Chromatin was prepared from mouse mammary epithelial cell nuclei and histones were removed by extraction with 2.0 M NaCl, pH 6.0. Electrophoresis of the residual acidic chromatin proteins in polyacrylamide gels containing sodium dodecyl sulfate revealed a heterogeneous banding pattern which was tissue specific. Six or more bands among the proteins present in differentiated, lactational cells were not detectable in undifferentiated, virginal cells.In organ culture studies insulin stimulated the incorporation of 3H-tryptophan or 3H-aspartic acid and 3H-leucine into all electrophoretic components of the mammary acidic chromatin protein. Two peak rates of synthesis, preceding and following the peak rate of DNA synthesis, were observed. Synthesis was unaffected by hydrocortisone or prolactin. Synthesis of the six bands characteristic of the lactational cells was initially undetectable in undifferentiated, virginal cells, but was induced after incubation with insulin for 42 h. These results, taken in correlation with previous studies on this system, are consistent with a proposed model in which new species of acidic chromatin proteins may participate in chromatin reconstitution during mammary cell differentiation.
Regulation of mouse mammary cell differentiation by extracellular milk proteins