Prolactin activation of mammary nitric oxide synthase: molecular mechanisms

The market for amino acids: understanding supply and demand of substrate for more efficient milk protein synthesis

For dairy production systems, nitrogen is an expensive nutrient and potentially harmful waste product. With three quarters of fed nitrogen ending up in the manure, significant research efforts have focused on understanding and mitigating lactating dairy cows’ nitrogen losses. Recent changes proposed to the Nutrient Requirement System for Dairy Cattle in the US include variable efficiencies of absorbed essential AA for milk protein production. This first separation from a purely substrate-based system, standing on the old limiting AA theory, recognizes the ability of the cow to alter the metabolism of AA. In this review we summarize a compelling amount of evidence suggesting that AA requirements for milk protein synthesis are based on a demand-driven system. Milk protein synthesis is governed at mammary level by a set of transduction pathways, including the mechanistic target of rapamycin complex 1 (mTORC1), the integrated stress response (ISR), and the unfolded protein response (UPR). In tight coordination, these pathways not only control the rate of milk protein synthesis, setting the demand for AA, but also manipulate cellular AA transport and even blood flow to the mammary glands, securing the supply of those needed nutrients. These transduction pathways, specifically mTORC1, sense specific AA, as well as other physiological signals, including insulin, the canonical indicator of energy status. Insulin plays a key role on mTORC1 signaling, controlling its activation, once AA have determined mTORC1 localization to the lysosomal membrane. Based on this molecular model, AA and insulin signals need to be tightly coordinated to maximize milk protein synthesis rate. The evidence in lactating dairy cows supports this model, in which insulin and glucogenic energy potentiate the effect of AA on milk protein synthesis. Incorporating the effect of specific signaling AA and the differential role of energy sources on utilization of absorbed AA for milk protein synthesis seems like the evident following step in nutrient requirement systems to further improve N efficiency in lactating dairy cow rations.

Prolactin alters blood pressure by modulating the activity of endothelial nitric oxide synthase

Increased levels of a cleaved form of prolactin (molecular weight 16 kDa) have been associated with preeclampsia. To study the effects of prolactin on blood pressure (BP), we generated male mice with a single-copy transgene (Tg; inserted into the hypoxanthine-guanine phosphoribosyltransferase locus) that enables inducible hepatic production of prolactin and its cleavage product. The Tg is driven by the indole-3-carbinol (I3C)-inducible rat cytochrome P450 1A1 promoter. When the Tg mice were fed normal chow (NC), plasma prolactin concentrations were comparable to those in female WT mice in the last third of pregnancy, and BP was lower than in WT mice (∼95 mm Hg vs. ∼105 mm Hg). When the Tg mice were fed chow containing IC3, plasma prolactin concentrations increased threefold, BP increased to ∼130 mm Hg, and cardiac function became markedly impaired. IC3 chow did not affect the WT mice. Urinary excretion of nitrite/nitrate and the amount of Ser1177-phosphorylated endothelial nitric oxide (NO) synthase (eNOS) were significantly greater in the Tg mice fed NC than in WT mice, as they are during pregnancy. However, when I3C was fed, these indicators of NO production became significantly less in the Tg mice than in WT mice. The effects of increased plasma prolactin were abolished by a genetic absence of eNOS. Thus, a threefold increase in plasma prolactin is sufficient to increase BP significantly and to markedly impair cardiac function, with effects mediated by NO produced by eNOS. We suggest that pregnant women with abnormally high prolactin levels may need special attention.

Staphylococcus aureus inhibits nuclear factor kappa B activation mediated by prolactin in bovine mammary epithelial cells

The hormone prolactin (PRL) regulates differentiation and lactation in the bovine mammary epithelium. This tissue is especially prone to contracting mastitis, a disease characterized by an inflammatory response in the mammary gland. Staphylococcus aureus is the infectious agent primarily responsible for mastitis. In a previous study, we have shown that bovine PRL (bPRL) stimulates S. aureus internalization in bovine mammary epithelial cells (bMECs) by regulating several host innate immune elements, which are often modulated by nuclear factor kappa B (NF-κB). However, it is unknown whether the activation of the NF-κB transcription factor is regulated by bPRL during S. aureus internalization. The objective of this study was to determine the role of NF-κB in bPRL-stimulated bMECs during S. aureus internalization. Our results showed that bPRL (5 ng/ml) induced NF-κB activation in bMECs; however, it was inhibited by S. aureus in presence of the hormone. When we blocked NF-κB activation with acetylsalicylic acid, we detected an inhibition in S. aureus internalization (48%) in bPRL-stimulated bMECs. The infection-induced inhibition of NF-κB activation in the presence of bPRL correlates with the downregulation in bPRL-mediated tumor necrosis factor (TNF)-α (27%) and tracheal antimicrobial peptide (TAP, 70%) mRNA expression and nitric oxide (NO) production in bMECs. We also detected an inhibition in the expression of the bPRL target gene κ-casein (50%) under these conditions. Interestingly, these effects are not achieved through increased PRL receptor expression (PRLR), as it was inhibited (48%) compared to control cells. In conclusion, NF-κB activation in bMECs is inhibited by S. aureus in the presence of bPRL, suggesting a mechanism by which the host innate immune response may be compromised during subclinical mastitis.

Mesenteric nitric oxide and superoxide production in experimental necrotizing enterocolitis

BACKGROUND

A proposed mechanism of intestinal injury in necrotizing enterocolitis (NEC) involves vascular dysfunction through altered nitric oxide synthase (NOS) activity. We hypothesize that this dysfunction results in an imbalance in nitric oxide (*NO) and superoxide (O(2)(*-)) production by the intestinal vascular endothelium, which contributes to the intestinal injury seen in NEC.

MATERIALS AND METHODS

Neonatal rat pups were divided into two groups. Control pups were breast fed and housed with their mother. Experimental NEC pups were housed separately and either exposed to formula feeding and 5% to 10% hypoxia alone (FF/H) or with the addition of lipopolysaccharide (FF/H/LPS). Mesenteries from each group were analyzed for *NO and O(2)(*-) production with and without NOS inhibition by N(G)-monomethyl-L-arginine (L-NMMA). Western blot analysis for eNOS, phosphorylated eNOS (phospho-eNOS), and inducible NOS (iNOS) was performed, and each terminal ileum was graded for intestinal injury by histology.

RESULTS

Histology revealed mild intestinal injury (grade 1-2 on a 4-point scale) in the FF/H group and severe injury (grade 3-4) in the FF/H/LPS group. The FF/H cohort had significantly increased *NO and lower O(2)(*-) production, while the FF/H/LPS group shifted to significantly decreased *NO and increased O(2)(*-) production. L-NMMA inhibited >50% of O(2)(*-) production in all three groups but only inhibited *NO production in control and FF/H pups. Western blot analysis revealed increased levels of phospho-eNOS in FF/H pups and increased iNOS in FF/H/LPS pups.

CONCLUSIONS

This study demonstrates in the progression of NEC, intestinal ischemia is associated with a shift from *NO to O(2)(*-) production, which is NOS-dependent. Potentially greater injury results from impaired vasodilatation and over-production of reactive oxygen species.

Sex hormones induce direct epithelial and inflammation-mediated oxidative/nitrosative stress that favors prostatic carcinogenesis in the noble rat

Oxidative and nitrosative stress have been implicated in prostate carcinogenesis, but the cause(s) of redox imbalance in the gland remains poorly defined. We and others have reported that administration of testosterone plus 17beta-estradiol to Noble rats for 16 weeks induces dysplasia and stromal inflammation of the lateral prostate (LP) but not the ventral prostate. Here, using laser capture microdissected specimens, we found that the combined hormone regimen increased the expression of mRNA of specific members of NAD(P)H oxidase (NOX-1, NOX-2, and NOX4), nitric-oxide synthase [NOS; inducible NOS and endothelial NOS], and cyclooxygenase (COX-2) in the LP epithelium and/or its adjacent inflammatory stroma. Accompanying these changes was the accumulation of 8-hydroxy-2'-deoxyguanosine, 4-hydroxynonenal protein adducts, and nitrotyrosine, primarily in the LP epithelium, suggesting that NOX, NOS, and COX may mediate hormone-induced oxidative/nitrosative stress in epithelium. We concluded that the oxidative/nitrosative damage resulting from the testosterone-plus-17beta-estradiol treatment is not solely derived from stromal inflammatory lesions but likely also originates from the epithelium per se. In this context, the up-regulation of COX-2 from epithelium represents a potential mechanism by which the hormone-initiated epithelium might induce inflammatory responses. Thus, we link alterations in the hormonal milieu with oxidative/nitrosative/inflammatory damage to the prostate epithelium that promotes carcinogenesis.

Prolactin stimulates the proliferation of normal female cholangiocytes by differential regulation of Ca2+-dependent PKC isoforms

Background

Prolactin promotes proliferation of several cells. Prolactin receptor exists as two isoforms: long and short, which activate different transduction pathways including the Ca²⁺-dependent PKC-signaling. No information exists on the role of prolactin in the regulation of the growth of female cholangiocytes. The rationale for using cholangiocytes from female rats is based on the fact that women are preferentially affected by specific cholangiopathies including primary biliary cirrhosis. We propose to evaluate the role and mechanisms of action by which prolactin regulates the growth of female cholangiocytes.

Results

Normal cholangiocytes express both isoforms (long and short) of prolactin receptors, whose expression increased following BDL. The administration of prolactin to normal female rats increased cholangiocyte proliferation. In purified normal female cholangiocytes, prolactin stimulated cholangiocyte proliferation, which was associated with increased [Ca²⁺]_i levels and PKCβ-I phosphorylation but decreased PKCα phosphorylation. Administration of an anti-prolactin antibody to BDL female rats decreased cholangiocyte proliferation. Normal female cholangiocytes express and secrete prolactin, which was increased in BDL rats. The data show that prolactin stimulates normal cholangiocyte growth by an autocrine mechanism involving phosphorylation of PKCβ-I and dephosphorylation of PKCα.

Conclusion

We suggest that in female rats: (i) prolactin has a trophic effect on the growth of normal cholangiocytes by phosphorylation of PKCβ-I and dephosphorylation of PKCα; and (iii) cholangiocytes express and secrete prolactin, which by an autocrine mechanism participate in regulation of cholangiocyte proliferation. Prolactin may be an important therapeutic approach for the management of cholangiopathies affecting female patients.

Effect of Stage of Lactation and Parity on Mammary Gland Cell Renewal

Milk production is a function of the number and activity of mammary epithelial cells, regardless of stage of lactation. Milk yield is generally higher in multiparous cows than in primiparous cows, but persistency is usually greater in the latter group. We compared several measures related to metabolic activity, apoptosis, and endocrine control of mammary cell growth in 8 primiparous and 9 multiparous cows throughout lactation. Mammary gland biopsies were taken in early [10 d in milk (DIM)], peak (50 DIM), and late (250 DIM) lactation to evaluate gene expression and determine DNA and fatty acid synthase (FAS) content. Milk samples taken the day before the biopsies were used to detect protease activities and to determine stanniocalcin-1 (STC) concentrations. Blood samples served to measure insulin-like growth factor-1, prolactin, and STC concentrations. Milk yield was higher in multiparous cows than in primiparous cows at the 10 DIM (32.8 +/- 1.3 and 25.2 +/- 0.8 kg/d) and 50 DIM (38.0 +/- 1.2 and 29.8 +/- 1.1 kg/d), but it was the same for both groups at 250 DIM (23.9 +/- 1.5 and 23.8 +/- 1.1 kg/d). Except for stearoyl-coenzyme A desaturase, expression of genes related to milk synthesis was not affected by stage of lactation. However, gene expression of acetyl-coenzyme A carboxylase, beta-casein, and FAS was lower in early lactation in primiparous cows. Expression of both proapoptotic bax and antiapoptotic bcl-2 genes was higher in primiparous cows, whereas the bax-to-bcl-2 ratio was not changed. Mammary DNA concentration was higher in multiparous cows, as was the amount of FAS protein in early lactation. Two bands of protease activity were found in milk samples, and one of the bands had an apparent molecular weight similar to gelatinase A and was dependent on the stage of lactation. Serum insulin-like growth factor-1 increased with day of lactation and was higher in primiparous cows. Serum prolactin decreased in late lactation, but peak values were observed in early lactation for primiparous cows and peak lactation for multiparous cows. Milk STC content increased with advancing lactation. The results are consistent with a lower degree of differentiation and a greater capacity for cell renewal in the mammary gland of primiparous cows.

Rapid eye movement sleep is reduced in prolactin-deficient mice

Prolactin (PRL) is implicated in the modulation of spontaneous rapid eye movement sleep (REMS). Previous models of hypoprolactinemic animals were characterized by changes in REMS, although associated deficits made it difficult to ascribe changes in REMS to reduced PRL. In the current studies, male PRL knock-out (KO) mice were used; these mice lack functional PRL but have no known additional deficits. Spontaneous REMS was reduced in the PRL KO mice compared with wild-type or heterozygous littermates. Infusion of PRL for 11-12 d into PRL KO mice restored their REMS to that occurring in wild-type or heterozygous controls. Six hours of sleep deprivation induced a non-REMS and a REMS rebound in both PRL KO mice and heterozygous littermates, although the REMS rebound in the KOs was substantially less. Vasoactive intestinal peptide (VIP) induced REMS responses in heterozygous mice but not in KO mice. Similarly, an ether stressor failed to enhance REMS in the PRL KOs but did in heterozygous littermates. Finally, hypothalamic mRNA levels for PRL, VIP, neural nitric oxide synthase (NOS), inducible NOS, and the interferon type I receptor were similar in KO and heterozygous mice. In contrast, tyrosine hydroxylase mRNA was lower in the PRL KO mice than in heterozygous controls and was restored to control values by infusion of PRL, suggesting a functioning short-loop negative feedback regulation in PRL KO mice. Data support the notion that PRL is involved in REMS regulation.

The compartmentalization of prolactin signaling in the mouse mammary gland

In mammary epithelial cells, prolactin (PRL) activates at least two signaling pathways: Jak/Stat and nitric oxide (NO). The former induces differentiation as measured by alpha-lactalbumin accumulation, while experiments with sodium nitroprusside (SNP) show that NO inhibits differentiation. In order to resolve this apparent contradiction, the kinetics, inducibility, and cellular localization of NO production and sensitivity in mammary cells were examined. First, mammary cells remained responsive to PRL throughout the incubation with respect to NO production. Second, although desensitization occurred with continuous PRL exposure, recovery began as quickly as 30 min after PRL withdrawal. Since PRL is secreted in pulses in vivo, complete desensitization was not a likely explanation for the cells' escape from NO inhibition. Finally, the cellular site of transduction was examined using the caveolar disrupting agent, methyl-beta-cyclodextrin (MBCD). MBCD inhibited the accumulation of PRL-induced NO but not alpha-lactalbumin. This finding was confirmed by membrane fractionation studies where the PRL-induced NO production occurred primarily in caveolae and PRL-stimulated tyrosine phosphorylation of Stat5, which transcribes the alpha-lactalbumin gene, occurred predominantly in noncaveolar membranes. Finally, endogenous elevations of NO by arginine did not inhibit differentiation. As such, the inhibition seen with SNP appeared to be an artifact of the ubiquitous generation of NO from SNP. Physiologically, PRL induces NO only in caveolae and this restricted distribution does not interfere with differentiation.

Brainstem prolactin mRNA is enhanced in mice with suppressed neuronal nitric oxide synthase activity

Prolactin (PRL) and vasoactive intestinal polypeptide (VIP) mRNA levels were elevated in the brainstem of neuronal nitric oxide synthase (nNOS) gene knockout (KO) mice compared to the levels in nNOS control mice. In addition, PRL mRNA levels increased in the hypothalamus and the brainstem of nNOS control mice after administration of 7-nitro-indazole (7-NI), a relatively selective nNOS inhibitor. The results suggest that NO inhibits PRL. No differences in the genes measured were observed in inducible NOS KO mice.