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Arrest of Synthesis of Specific Proteins at the Onset of Mammary Tumor Regression¹

Laboratory of Pathophysiology, National Cancer Institute, NIH, Bethesda, Maryland 20014

MTW9 mammary carcinoma regressing after removal of mammotropin stimulation was used as a model to test the hypothesis that the augmented activity of lysosomal enzymes observed during regression may be triggered by a modification of the protein composition of the cytoplasm. In support of the hypothesis, we observed that: (a) the pattern of leucine incorporation into cytosol proteins differs between growing and regressing tumors; (b) the difference is localized in three bands of the electrophoretic pattern; and (c) the change in pattern appears within 6 hr after hormone removal, about 4 hr after prolactin levels in blood were below the concentration needed by MTW9 to grow. These observations are in line with our previous finding of an increased susceptibility of cytosol proteins to proteolytic digestion during MTW9 regression.

¹ This is Paper 9 in a series on mammary tumor regression from the Laboratory of Pathophysiology, National Cancer Institute, NIH, Bethesda, Md.

² Present address: Centre de Recherche en Reproduction Animale, Faculté de Médecine Vétérinaire, Université de Montréal, C.P. 5000, Saint-Hyacinthe, Québec, Canada J2S 7C6.

³ To whom requests for reprints should be addressed.

Received 9/ 1/77. Accepted 1/ 9/78.