Prolactin stimulation of alpha-lactalbumin in normal primate mammary gland

Effects of recombinant human prolactin on breast milk composition

OBJECTIVE

The objective of this study was to determine the impact of recombinant human prolactin (r-hPRL) on the nutritional and immunologic composition of breast milk.

METHODS

We conducted 2 trials of r-hPRL treatment. In the first study, mothers with documented prolactin deficiency were given r-hPRL every 12 hours in a 28-day, open-label trial. In the second study, mothers with lactation insufficiency that developed while they were pumping breast milk for their preterm infants were given r-hPRL daily in a 7-day, double-blind, placebo-controlled trial. Breast milk characteristics were compared before and during 7 days of treatment.

RESULTS

Among subjects treated with r-hPRL (N = 11), milk volumes (73 ± 36 to 146 ± 54 mL/day; P < .001) and milk lactose levels (155 ± 15 to 184 ± 8 mmol/L; P = .01) increased, whereas milk sodium levels decreased (12.1 ± 2.0 to 8.3 ± 0.5 mmol/L; P = .02). Milk calcium levels increased in subjects treated with r-hPRL twice daily (2.8 ± 0.6 to 5.0 ± 0.9 mmol/L; P = .03). Total neutral (1.5 ± 0.3 to 2.5 ± 0.4 g/L; P = .04) and acidic (33 ± 4 to 60 ± 6 mg/L; P = .02) oligosaccharide levels increased in r-hPRL-treated subjects, whereas total daily milk immunoglobulin A secretion was unchanged.

CONCLUSIONS

r-hPRL treatment increased milk volume and induced changes in milk composition similar to those that occur during normal lactogenesis. r-hPRL also increased antimicrobially active oligosaccharide concentrations. These effects were achieved for women with both prolactin deficiency and lactation insufficiency.

The Mammary Glands of Macaques

This review describes the normal biology and physiology of the mammary gland in macaques, including the typical histologic appearance across the life span (development, reproductive maturity, lactation, and senescence). The molecular events regulating breast morphogenesis are described, as well as systemic and local hormonal regulators of mammary gland proliferation, differentiation, and function. Similarities and differences to the human breast are described. Regulatory events are illuminated by discussion of genetically modified mouse models. Tissue response markers, including immunohistochemical markers of proliferation and other hormonally induced changes and studies to date, regarding the effects of exogenous hormones, are briefly summarized. In general, estrogens stimulate progesterone receptor expression and proliferation in the mammary gland, and combinations of estrogens and progestogens cause greater proliferation than estrogens alone. Evaluation of novel chemical agents in macaques requires careful evaluation of age and hormonal context to avoid the confounding effects of mammary gland development, past reproductive history, and other influences on mammary gland morphology. The expression of proliferation markers and progesterone receptors may be used as biomarkers to measure chemically induced hormonal effects.

Validation of radioimmunoassay for human lactalbumin in the serum by testing the endogenous antibodies interference

For re-establishing the value of human lactalbumin as a functional marker of normal and pathological activity of the breast a sensitive and specific radioimmunoassay was established with the prior important control of the interference of endogenous antibodies. The specificity of the assay was assessed by the absence of interference from other proteins in milk or in breast cyst fluid, various hormones and tumor markers. Bovine lactalbumin showed incomplete and weak cross-reactivity. By an enzymoimmunoassay method it was shown that all the 222 human sera studied contain IgG immunoglobulins which bind bovine and human lactalbumin with greater reactivity of children's serum and without relationship to the blood groups. The maximum affinity constant of these endogenous immunoglobulins determined by the radioimmunoassay method is 4.5 times greater for bovine (Kd = 18 X 4(-11) M) than for human (Kd = 4 X 10(-11) M) lactalbumin. These endogenous anti-lactalbumin immunoglobulins caused no interference in the radioimmunoassay as shown by the complete correlation between the concentrations of human lactalbumin previously incubated and added to sera containing high-affinity antibodies and those measured directly in the radioimmunoassay. This lack of interference was explained by the higher (22-fold) affinity constant of the rabbit antiserum against human lactalbumin (Kd = 9 X 10(-12) M). The study of endogenous antibodies by the two enzymes and radioimmunoassay methods is needed before assessing and using a radioimmunoassay of human lactalbumin in serum.

Primate mammary development. Effects of hypophysectomy, prolactin inhibition, and growth hormone administration

The pituitary gland has been found to be an important factor in mammary development in primates. Hypophysectomy in 12 sexually immature monkeys caused significant inhibition of estradiol (E2)-induced mammary growth and development. A histological index of mammary development in sexually immature hypophysectomized animals was lower (0.82) than in intact E2-treated controls (3.4; P less than 0.008). Hypophysectomy also inhibited growth of the mammary gland as judged by a size index. Despite the hypophysectomy, E2 stimulated some, albeit blunted, mammary growth and development, which may have been due to incomplete hypophysectomy. Selective inhibition of prolactin by ergot drugs in intact animals did not prevent full mammary development, suggesting that there may be pituitary mammogens other than prolactin, or that very low or unmeasurable concentrations of prolactin were sufficient to synergize with E2 to cause full acinar development. The mean histological index was 3.08 in E2-treated animals and 3.16 in animals treated with E2 plus pergolide. There was also no difference in the size of the glands. We evaluated the effect of growth hormone on mammary development by treating three hypophysectomized animals with pure 22,000 mol wt human growth hormone (hGH) (Genentech, Inc., South San Francisco, CA). We found that physiological or slightly supraphysiological concentrations of hGH in animals with unmeasurable prolactin were incapable of restoring the capacity of E2 to induce full mammary growth. These findings suggest that, if growth hormone is a mammary mitogen, that physiological concentrations are insufficient to synergize with E2 to induce full mammary growth or that other forms of hGH are mammogenic. Our studies suggest that the role of the pituitary gland in mammary mitogenesis in primates is more complicated than previously thought. They also raise the possibility that heretofore unidentified pituitary substances may be mammogenic.

Inhibition by estradiol of the lactogenic effect of prolactin in primate mammary tissue: reversal by antiestrogens LY 156758 and tamoxifen

Increasing concentrations of estradiol (E2) ranging from 0.01 to 10 nM were found to inhibit partially but significantly the lactogenic effect of ovine prolactin (oPRL) on alpha-lactalbumin production in primate mammary tissues maintained in organ culture for 9 days. E2 at 10 nM inhibited by 38% (mean) PRL-stimulated alpha-lactalbumin production measured by radioimmunoassay. E2 antagonized the effect of oPRL by reducing new alpha-lactalbumin synthesis as determined by specific immunoprecipitation of alpha-lactalbumin and by analysis with NaDodSO4 gel electrophoresis. In immunoprecipitation studies, the mean inhibition of alpha-lactalbumin production was 57.6%. E2 in the absence of oPRL had no effect on alpha-lactalbumin production. In contrast to previous observations in rodents, progesterone was found to be a much weaker inhibitor of PRL-induced alpha-lactalbumin production than was E2 in primate breast tissues. Mean inhibition of oPRL-stimulated alpha-lactalbumin production was 32.3% with 10 microM progesterone and 8.3% with 10 nM. The inhibitory effect of E2 on oPRL-stimulated alpha-lactalbumin production was significantly reversed by both tamoxifen and a new antiestrogen, LY 156758. Although exact comparison of the effects of these two antiestrogens was not possible, it was apparent that LY 156758 was more potent in blocking the E2 inhibitory effect. In summary, these studies provide evidence that physiologic concentrations of estradiol partially block the lactogenic effect of PRL in primate mammary glands, suggesting a new role for estrogen in mammary physiology. The inhibitory effect of estrogen treatment on milk production in women after parturition may possibly be explained by this direct antagonism between E2 and PRL.

Glucocorticoid regulation of alpha-lactalbumin production in long-term cultures of normal rat mammary cells

Cultures of normal rat mammary cells have been developed which produce alpha-lactalbumin (a-LA) for several months in response to physiological levels of corticosterone whereas none is measureable without glucocorticoid. In contrast to recent studies in short-term organ cultures, these long-term cell cultures derived from virginal or mid-pregnant rats show no inhibition of a-LA production at high glucocorticoid concentrations. Comparable a-LA production was observed after a few months whether cells were initially cultured into media with various corticosterone levels or initially into high levels of corticosterone for 3 weeks prior to the shift to various lower doses.

A radioimmunoassay for mouse alpha-lactalbumin

alpha-Lactalbumin has been purified from mouse milk by the use of ammonium sulfate precipitation, Bio-Gel P-100 chromatography and DEAE-cellulose chromatography. Mouse alpha-lactalbumin exists as two major charge forms with the same molecular weight. Both charge forms have been used in the development of a radioimmunoassay to mouse alpha-lactalbumin. The assay is specific for mouse alpha-lactalbumin, reacting identically with both forms, and sensitive with a minimal detectable concentration of 0.35 ng. alpha-Lactalbumin induced in mouse mammary gland explants has been measured with the use of the radioimmunoassay and is detectable in both the explants and media of the cultures.

Prolactin in human milk: correlation with lactose, total protein, and alpha-lactalbumin levels

To determine whether prolactin (PRL) could be conclusively demonstrated in human milk, samples from nine puerperal women were examined by radioimmunoassay techniques. Recovery of 78 +/- 5% (mean +/- SE) PRL added to eight milk samples and mean recoveries of 83, 92, and 92% of PRL after 1, 2, and 3 h incubation at 21 degrees C indicated that quantitative recovery was possible. PRL immunoreactivity was not lost by dialysis for 96 h. Serial dilution of milk samples before and after dialysis gave inhibition curves parallel to PRL standards. Milk PRL concentrations were high for the first 3 days after birth, with a peak mean value of 157 +/- 18 ng/ml (SE) on the third day. Milk PRL levels then fell sharply to a mean of 24 ng/ml 13 days after delivery. Milk PRL concentrations showed a significant negative correlation (P less than 0.001) with milk lactose (r = -0.59) and positive correlations (P less than 0.01) with total milk protein (r = 0.49) and alpha lactalbumin (r = 0.33) estimations. We conclude that i) PRL is a normal constituent of human milk, ii) high concentrations of PRL are present in human milk for the first 3 days postpartum but subsequently fall rapidly, and iii) milk PRL levels correlate significantly with milk lactose, total protein, and alpha-lactalbumin values.