhanced invasiveness of PC-3ML and MDA-MB-231 cells in concert with elevated secretion of CTSL. Molecular assessments revealed that the observed increase in CTSL secretion was a consequence of increased CTSL expression and exocytosis of lysosomes containing active CTSL into the extracellular milieu. Immunostaining with the lysosomal/endosomal marker LAMP-1 revealed that in contrast to the peri-nuclear localization observed in control cells, lysosomes in cells exposed to low oxygen tension or pH traffic toward the cell periphery. Observations based on immunostaining were confirmed through biochemical quantification of the release of lysosomal enzyme β-Hexosaminidase into the incubation media after hypoxic or acidic exposures. KGP94 treatment of PC-3ML and MDA-MB-231 cells exposed to aberrant microenvironmental conditions reduced the enhanced invasive phenotype of these cells below basal invasion levels observed under normoxic conditions. Our findings indicate that (i) the hypoxia and acidosis potentiated metastatic phenotype of PC-3ML and MDA-MB-231 cells is associated with elevated CTSL secretion and (ii) KGP94 mediated CTSL inhibition effectively impairs the microenvironmentally induced enhancement of the metastatic phenotype of prostate and breast cancer cells. Citation Format: Dhivya R. Sudhan, Dietmar W. Siemann. Small molecule cathepsin L inhibitor KGP94 suppresses hypoxia and acidosis potentiated metastatic capacity of prostate and breast cancer cells. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Invasion and Metastasis; Jan 20-23, 2013; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2013;73(3 Suppl):Abstract nr A33.