Insulin is essential for accumulation of casein mRNA in mouse mammary epithelial cells.

Abstract

In the presence of cortisol and prolactin, insulin at concentrations as low as 1 ng/ml significantly stimulates casein synthesis in mammary explants from midpregnant mice; maximal synthesis occurs at 10 ng/ml. However, in the absence of insulin, no detectable immunoprecipitable casein is produced. Insulin also supports enhanced accumulation of casein mRNA in the presence of cortisol and prolactin; neither epidermal growth factor nor somatomedin C has this effect. These inductive actions ofinsulin are not secondary to a general maintenance effect on the mammary epithelial cell; insulin, epidermal growth factor, and somatomedin C can support the accumulation of RNA in rough endoplasmic reticulum equally well. In addition, these effects do not reflect a specific insulin requirement for prolactin sensitivity; epidermal growth factor can support prolactin-induced total RNA synthesis as well as insulin can. The results demonstrate that, although insulin, epidermal growth factor, and somatomedin C can all function as cell maintenance agents, only insulin, together with cortisol and prolactin, can induce casein mRNA accumulation.Casein can be induced in mouse mammary explants in the presence of insulin, cortisol, and prolactin. Placental lactogen (1) and, in some instances, growth hormone (2, 3), can substitute for prolactin in this regard. Other glucocorticoids (4) can substitute for cortisol in this induction. On the other hand, other agents tested as possible substitutes for insulin have been found ineffective. Neither serum (5) nor epidermal growth factor (6) supports the induction of casein synthesis when used in the presence of cortisol and prolactin, although both are active mitogenic agents for mammary epithelial cells. Despite these findings, insulin has not been regarded as essential physiologically for phenotypic expression of casein genes in mammary epithelial cells (for review, see ref. 7). Rather, it has been considered to function in cell maintenance in vitro. In more recent studies, it has been reported that prolactin (8) tained from Calbiochem. The somatomedin C used was derived from the final high-pressure liquid chromatography purification step described previously (11) and was free of both insulin and insulin-like growth factor II. This material was 33% pure, as determined by radioimmunoassay; the amounts given below take this factor into account. Medium 199/Hanks' salts was purchased from GIBCO, Hepes was obtained from Sigma, and rabbit anti-sheep IgG antiserum was from Miles-Yeda. aip1, carrier-free, and [5_3H]uridine (30 Ci/mmol; 1 Ci = 3.7 X 1010 becquerels) were purchased from New England Nuclear.Organ Culture. The abdominal mammary glands from C3H/ HeN mice, 10-12 days into theirfirst pregnancy, were removed under sterile conditions and explants were prepared as described (12). The culture medium (medium 199/Hanks' salts/ 20 mM Hepes, pH 7.6) contained cortisol (1 jug/ml), prolactin (1 ug/ml), and insulin, epidermal growth factor, or somatomedin C at 50 ng/ml, unless otherwise stated....

Induction of Differentiation in a Rat Mammary Epithelial Stem Cell Line by Dimethyl Sulphoxide and Mammotrophic Hormones

“…Earlier evidence suggested that addition of prolactin to either explants or dissociated cells from mid-pregnant mouse or rat mammary glands is sufficient to induce the synthesis of casein [7,31], whereas addition of prolactin to mammary explants from mature mouse virgin gland results in increased levels of casein mRNA, but these are not accompanied by an increase in the synthesis of casein unless hydrocortisone is also present [32,33]. More recent evidence suggests that hydrocortisone promotes the accumulation of casein mRNA in mice and rabbits [3,34] whereas insulin promotes both the accumulation of casein mRNA and its subsequent translation [2].…”

Contrôle hormonal du développement et de l'activité de la glande mammaire

“…En effet, la suppression transitoire de la prolactine se traduit par une production laitière particulièrement basse (Kann et al, 1978 Bolander et al, 1981 ;Chomczynski et a/., 1984 ;Houdebine et al, 1985a). S'il n'est pas douteux que l'insuline exerce un effet amplificateur puissant sur l'action inductrice de la prolactine, son rôle exact reste mal défini.…”

Growth factors in mammary gland function

“…It would also explain why diabetic animals show no significant impairment of mammary growth (Norgren, 1968;Topper & Freeman, 1980). In addition to effects on cell replication, Prosser & Topper (1986) (Posner et al, 1974;Campbell et al, 1987 (Bolander et al, 1981;Chomczynski et al, 1984).…”

“…Thyroid hormones synergize with pro¬ lactin with respect to a-lactalbumin synthesis (see Topper & Freeman, 1980). Insulin is essential for transcription of mouse and rat casein messenger RNA (Bolander et al, 1981;Chomczynski et al, 1984) but insulin-like growth factors can substitute in other roles, which require supraphysiological insulin concentrations in vitro (see below for further discussion).…”