Altered Expression of a Third Actin Accompanying Malignant Progression in Mouse B16 Melanoma Cells

The expression of actin was examined and compared in several mouse B16 melanoma cell lines with different metastatic ability, by the use of two‐dimensional gel electrophoresis or horizontal isoelectric focusing. In the mouse B16 melanoma cell lines, the expression of newly found A x actin (Mr = 43,000, pl = 5.2) decreased with the increase in in vitro and in vivo selection cycles (F number) for high‐metastatic cells. On the contrary, the metastatic ability of each mouse cell line, assessed by lung colony‐forming ability following iv administration, increased with increase in the F number. The half life of A x actin was much the same as that of β‐and γ‐actin and the different expressions of A x actin between the low‐ (F=l) and high‐metastatic (F=10) cell lines were attributed to differences in the rate of synthesis but not in the decay rate of A x actin. The A x actin was incorporated into the cytoskeletal fraction with the same efficiency as β‐and γ‐actin. The invasiveness of the cells, assessed in vitro using matrigel, was increased with the decrease in A x expression. The actin stress fibers, observed staining with rhodamine‐conjugated phalloidin, were organized better in a low‐metastatic cell line (F=l) than in a high‐metastatic one (F = 10). These results suggest to us that depression of A x actin is involved in disorganization of the cytoskeletal system, the cellular flexibility and motility are enhanced and there is a consequent increase in the invasiveness and metastatic potential.