Treatment of Ras-induced cancers by the F-actin cappers tensin and chaetoglobosin K, in combination with the caspase-1 inhibitor N1445

BACKGROUND For transforming normal fibroblasts to malignant cells, oncogenic Ras mutan ts such as v-Ha-ras require Rho family GTPases (Rho, Pac, and CDC42) that a re responsible for controlling actin-cytoskeleton organization. Pas activat es Rac through a PI-3 kinase-mediated pathway. Pac causes uncapping of acti n filaments (F-actin) at the plus-ends, through phosphatidylinositol 4,5 bi sphosphate (PIP2), and eventually induces membrane ruffling. Several distin ct F-actin/PIP2-binding proteins, such as gelsolin, which severs and caps t he plus-ends of actin filaments, or HS1, which cross-links actin filaments, have been shown to suppress v-Ha-Ras-induced malignant transformation when they are overexpressed. Interestingly an F-actin cross-linking drug (photo sensitizer) called MKT-077 suppresses Ras transformation. Thus, an F-actin capping/severing drug might also have an anticancer potential. PURPOSE This study was conducted to determine first whether Pas-induced malignant p henotype (anchorage-independent growth) is suppressed by overexpression of the gene encoding a large plus-end F-actin capping protein called tensin an d second to test the anti-pas potential of a unique fungal antibiotic (smal l compound) called chaetoglobosin K (CK) that also caps the plus-ends of ac tin filaments. METHODS AND RESULTS DNA transfection with a retroviral vector carrying the tensin cDNA was used to overexpress tensin in v-Ha-Ras-transformed NIH 3T3 cells. All stable te nsin transfectants rarely formed colonies in soft agar, indicating that ten sin suppresses the anchorage-independent growth. The anti-Ras action of CK was determined by incubating the Ras-transformants in the presence of CK in soft agar, Two mu M CK almost completely inhibited their colony formation, indicating that CK also suppresses the malignant phenotype. However, unlik e tensin, CK causes an apoptosis of Ras-transformed NIH 3T3 cells and, less effectively, of normal NIH 3T3 cells, indicating that CK has an F-actin ca pping-independent side effect(s). CK-induced apoptosis is at least in part caused by CK-induced inhibition of the kinase PKB/AKT. However, a specific ICE/caspase-1 inhibitor called N1445 completely abolished the CK-induced ap optosis by reactivating PKB, but without affecting the CK-induced suppressi on of Ras transformation. CONCLUSIONS Like the F-actin cross-linking drug MKT-077, the F-actin capping drug CK ma y be useful for the treatment of Ras-associated cancers if it is combined w ith the ICE inhibitor N1445, which abolishes the side effect of CK. Our obs ervations that two distinct F-actin capping molecules (i.e., tensin and CK) suppress Ras-induced malignant phenotype strongly suggest, if not prove, t hat capping of actin filaments at the plus-ends alone is sufficient to bloc k one of the Pas signaling pathways essential for its oncogenicity, This no tion is compatible with the fact that Ras induces the uncapping of actin fi laments at the plus-ends through the Rac/PIP2 pathway.