Mammary type I deiodinase is dependent on the suckling stimulus: differential role of norepinephrine and prolactin

Effect of hypothyroidism on the expression of nuclear receptors and their co-regulators in mammary gland during lactation in the rat

Thyroid hormones (TH) regulate mammary function. Hypothyroidism (HypoT) has deleterious effects on lactation, litter growth and survival. We analyzed the effect of chronic 6-propyl-2-thiouracil (PTU)-induced HypoT in the expression of nuclear receptors, co-regulators and oxytocin receptor (OTR) on lactation (L) days 2, 7 and 14. TH receptors (TRs) were increased on L7 at mRNA and protein levels, except TRα protein, that fell on L14. HypoT decreased TRα2 mRNA on L7 and TRα1 protein on L2, while TRβ1 protein increased on L14. HypoT increased estrogen receptor β (ERβ) mRNA on L7 but decreased its protein levels on L14. Progesterone receptor A (PRA) mRNA decreased from L2 to L14 while PRB increased, and at protein levels PRA levels showed a nadir on L7, while PRB peaked. HypoT decreased PRA mRNA and protein and increased PRB mRNA at L14. Nuclear receptor co-activator (NCOA) 1 and RXRα mRNA showed an opposite pattern to the TRs, while NCOA2 increased at L14; HypoT blocked the variations in NCOA1 and NCOA2. HypoT increased NCOR1 on L2 and decreased OTR at L2 and circulating estradiol and NCOR2 at L14. In controls the most notable changes occurred on L7, suggesting it is a key inflection point in mammary metabolism. The low levels of TRα1, NCOA1 and OTR, and increased NCOR1 produced by HypoT on L2 may hinder the mammary ability to achieve normal milk synthesis and ejection, leading to defective lactation. Later on, altered ER and PR expression may impair further mammary function.

Type 1 deiodinase activity and generation of triiodothyronine (T3) in prostate of sexually active rats

BACKGROUND

Thyroxine (T(4)) and triiodothyronine (T(3)) are involved in the development and function of the male reproductive system. The type 1 deiodinase enzyme (D1) plays a major role in the intracellular conversion of T(4) to the active form, T(3). D1 is expressed in the prostate of pubescent rats, but it is unknown whether locally generated T(3) is involved in the development and function of this gland.

METHODS

D1 activity was analyzed in prostates from neonatal to old rats. Local T3 generation (D1 and T3 levels) was evaluated in adult animals with 1-5 months of continuous sexual activity. D1 activity was measured by the radiolabeled-iodide-release method and T(3) concentration by radioimmunoassay. Secretory activity of the prostate was evaluated by a morphological analysis of epithelium (hematoxilin-eosin stain) and by measuring the activity of acid phosphatase as a marker enzyme for secretion.

RESULTS

The highest prostate D1 activity was expressed around puberty, and it was almost undetectable during the neonatal period and with aging. Interestingly, 1 and 4 months of sexual activity avoided the decrease of D1 activity associated with aging. Sexual activity provoked a hypertrophy and functional hyperplasia in all lobes, but D1 and acid phosphatase activity increased only in the ventral lobe. D1 activity correlated with an increase in the prostatic T(3) concentration.

CONCLUSIONS

The increased local generation of T(3) in prostate might be related to: (1) the differentiation/maturation that occurs at puberty and (2) the energy expenditure associated with maintaining the secretory activity of the glandular epithelium.

Epididymis expresses the highest 5'-deiodinase activity in the male reproductive system: kinetic characterization, distribution, and hormonal regulation

We characterized the enzymes that catalyze the deiodination of T(4) to T(3) in the male reproductive tract. Testis, epididymis (EPI), seminal vesicles, prostate, bulbourethral glands, spermatozoa, and semen were taken from sexually mature rats (300 g). Iodothyronine 5'-deiodinase (5'-D) activity was quantified by the radiolabeled-iodide-release method. 5'-D activity was 10-fold higher in EPI and semen than in the rest of the tissues. In EPI, semen, and prostate, the enzymatic activity was completely inhibited by 1 mm 6-n-propyl-2-thiouracil, whereas in the other tissues the inhibition was partial (50%). The high susceptibility to 6-n-propyl-2-thiouracil inhibition, a ping-pong kinetic pattern, and low cofactor (Michaelis Menten constant for dithiothreitol=0.7 mm) and high substrate (Michaelis Menten constant for reverse T(3)=0.4 microm) requirements indicate that EPI 5'-D corresponds to type 1 deiodinase (D1). Real-time RT-PCR amplification of D1 mRNA in this tissue confirms this conclusion. The highest EPI D1 expression occurred at the onset of puberty and sexual maturity, and in the adult, this activity was more abundant in corpus and caput than in the caudal region. EPI D1 expression was elevated under conditions of hyperthyroidism and with addition of 17beta-estradiol. Our data also showed a direct association between D1 and a functional epididymis marker, the neutral alpha-glucosidase enzyme, suggesting that local generation of T(3) could be associated with the development and function of EPI and/or spermatozoa maturation. Further studies are necessary to analyze the possible physiological relevance of 5'-D in the male reproductive system.

Selenium, the thyroid, and the endocrine system

Recent identification of new selenocysteine-containing proteins has revealed relationships between the two trace elements selenium (Se) and iodine and the hormone network. Several selenoproteins participate in the protection of thyrocytes from damage by H(2)O(2) produced for thyroid hormone biosynthesis. Iodothyronine deiodinases are selenoproteins contributing to systemic or local thyroid hormone homeostasis. The Se content in endocrine tissues (thyroid, adrenals, pituitary, testes, ovary) is higher than in many other organs. Nutritional Se depletion results in retention, whereas Se repletion is followed by a rapid accumulation of Se in endocrine tissues, reproductive organs, and the brain. Selenoproteins such as thioredoxin reductases constitute the link between the Se metabolism and the regulation of transcription by redox sensitive ligand-modulated nuclear hormone receptors. Hormones and growth factors regulate the expression of selenoproteins and, conversely, Se supply modulates hormone actions. Selenoproteins are involved in bone metabolism as well as functions of the endocrine pancreas and adrenal glands. Furthermore, spermatogenesis depends on adequate Se supply, whereas Se excess may impair ovarian function. Comparative analysis of the genomes of several life forms reveals that higher mammals contain a limited number of identical genes encoding newly detected selenocysteine-containing proteins.

Potential role of the cannabinoid receptor CB1 in rapid eye movement sleep rebound

Sleep is an unavoidable activity of the brain. The delay of the time to sleep (sleep deprivation), induces an increase of slow-wave sleep and rapid-eye-movement (REM) sleep (rebound) once the subject is allowed to sleep. This drive to sleep has been hypothesized to be dependent on the accumulation of sleep-inducing molecules and on the high expression of these molecule receptors. In this study we selectively deprived rats of REM sleep for 24 h by using the flowerpot technique. One group deprived of REM sleep was treated with SR141716A, a cannabinoid receptor 1 (CB1) receptor antagonist and then allowed to sleep for the next 4 h. Two other groups were killed, one immediately after the REM sleep deprivation period and the other after 2 h of REM sleep rebound (REM sleep deprivation plus 2 h of rebound). In both groups we determined the expression of the CB1 receptor and its mRNA. Results indicated that SR141716A prevents REM sleep rebound and REM sleep deprivation does not modify the expression of the CB1 protein or mRNA. However, REM sleep deprivation plus 2 h of sleep rebound increased the CB1 receptor protein and, slightly but significantly, decreased mRNA expression. These results suggest that endocannabinoids may be participating in the expression of REM sleep rebound.

Regulation of type II deiodinase expression by EGF and glucocorticoid in HC11 mouse mammary epithelium

Thyroid hormones are important for mammary gland growth and development. The iodothyronine deiodinases play a key role in thyroid hormone metabolism. We have showed that type II 5'-deiodinase (5'D2) activity and mRNA are present in the mouse mammary gland and that their levels are reduced in the lactating gland. To investigate the regulatory mechanism of mouse 5'D2 gene (mdio2) expression in mammary epithelium, we employed the HC11 cell line, which is derived from mouse mammary epithelial cells and retains the ability to express differentiated function. HC11 cells were treated with combinations of insulin, glucocorticoid (GC, dexamethasone), prolactin, and epidermal growth factor (EGF), and 5'D2 activity and the D2-to-GAPDH mRNA ratio were measured by (125)I(-) release from (125)I-labeled thyroxine and semiquantitative RT-PCR, respectively. EGF increased both 5'D2 activity and mRNA levels about twofold. GC reduced both 5'D2 activity and mRNA in a dose-dependent manner, and their levels were decreased to approximately one-tenth and one-fifth, respectively, of control levels. These data demonstrated that mdio2 expression in HC11 cells is upregulated by EGF mainly at the pretranslational level and downregulated by GC at both pre- and posttranslational levels. Furthermore, we showed that GC reduced the promoter activity of the 627- bp 5'-upstream region of the mdio2/luciferase chimeric reporter gene, suggesting that GC exerts its effect, at least in part, at the transcriptional level.

Suckling-induced activation of neural c-fos expression at lower thoracic rat spinal cord segments

Suckling stimulation is essential for neuroendocrine and sympathetic reflex activation during lactation. In the present study, the induction of c-fos gene expression was used to identify neuronal populations in the spinal cord activated by acute 5 min suckling or by electrical stimulation of the central stump of the first abdominal mammary nerve in lactating rats previously separated from their litters for 6 or 18 h. In addition, to investigate whether spinal sympathetic preganglionic neurons are activated by suckling, dual immunostaining (Fos and choline acetyltransferase) was performed. Fos was expressed at low levels in continuously suckled and 6 h nonsuckled mothers, but no expression was found after 18 h of nonsuckling. On the other hand, in 6 h nonsuckled rats, significant increments in Fos expression occurred in several regions after acute suckling and after electrical stimulation. Also, the pattern of Fos expression in each spinal laminae was different for the two stimuli, i.e. more intense effects of suckling in deep laminae V-X and more intense effects in laminae I-IV with electrical stimulation. Double-labeling after suckling was found only in sympathetic preganglionic neurons from the intermedio-medial cell column, whereas after electrical stimulation, double label was observed only in neurons from the intermedio-lateral cell column. On the other hand, no effect upon Fos protein expression was observed after suckling and only a minor effect after electrical stimulation of mammary nerve in 18 h nonsuckled rats. These results are consistent with previous findings on the sympathetic reflex regulation of the mammary gland, as well as on the importance of the nonsuckling interval for optimal functioning of lactation.

Transneuronal labelling of nerve cells in the CNS of female rat from the mammary gland by viral tracing technique

Using the viral transneuronal tracing technique, the cell groups in the CNS transneuronally connected with the female mammary gland were detected. Lactating and non-lactating female rats were infected with pseudorabies virus injected into the mammary gland. The other group of animals was subjected to virus injection into the skin of the back. Four days after virus injection, infected neurons detected by immunocytochemistry, were present in the dorsal root ganglia ipsilateral to inoculation and in the intermediolateral cell column of the spinal cord. In addition, a few labelled cells could be detected in the dorsal horn and in the central autonomic nucleus (lamina X) of the spinal cord. At this survival time several brain stem nuclei including the A5 noradrenergic cell group, the caudal raphe nuclei (raphe obscurus, raphe pallidus, raphe magnus), the A1/C1 noradrenergic and adrenergic cell group, the nucleus of the solitary tract, the area postrema, the gigantocellular reticular nucleus, and the locus coeruleus contained virus-infected neurons. In some animals, additional cell groups, among others the periaqueductal gray and the red nucleus displayed labelling. In the diencephalon, a significant number of virus-infected neurons could be detected in the hypothalamic paraventricular nucleus. In most cases, virus-labelled neurons were present also in the lateral hypothalamus, in the retrochiasmatic area, and in the anterior hypothalamus. In the telencephalon, in some animals a few virus-infected neurons could be found in the preoptic area, in the bed nucleus of the stria terminalis, in the central amygdala, and in the somatosensory cortex. At the longer (5 days) survival time each cell group mentioned displayed immunopositive neurons, and the number of infected cells increased. The pattern of labelling was similar in animals subjected to virus inoculation into the mammary gland and into the skin. The distribution and density of labelling was similar in lactating and non-lactating rats. The present findings provide the first morphological data on the localization of CNS structures connected with the preganglionic neurons of the sympathetic motor system innervating the mammary gland. It may be assumed that the structures found virus-infected belong to the neuronal circuitry involved in the control of the sympathetic motor innervation of the mammary gland.

Effect of somatotropin on thyroid hormones and cytokines in lactating dairy cows during ad libitum and restricted feed intake

Twelve Holsteins in first lactation were used to investigate the relationship between energy balance and effects of bovine somatotropin (bST) on thyroid hormone metabolism and cytokine concentrations in serum. Six cows were fed for ad libitum intake and six cows were feed restricted to induce negative energy balance during two treatment periods of 6 d. During treatment periods, cows were administered vehicle or 40 mg of bST/d according to a crossover design. Between treatment periods was a 15-d recovery period, during which all cows were fed ad libitum. Cows that were fed ad libitum remained in positive energy balance during control and bST treatments, whereas cows that were fed for restricted intake were in negative energy balance during control and bST treatment periods. In both dietary groups, bST decreased energy balance. Milk production and the fat percentage of milk increased during bST treatment in both dietary groups. Fat-corrected milk yield was increased 13% by bST treatment. Serum concentrations of IGF-I did not differ between dietary groups but were greater during bST than control periods. Serum thyroxine concentration was decreased by bST treatment. Serum triiodothyronine and reverse-triiodothyronine were not altered by hormone treatment, but circulating concentrations of thyroid hormones were apparently reduced by dietary restriction. Neither hepatic nor mammary thyroxine 5'-deiodinase was affected by bST treatment. Plasma concentration of tumor necrosis factor-alpha, a potential regulator of thyroxine 5'-deiodinase, was not affected by bST treatment. Short-term treatment with bST did not influence thyroid hormone metabolism in lactating cows in positive or negative energy balance.