Aromatase (CYP19) is a cytochrome P450 enzyme that catalyzes the formation of aromatic C18 estrogens from C19 androgens. It is expressed in various tissues and contributes to sex-specific differences in cellular metabolism. We have generated aromatase-knockout (ArKO) mice in order to study the role of estrogen in the regulation of glucose metabolism. The mean body weights of male ArKO (-/-) mice (n=7) and wild-type littermates (+/+) (n=7) at 10 and 12 weeks of age were 26.7+/-1.9 g vs 26.1+/-0.8 g and 28.8+/-1.4 g vs 26.9+/-1.0 g respectively. The body weights of the ArKO and wild-type mice diverged between 10 and 12 weeks of age with the ArKO males weighing significantly more than their wild-type littermates (P<0.05). The ArKO males showed significantly higher blood glucose levels during an intraperitoneal glucose tolerance test compared with wild-type littermates beginning at 18 weeks of age. By 24 weeks of age, they had higher fasting blood glucose levels compared with wild-type littermates (133.8+/-22.8 mg/dl vs 87.8+/-20.3 mg/dl respectively; P<0.01). An intraperitoneal injection of insulin (0.75 mU insulin/g) caused a continuous decline in blood glucose levels in wild-type mice whereas ArKO males at 18 weeks and older exhibited a rebound increase in glucose levels 30 min after insulin injection. Thus, ArKO male mice appear to develop glucose intolerance and insulin resistance in an age-dependent manner. There was no difference in fasting serum triglyceride and total cholesterol levels between ArKO male mice and wild-type littermates at 13 and 25 weeks of age. However, serum triglyceride and cholesterol levels were significantly elevated following a meal in ArKO mice at 36 weeks of age. Serum testosterone levels in ArKO male mice were continuously higher compared with wild-type littermates. Treatment of ArKO males with 17beta-estradiol improved the glucose response as measured by intraperitoneal glucose and insulin tolerance tests. Treatment with fibrates and thiazolidinediones also led to an improvement in insulin resistance and reduced androgen levels. As complete aromatase deficiency in man is associated with insulin resistance, obesity and hyperlipidemia, the ArKO mouse may be a useful animal model for examining the role of estrogens in the control of glucose and lipid homeostasis.

Aromatase P450 (CYP19) is an enzyme responsible for the conversion of androgens to oestrogens. We generated CYP19 knockout (ArKO) mice by targeted disruption of Cyp19 and studied the role of oestrogens in male reproductive ability. Approximately 85% of ArKO males were unable to sire offspring. However, no obvious difference was found in testicular and epididymal weights, numbers of sperm in the epididymis or the ability of sperm to fertilize eggs in vitro between wild-type and ArKO males. An examination of mating behaviour demonstrated that ArKO males showed an impairment in mounting behaviour against sexually mature females. The inability of more than 90% of ArKO males to sire offspring was reversed by repeated subcutaneous injections of 17beta-oestradiol when initiated on the day of birth. The effects of 17beta-oestradiol on reproduction were concentration dependent and evident when supplementation was initiated on day 7, but not on day 15 after birth. These findings suggest that oestrogens acting during neonatal life are required for normal mating behaviour in adulthood.

Cytokines and steroid hormones use different sets of signal transduction pathways, which seem to be unrelated. Interleukin-6 (IL-6) uses JAK tyrosine kinase and STAT (signal transducer and activator of transcription) transcription factor. Glucocorticoid binds glucocorticoid receptor (GR), which is a member of the steroid receptor superfamily. We have studied the crosstalk between the IL-6-JAK-STAT and glucocorticoid-nuclear receptor pathways. IL-6 and glucocorticoid synergistically activated the IL-6 response element on the rat alpha2-macroglobulin promoter (APRE)-driven luciferase gene. The exogenous expression of GR enhanced the synergism. The exogenous expression of dominant negative STAT3 completely abolished the IL-6 plus glucocorticoid-induced activation of the APRE-luciferase gene. Tyrosine phosphorylation of STAT3 stimulated by IL-6 alone was not different from that by IL-6 plus glucocorticoid. The protein level of STAT3 was also not increased by glucocorticoid stimulation. The time course of STAT3 tyrosine phosphorylation by IL-6 plus glucocorticoid was not different from that by IL-6 alone. The synergism was studied on the two other IL-6 response elements, the junB promoter (JRE-IL-6) and the interferon regulatory factor-1 (IRF-1) promoter (IRF-GAS) which could be activated by STAT3. The synergistic activation by glucocorticoid on the IL-6-activated JRE-IL-6 and the IRF-GAS-driven luciferase gene was not detected. Glucocorticoid did not change the mobility of IL-6-induced APRE-binding proteins in a gel shift assay. These results suggest that the synergism was through the GR and STAT3, and the coactivation pathway which was specific for APRE was the target of glucocorticoid.

Thyroid hormone receptors (TR) are members of the nuclear receptor superfamily. There are at least two TR isoforms, TRalpha and TRbeta, which act as mediators of thyroid hormone in tissues. However, the relative expression of each TR isoform in target tissues is still elusive. Herein, we have developed an RT-PCR and restriction enzyme digestion method to determine the expression of TRalpha1 and TRbeta1. We analyzed the expression of TR isoforms in 3T3-L1 preadipocytes induced to differentiate by an adipogenic cocktail in the presence or absence of 100 nM triiodothyronine (T(3)). The TRalpha1 isoform was predominantly expressed in 3T3-L1 adipocytes, and its expression was increased at the stage of development concomitant with the emergence of lipid droplets. Little, if any, TRbeta1 mRNA was detected in adipocytes. Administration of T(3) to the differentiating 3T3-L1 cells enhanced the accumulation of triglyceride. The expression profile of TRalpha1 in T(3)-treated adipocytes was similar to that in non-treated cells. The transcripts of adipogenic factors, CCAAT/enhancer binding protein beta (C/EBPbeta) and peroxisome proliferator activated receptor gamma (PPARgamma), were not altered by T(3). Lipid binding protein, aP2, that is downstream of these transcription factors was also unaffected by T(3). In contrast, the lipogenic enzyme, glyceraldehyde-3-phosphate dehydrogenase mRNA was significantly increased in the presence of T(3). Therefore, T(3) appears to be a hormone capable of modulating the expression of lipogenic enzyme and augments the accumulation of lipid droplets. We conclude that the TRalpha isoform might play an important role in the generation and maintenance of the mature adipocyte phenotype, regulating the expression of lipogenic enzymes.

Breast milk has non-nutritional protective effects on recipient infants. It has been speculated that bioactive substances present in human milk have important roles in protecting infants. However, the mechanisms by which such substances protect newborns are unclear. Therefore, we analyzed the growth-promoting activity of human milk and the intracellular signaling mechanism thereof using human fetal small intestinal (FHS 74 Int) cells. Epidermal growth factor (EGF) stimulated the proliferation of these cells. However, this stimulation was less effective than that of aqueous milk (5% vol/vol). The bioactivity of human milk was heat stable but protease sensitive. EGF receptor tyrosine kinase inhibitor did not repress the milk-induced growth-promoting effect on fetal small intestinal cells. Regarding the intracellular signaling pathway, the milk-induced cell proliferation pathway was tyrosine kinase dependent but was neither mitogen-activated protein (MAP) kinase nor phosphatidylinositol-3 (PI-3) kinase dependent. On the other hand, EGF-induced cell proliferation was tyrosine kinase, MAP kinase, and PI-3 kinase dependent. Rapid tyrosine phosphorylation of several intracellular proteins was detected after milk stimulation. Furthermore, the time course of phosphorylation induced by milk was different from that induced by EGF. The sizes of the proteins phosphorylated in response to milk were different from those of the Shc proteins phosphorylated in response to EGF. These results suggest that human milk induces fetal intestinal cell proliferation through a unique tyrosine kinase pathway different from the EGF receptor signaling pathway.

ABSTRACT

Changes in the amount of cytosolic 3,5,3′-tri-iodo-l-thyronine (T₃)-binding protein (CTBP) and its activator during administration of l-thyroxine (T₄) to thyroidectomized rats were investigated. Thyroidectomy decreased the amount of CTBP in the kidney, whereas the activator was not significantly modified by thyroidectomy. The activator was increased by administration of T₄ to thyroidectomized rats. The amount of CTBP was also increased by administration of T₄. The activator increased the maximal binding capacity (MBC) without changes in the affinity constant for T₃ binding in CTBP. A T₄-induced increase in MBC in cytosol inhibited nuclear T₃ binding in vitro by competition of T₃ binding between CTBP and the nuclear receptor.

These results suggest that thyroid hormone increases the capacity for cytosolic T₃ binding through increasing the amount of CTBP and its activator, and that these increases play a role in regulating the amount of T₃ that binds to its nuclear receptor.

Journal of Endocrinology (1989) 123, 99–104

Abstract

In order to study whether peripheral action of thyroid hormones is altered in insulin deficiency and to elucidate the biological consequences of alteration of the cytosolic 3,5,3′-tri-iodo-l-thyronine (T₃) binding protein (CTBP), we measured malic enzyme, T₃-responsive nuclear n protein, CTBP and nuclear thyroid hormone receptor in the liver and kidney of streptozotocin (STZ)-induced diabetic rats that were treated with or without insulin and/or a receptor-saturating dose of T₃. The following results were obtained. 1. Induction of malic enzyme by T₃ was apparently diminished in diabetic rats. However, supplementary injection of insulin enabled previously given T₃ to take effect in diabetic rats. 2. T₃-responsiveness of other hepatic proteins (n protein and CTBP) was not altered by insulin in diabetic rats. 3. The level of n protein was increased by insulin in diabetic rats in vivo and in perfused rat liver, indicating that the hepatic n protein is a novel insulin-responsive protein. T₃ and insulin increased the level of n protein non-synergistically in diabetic rat liver. 4. Hepatic nuclear receptor levels were not altered in diabetic rats. 5. Hepatic CTBP levels were decreased in diabetic rats. This was not due to the toxic effect of STZ. Low CTBP level was only partially increased by insulin after 30 days of diabetic period. Renal CTBP levels were not altered in diabetic rats with or without insulin treatment. These results indicate that reduction of CTBP did not influence the hepatic response to a receptor-saturating dose of T₃, although CTBP may regulate the nuclear T₃ transport, and that fundamental action of a receptor-saturating dose of T₃ was not attenuated in diabetic rat liver.

Journal of Endocrinology (1994) 143, 55–63

Aromatase P450 (CYP19) is an enzyme catalysing the conversion of androgens into oestrogens. We generated mice lacking aromatase activity (ArKO) by targeted disruption of Cyp19 and report the characteristic features of the ArKO ovaries and uteri as revealed by histological and biochemical analyses. ArKO females were totally infertile but there were as many developing follicles in their ovaries at 8 weeks of age as in wild-type ovaries. Nevertheless, no typical corpus luteum was observed in the ArKO ovaries. Electron microscopy revealed the presence of well-developed smooth endoplasmic reticulum, few lipid droplets and mitochondria with less organized tubular structures in the ArKO luteinized interstitial cells. These ultrastructural features were different from those of the wild-type interstitial cells, where there are many lipid droplets and mitochondria with well-developed tubular structures, characteristic of steroid-producing cells. When ArKO mice were supplemented with 17beta-oestradiol (E(2); 15 microg/mouse) every fourth day from 4 weeks of age for 1 month, increased numbers of follicles were observed in the ovaries as compared with those of untreated ArKO mice, although no typical corpus luteum was detectable. Ultrastructural analysis revealed the disappearance of the accumulated smooth endoplasmic reticulum in the luteinized interstitial cells after E(2 )supplementation. Transcripts of pro-apoptotic genes such as p53 and Bax genes were markedly elevated in the ArKO ovaries as compared with those of wild-type mice. Although E(2) supplementation did not cause suppression of the elevated expression of p53 and Bax mRNAs, it caused marked enhancement of expression levels of lactoferrin and progesterone receptor mRNAs in the uteri as well as increases in uterine wet weight. At 8 months of age, ArKO mice developed haemorrhages in the ovaries, in which follicles were nearly depleted, while age-matched wild-type females still had many ovarian follicles. Furthermore, macrophage-like cells were occasionally observed in the ArKO ovarian follicles. These results suggested that targeted disruption of Cyp19 caused anovulation and precocious depletion of ovarian follicles. Additionally, analysis of mice supplemented with E(2) demonstrated that E(2) apparently supports development of ovarian follicles, although it did not restore the defect in ovulation.

The thyromimetic compound SK&F L-94901 shows more potent thyromimetic activity in the liver than in the pituitary gland or heart when administered to rats. The mechanisms of liver-selectivity of SK&F L-94901 were examined using cultured rat hepatoma cells (dRLH-84) and rat pituitary tumor cells (GH3), both of which showed saturable cellular uptake of tri-iodothyronine (T(3)). When isolated nuclei with partial disruption of the outer nuclear membrane were used, SK L-94901 competed for [(125)I]T(3) binding to nuclear receptors almost equally in dRLH-84 and GH3 cells. SK L-94901 also did not discriminate thyroid hormone receptors (TR) alpha1 and beta1 in terms of binding affinity and activation of the thyroid hormone responsive element. In intact cells, however, SK L-94901 was a more potent inhibitor of nuclear [(125)I]T(3) binding in dRLH-84 cells than in GH3 cells at an early phase of the nuclear uptake process and after binding equilibrium. These data suggest that SK L-94901 is more effectively transported to nuclear TRs in hepatic cells than in pituitary cells and therefore shows liver-selective thyromimetic activity. In conclusion, SK L-94901 discriminates hepatic cells and pituitary cells at the nuclear transport process. The cellular transporters responsible for this discrimination were not evident.