and pH of the nucleus, contributes to cell cycle arrest and the specific gene expression program associated with keratinocyte terminal differentiation.

mtCLIC/CLIC4 is a p53 and tumor necrosis factor α (TNFα) regulated intracellular chloride channel protein that localizes to cytoplasm and organelles and induces apoptosis when overexpressed in several cell types of mouse and human origin. CLIC4 is elevated during TNFα-induced apoptosis in human osteosarcoma cell lines. In contrast, inhibition of NFκB results in an increase in TNFα-mediated apoptosis with a decrease in CLIC4 protein levels. Cell lines expressing an inducible CLIC4-antisense construct that also reduces the expression of several other chloride intracellular channel (CLIC) family proteins were established in the human osteosarcoma lines SaOS and U2OS cells and a malignant derivative of the mouse squamous papilloma line SP1. Reduction of CLIC family proteins by antisense expression caused apoptosis in these cells. Moreover, CLIC4-antisense induction increased TNFα-mediated apoptosis in both the SaOS and U2OS derivative cell lines without altering TNFα-induced NFκB activity. Reducing CLIC proteins in tumor grafts of SP1 cells expressing a tetracycline-regulated CLIC4-antisense substantially inhibited tumor growth and induced tumor apoptosis. Administration of TNFα i.p. modestly enhanced the antitumor effect of CLIC reduction in vivo. These results suggest that CLIC proteins could serve as drug targets for cancer therapy, and reduction of CLIC proteins could enhance the activity of other anticancer drugs.