Quantitative changes in cytoskeletal and nuclear actins during cellular transformation.

ABSTRACT

Actin, a highly conserved protein comprising cell stress fibers and other cellular structures, is found in both the cytoplasm and nucleus of cells and responds to both epigenetic signals and altered gene expression occurring during tumorigenesis. We have previously shown that changes in the cytoplasmic F- and G-actin ratios reflect bladder cancer risk. To determine whether nuclear actin is also altered and how nuclear and cytoplasmic actin alterations are interrelated in transformation, an in vitro model of carcinogen-induced transformation consisting of 2 human uroepithelial cell lines immortalized by infection with SV-40 was studied. One line, HUC-PC, is tumorigenic in nude mice after incubation with the carcinogen 4-ABP, the other, HUC-BC, is not. Cytoplasmic and nuclear F- and G-actin were determined by QFIA on individual cells using fluorochrome-labeled phallicidin and DNase, I, respectively. Before exposure to 4-ABP, the PC cells had lower cytoplasmic F-actin content, higher cytoplasmic G-actin content, but similar levels of nuclear G- and F-actin in comparison to the BC cells. After incubation with 4-ABP, F-actin decreased and G-actin increased in both cytoplasm and nuclei of PC cells and cytoplasmic F-actin fibers were lost, but only cytoplasmic actin was altered in the BC cells. Northern blot analysis showed the expression of the beta-actin gene was only approximately 20% lower in 4-ABP-treated PC cells than in untreated controls, indicating the cellular change in actin was attributed to a shift between F- and G-actin proteins rather than to net actin synthesis.