Studies of beta-casein content of normal and neoplastic rat mammary tissues by a homologous radioimmunoassay.

We have developed a sensitive homologous radioimmunoassay for rat beta-casein (r-beta-casein) capable of measuring concentrations of beta-casein as low as 0.2 ng/mg of tissue and have measured casein content of rats under several physiological conditions as well as in experimentally induced mammary tumor explants. Changes in content of beta-casein were followed during pregnancy and lactation. Virgin females had 10 ng beta-casein per micrograms of DNA, with a continuous increase in casein content observed throughout pregnancy. At the outset of lactation, beta-casein increased markedly and remained relatively constant throughout lactation. These variations in beta-casein levels indicate that in the rat the synthesis of this protein is under hormonal control and related to the state of differentiation of the mammary gland. The concentration of beta-casein was very low in mammary tumors (0.6 ng/microgram of DNA). Addition of prolactin to organ culture of NMU-induced tumors did not induce significant changes in beta-casein content. These results indicate that NMU-induced tumors have lost at least one of their hormonally regulated functions of cell differentiation, although their hormonal dependence for growth is maintained.