Mammary Tumor Regression

Studies have been made on the role of two hydrolases, acid RNase and cathepsin, in the regression of hormone-dependent rat mammary carcinomas. A burst of acid RNase activity as early as 6 hours after hormonal deprivation was followed by a significant increase in cathepsin activity that began 48 hours after hormone removal. The ratios of free to bound activities of these enzymes increased with tumor regression, reaching their highest ratios of 4- to 5-fold that of growing tumors 72 to 96 hours after hormone removal when most tumors had regressed to one-half of their original size. The possible mechanisms leading to the increased free activity in regressing tumors are discussed. Isolation of acid RNase in a homogeneous state has made possible quantitation of the enzyme in tumors by immuno-diffusion techniques. The increase in enzyme quantity was closely correlated with its increased activity in regressing tumors. New synthesis of acid RNase during the early phase of tumor regression is indicated by an increased concentration of the enzyme which is partially blocked in vivo by inhibitors of protein synthesis.