Effects of an acute bacterial infection on serum thyroid hormones and nuclear triiodothyronine receptors in mice

Beyond low plasma T3: local thyroid hormone metabolism during inflammation and infection

Decreased serum thyroid hormone concentrations in severely ill patients were first reported in the 1970s, but the functional meaning of the observed changes in thyroid hormone levels, together known as nonthyroidal illness syndrome (NTIS), remains enigmatic. Although the common view was that NTIS results in overall down-regulation of metabolism in order to save energy, recent work has shown a more complex picture. NTIS comprises marked variation in transcriptional and translational activity of genes involved in thyroid hormone metabolism, ranging from inhibition to activation, dependent on the organ or tissue studied. Illness-induced changes in each of these organs appear to be very different during acute or chronic inflammation, adding an additional level of complexity. Organ- and timing-specific changes in the activity of thyroid hormone deiodinating enzymes (deiodinase types 1, 2, and 3) highlight deiodinases as proactive players in the response to illness, whereas the granulocyte is a novel and potentially important cell type involved in NTIS during bacterial infection. Although acute NTIS can be seen as an adaptive response to support the immune response, NTIS may turn disadvantageous when critical illness enters a chronic phase necessitating prolonged life support. For instance, changes in thyroid hormone metabolism in muscle during critical illness may be relevant for the pathogenesis of myopathy associated with prolonged ventilator dependence. This review focuses on NTIS as a timing-related and organ-specific response to illness, occurring independently from the decrease in serum thyroid hormone levels and potentially relevant for disease progression.

Thyroid hormone (3,5,3'-triido-L-thyronine) masking/inversion of stimulatory effect of androgen on expression of mk1, a true tissue kallikrein, in the mouse submandibular gland

We studied hormonal regulation of the expression of mkl, a true tissue kallikrein, in the submandibular gland (SMG) of ICR, C3H/ HeN, and F1 (mice from male C3H/HeN x female ICR and in the ones from male ICR x female C3H/HeN). In these mouse strains, mk1 was low in content in males, abundant in females, and increased remarkably by castration of males. In the case of ICR and both F1 mice, injection of 5alpha-dihydrotestosterone (DHT) reduced the mkl level of castrated and female mice. However, the mkl content in female C3H/ HeN mice (or castrated C3H/HeN) was further increased by DHT. To investigate the real action of DHT on mk1 expression, we examined the effects of adrenoectomy/glucocorticoid (dexamethasone, Dex) administration; DHT administration into castrated and adrenoectomized mice; ovariectomy/female hormone (17beta-estradiol, progesterone) administration; and hypophysectomy/combinatory administration of DHT, Dex, and thyroid hormone (3,5,3'-triiodo-L-thyronine, T3) on the mk1 expression in the SMG of ICR mice. Adrenoectomy or ovariectomy did not change the characteristic pattern of mk1 expression in male and female ICR mice. In hypophysectomized (Hypox) ICR male mice, the mk1 content was increased to the same level as in normal ICR females, and DHT administration into the Hypox mice further increased the mk1 level. However, combinatory administration of DHT + T3 or of DHT + T3 + Dex into the Hypox mice lowered the mkl content to the level of normal ICR males, whereas T3 single administration had no effect. Dex single administration into the Hypox mice increased the mkl level to an even higher than that observed with DHT administration. The mk1 level in Hypox mice was not significantly changed by coadministration of Dex with T3. From these results, we conclude that 1) mk1 expression is fundamentally stimulated by androgen (DHT) as are other mk isozymes, such as mk9, mk13, mk22, and mk26 in the mouse SMG, 2) the effect (stimulatory) of DHT on mk1 expression becomes, however, inverted (inhibitory) in the presence of T3. Although the serum T3 level of C3H/HeN female (0.52 ng/ml) was not significantly different from that of C3H/HeN males or ICR mice, coadministration of T3 into C3H/HeN females with a fixed amount of DHT (20 mg/kg body weight) dose dependently repressed the DHT-induced increase in mkl expression, suggesting the lower sensitivity of C3H/HeN females to T3.

Flavin metabolism during respiratory infection in mice

Previous control studies carried out in children showed that respiratory infection alters riboflavin metabolism and leads to excessive urinary losses of the vitamin. In order to understand the nature of biochemical changes in riboflavin metabolism during respiratory infection, a study was carried out using the mouse as the experimental model, and Klebsiella pneumoniae as the infective organism. Mice were fed on either a low (0.5 mg/kg)- or high (13.3 mg/kg)-riboflavin semi-synthetic diet. Infection resulted in a 5-6-fold higher excretion of riboflavin in the urine of mice fed on the low-riboflavin diet. Higher erythrocyte FAD levels and lower liver FAD levels were also observed during infection. Of the four enzymes involved in the synthesis and breakdown of the flavin coenzymes studied, the activity of hepatic flavokinase (ATP: riboflavin 5'-phosphotransferase; EC 2.7.1.26) was significantly lower, and that of FAD synthetase (ATP: FMN adenylyltransferase; EC 2.7.7.2) was higher during riboflavin restriction and infection. The activity of FMN (acid) phosphatase (EC 3.1.3.2) was unchanged, whereas FAD (nucleotide) pyrophosphatase (EC 3.6.1.9) activity was significantly higher both with the low-riboflavin diet and during infection. Thyroid hormone is known to modulate flavokinase activity and, hence, thyroid status was assessed. Plasma triiodothyronine (T3) levels were not affected, but thyroxine levels were lower in the mice fed on the low-riboflavin diet. However, plasma T3 was significantly lower during infection, suggesting a mechanistic role for the hormone in the reduction of flavokinase activity.

Changes in thyroid hormones and thyroxine glucuronidation in hamsters compared with rats following treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin

In rats exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds, serum thyroxine (T4) is depressed. Since hamsters are relatively insensitive to TCDD-induced lethality, the effects of TCDD on several parameters of thyroid status were measured in hamsters as a comparison with the more sensitive rat. At 7 days after ip injection of TCDD, there was a dose-dependent increase in serum 3,5,3'-triiodothyronine (T3) and T4 in hamsters to a maximum level 200% of control; the ED50 was approximately 10 micrograms/kg. Hamsters receiving 100 micrograms/kg lost up to 4% of their body weight but began to recover after about 3 weeks. Serum T4 in these animals was elevated compared to pair-fed and ad libitum controls throughout the 53-day experiment, although it also began to recover after Day 21. This was in direct contrast to the marked reduction of T4 in rats exposed to lower doses of TCDD. T3 was significantly higher in TCDD-treated hamsters than in pair-fed controls on Days 2-7, and TSH was also elevated on Days 2-21. Reverse T3, like T4, was increased by TCDD in hamsters whereas it was decreased in rats. Hepatic microsomal UDP-glucuronosyltransferase (GT) activity was measured using T4 as substrate (T4-GT). On a whole liver basis, T4-GT was induced by TCDD by the same proportion in both rats and hamsters (170-180% of controls) although absolute activities in rats were 3- to 4-fold higher than in hamsters. This similarity in T4-GT inducibility by TCDD suggests that there are likely mechanisms in addition to T4-GT induction which account for the species-specific alterations in T4. Thus, while the response of thyroid hormones to TCDD differed qualitatively, effective doses in hamsters were higher than in rats, suggesting that these changes, although secondary, may correlate more directly with toxicity than does enzyme induction (whose ED50s are similar in both species). An understanding of the mechanism of this species difference may be helpful in unravelling the primary mechanisms of TCDD toxicity.