Hormone-dependent Formation of Polysomes in Mammary Cells in Vitro

The mechanisms by which insulin, hydrocortisone, and prolactin cause cell differentiation and the induction of specific milk proteins in mammary cells were studied further in relation to the formation of ribosomes and their organization into polysomes. Insulin stimulated the formation of ribosomes and their organization into polysomes in mammary epithelial stem cells in vitro. The newly formed ribosomes were largely conserved in the daughter cells after cell division. In cells previously treated with insulin and hydrocortisone, insulin and prolactin acted synergistically to stimulate the formation of new ribosomes and polysome aggregations. The pre-formed and the newly formed ribosomes were equivalent in their distribution among monosomes and polysomes. Synthesis of specific casein proteins was undetectable in the cells formed in vitro until 4 to 6 hours of prolactin stimulation, at which time an increase in polysomal concentration and the initiation of 32P- or 14C-casein synthesis occurred concomitantly. This result is interpreted as the translation of casein messenger RNA in polysomes, an event which is coupled to preceding hormone-mediated increases in transcriptional activity.