Cyclic AMP regulation of mammary tumor virus production

Addition of dibutyryl cyclic AMP (cAMP) and agents (isoproterenol and epinephrine) that stimulate the activities of adenylate cyclase enhance the stimulation of mammary tumor virus (MTV) production two- to threefold by glucocorticoid in short-term primary cultures of mammary tumors. This cAMP potentiation seems to depend on the stimulated level of MTV production by glucocorticoid alone, which increases MTV production 5- to 10-fold over basal level but varies greatly in absolute terms. When the stimulated level by glucocorticoid alone is suboptimal, cAMP seems to restore the sensitivity of the cells to the stimulatory effect of glucocorticoid to its maximum.

Identification of a precursor protein to the major glycoproteins of mouse mammary tumor virus

Mouse mammary tumor virus-producing cultures of mouse mammary tumor cells synthesize a viral-related polypeptide of molecular weight of 73,000 (gp 73) which is rapidly labeled during a short pulse but disappears during the chase concomitantly with the appearance of label in the virion glycoproteins gp 49 and gp 37.5/33.5. The addition of the protein synthesis-inhibitor cycloheximide to the chase medium has little effect on this conversion. Treatment of the proposed precursor with alpha-chymotrypsin leads to the formation of a polypeptide of molecular weight 49,000, similar to the major virion glycoprotein. A comparison of tryptic digest maps of the glycoproteins involved supports the hypothesis that both the viral glycoproteins gp 49 and gp 37.5/33.5 are derived from gp 73.

Biochemical studies on mitochondria isolated from Normal and Neoplastic Tissues of the Mouse Mammary Gland

Mitochondria isolated from spontaneous and transplanted mammary adenocarcinomas of two strains of mice were compared, by various biochemical criteria, to mitochondria from mammary glands of midpregnant or hormonally stimulated, cancer-free mice. The specific activities of several mitochondrial enzymes including cytochrome oxidase, alpha-glycerophosphate oxidase, and succinate dehydrogenase were twofold to threefold lower, whereas the activity of monoamine oxidase was two fold higher in tumor mitochondria. Malate dehydrogenase, adenylate kinase, and NADH oxidase showed similar levels of activity in tumor and midpregnant mammary gland mitochondria. In addition, mitochondrial polypeptide composition was analyzed by electrophoresis on sodium dodecyl sulfate-urea polyacrylamide gels. Midpregnant mammary gland and mammary tumor mitochondria were similar in polypeptide composition; however, several differences were observed. A high-molecular-weight polypeptide, present in mid-pregnant mammary gland mitochondria was absent from tumor mitochondria. Also, tumor mitochondria contained an additional high-molecular-weight polypeptide not found in the midpregnant mammary gland. There were numerous differences in the relative proportions of many polypeptides common to both tumor and midpregnant mammary gland mitochondria.

Intracellular synthesis of mouse mammary tumor virus polypeptides: indication of a precursor glycoprotein

Mouse mammary tumor virus polypeptides were detected in the cytoplasm of mouse mammary tumor cell cultures using immunological precipitation techniques. The anti-mouse mammary tumor virus serum precipitated the major virion glycoproteins gp49 and gp37.5/33.5 and a viral-related nonvirion glycoprotein of 76,000 daltons. Subcellular fractionation studies revelaed that the cell-associated virion glycoproteins were present in the membrane fraction. Pulsechase experiments indicated that a viral-related nonvirion glycoprotein of 76,000 daltons may be a precursor to one or more of the virion glycoproteins.

Relationship between organization of mammary tumors and the ability of tumor cells to replicate mammary tumor virus and to recognize growth-inhibitory contact signals in vitro

Mammary tumor virus (MTV) replication was confined primarily to cells organized as acini in intact mouse mammary glands. Primary mammary tumors maintained a high degree of acinar organization and cells therein continued to replicate MTV vegetatively. Nonacinar mammary cells, derived by serial transplantation of acinar tumor cells, no longer actively replicated MTV. This suggests that phenotypic differences exist among mammary epithelial cells in their ability to support virus replication, that a fundamental relationship exists between the organization of epithelium for secretion and active virus replication, and that this relationship is not altered as a primary consequence of neoplastic transformation. Mammary epithelial cells from pregnant, non-tumor-bearing, MTV-infected BALB/cfC3H mice or from acinar mammary tumors from a number of mouse strains were grown in primary monolayer cultures. Such cell cultures under the influence of insulin and cortisol exhibited the ability to organize into discrete three-dimensional structures called "domes." MTV replication in such cultures took place primarily in cells within the organized domes. Cells cultured from nonacinar tumors did not exhibit any propensity to organize into domes, nor did they replicate MTV in primary culture. This suggests that the cell organizational requirement for MTV replication observed in vivo is conserved in primary culture. Dome formation is not an effect of virus replication, as cells from uninfected BALB/c animals organized into domes in culture without concomitant MTV replication. Growth-regulating signals, exerted between contiguous cells in cultures of non-MTV-infected mammary epithelium, were not modified by the occurrence of active virus replication nor as a direct consequence of neoplastic transformation. Cells derived from nontumor BALB/cfC3H glands and from spontaneous tumors exhibited cell growth kinetics, saturation densities, and deoxyribonucleic acid synthesis kinetics nearly identical to those of noninfected normal mammary epithelium in primary culture. Cell to cell growth regulatory signals were modified in cultures of nonalveolar tumor cells wherein evidence of overgrowth is documented.

Rat mammary gland differentiation in vitro in the absence of steroids

Mammary glands from several strains of immature female rats will form small lobules of alveoli when cultured in chemically defined media lacking steroid hormones but containing insulin and prolactin. The degree of lobulo-alveolar differentiation is increased if estradiol, progesterone, and aldosterone are also included in the culture media.

Influence of hormones on lobulo-alveolar differentiation of mouse mammary glands in vitro

(1) To induce mammary differentiation in vitro, BALB/cCrgl mice were given hormones before incubation of their mammary glands in defined medium containing steroid hormones (oestradiol, progesterone and aldosterone), anterior pituitary hormones (prolactin and growth hormone), and insulin. The sequence of lobule development in vitro obtained in this way was determined by cultivation of tissues for 1, 2, 3, 4 and 5 days. Lobules appeared on the 3rd day of culture, although proliferation occurred throughout the 5-day period.

(2) In two other sets of experiments, mammary glands were exposed to various combinations of hormones in culture. Survival of cultured tissues required the presence of insulin, whereas differentiation required the presence of steroid hormones, anterior pituitary hormones, and insulin. Of the steroid hormones used, aldosterone alone fulfilled the minimum steroid hormone requirement for lobule formation in vitro.