mtCLIC/CLIC4 is a p53 and
tumor necrosis factor alpha (
TNFalpha) 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
TNFalpha-induced apoptosis in human
osteosarcoma cell lines. In contrast, inhibition of NFkappaB results in an increase in
TNFalpha-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
TNFalpha-mediated apoptosis in both the SaOS and U2OS derivative cell lines without altering
TNFalpha-induced NFkappaB 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
TNFalpha 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.