The effect of tumor growth on the amount, state of polymerization, and synthesis of liver actin was investigated in fed and fasted rats bearing a Walker 256 carcinoma. The increase in liver size in the tumor-bearing animal was accompanied by a rise in the total amount of actin and protein, although the amounts present per g liver fell in both cases. Soluble actin increased in both concentration and total amount in the tumor-bearing animals. While the ratio of total actin to total protein in liver was unaltered by tumor growth, the ratio of soluble actin to total actin was increased. The incorporation of [3H]leucine into liver actin relative to that into liver protein, in vivo, was not affected by tumor growth, but the radioactivity incorporated into soluble actin relative to total actin in the livers of the tumor-bearing rats was increased. Liver polysome preparations from tumor-bearing rats showed an increased ability to synthesize actin and total protein, whereas polysomes from skeletal muscle of tumor-bearing rats exhibited a decreased synthesis of actin and total protein. These results suggest that, in the liver of the tumor-bearing rat, while there is an increase in actin synthesis in parallel with a net increase in protein synthesis, there is a decrease in the polymerization of actin.

In livers of both control and tumor-bearing rats, the consumption of a meal was accompanied by a decrease in soluble actin relative to total actin and an increase in the synthesis of actin relative to total protein.

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This work was supported by a grant from the Cancer Research Campaign.

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