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Altered Populations of Acidic Chromatin Proteins in Breast Cancer Cells¹

Department of Medicine, University of Wisconsin, Madison, Wisconsin 53706

The acidic chromatin proteins extracted from normal and neoplastic mammary cells of C3H mice and Fischer rats have been compared by polyacrylamide gel electrophoresis, amino acid analysis, and radioactivity labeling patterns of synthesis in vitro. Following purification of chromatin from purified nuclei and extraction of histones in 2 M sodium chloride, the acidic proteins associated with DNA were subjected to electrophoresis on polyacrylamide gels containing sodium dodecyl sulfate, and the stained gels were compared by absorbance scanning. Striking differences were observed in the banding patterns of proteins derived from the normal or neoplastic cells. Among a group of high-molecular-weight components characteristic of the lactational mammary gland, several were preserved while others were deficient or undetectable in the neoplastic cells. In addition, several components of intermediate molecular weight that were present in the neoplastic cells were undetectable in the normal pattern. Differences between the protein populations derived from normal and neoplastic cells were also demonstrated by amino acid analysis. Organ cultures of carcinoma and gland explants demonstrated that the normal mammary and gland synthesized lower-molecular-weight species at a higher net rate, whereas the carcinomas exhibited a relatively higher net rate of synthesis of high-molecular-weight components. ³²P was incorporated as phosphoserine and phosphothreonine residues in the major portion of the proteins of all preparations, although the carcinomas exhibited a relatively lower net rate of phosphorylation of the high-molecularweight proteins. The differences between preparations derived from neoplastic and normal tissues could not be attributed merely to the physiological state of the gland, since the patterns of the proteins from the carcinomas were also significantly different from previous patterns derived from the mammary glands of virginal or pregnant animals. The results demonstrate that the neoplastic characteristics of mammary carcinomas are associated with an altered content and pattern of synthesis of acidic chromatin proteins.

¹ This work was supported by USPHS Grant CA12904-1 from the National Cancer Institute and Grant VC-62A from the American Cancer Society, Inc.

Received 10/31/72. Accepted 3/ 1/73.