Joan Kwakkel

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.

Both immunisation with a formalin-inactivated respiratory syncytial virus (RSV) vaccine and a mock antigen vaccine induce severe lung pathology and a Th2 cytokine profile in RSV-challenged mice.

Respiratory syncytial virus (RSV) is the most important cause of bronchiolitis and pneumonia in infants and young children. Immunopathology may play a role in RSV-induced disease and a severe RSV infection may also be associated with an increased risk of developing asthma. Vaccination with formalin-inactivated RSV (FI-RSV) prior to infection resulted both in human and in the mouse model in extensive lung pathology. In the mouse model, it has been shown that this aggravation of disease was associated with a shift in the balance between Th1 and Th2 cytokines towards a Th2-type response. The aim of the present study was to characterise the immunological and inflammatory responses in BALB/c mice upon RSV infection with or without prior vaccination with aluminium-adjuvanted FI-RSV or control antigens (FI-Mock). As previously reported by others, we also observed that a primary RSV infection in BALB/c mice resulted in a predominant Th1-type cytokine response, which was associated with slight bronchiolitis and alveolitis. FI-RSV vaccination prior to RSV challenge prevented virus replication and was associated with an aggravation of pulmonary histopathology and a shift towards a Th2-type response. Vaccination with FI-Mock did not prevent RSV replication in the lung but resulted in an even more pronounced Th2 response after infection while these mice were not sensitised to specific viral antigens. Thus, viral replication in a Th2 responding animal (induced by aluminium-adjuvanted mock vaccine) appears to boost the Th2 response upon RSV infection.

Hypothalamic Neuropeptide Y (NPY) Controls Hepatic VLDL-Triglyceride Secretion in Rats via the Sympathetic Nervous System

Excessive secretion of triglyceride-rich very low-density lipoproteins (VLDL-TG) contributes to diabetic dyslipidemia. Earlier studies have indicated a possible role for the hypothalamus and autonomic nervous system in the regulation of VLDL-TG. In the current study, we investigated whether the autonomic nervous system and hypothalamic neuropeptide Y (NPY) release during fasting regulates hepatic VLDL-TG secretion. We report that, in fasted rats, an intact hypothalamic arcuate nucleus and hepatic sympathetic innervation are necessary to maintain VLDL-TG secretion. Furthermore, the hepatic sympathetic innervation is necessary to mediate the stimulatory effect of intracerebroventricular administration of NPY on VLDL-TG secretion. Since the intracerebroventricular administration of NPY increases VLDL-TG secretion by the liver without affecting lipolysis, its effect on lipid metabolism appears to be selective to the liver. Together, our findings indicate that the increased release of NPY during fasting stimulates the sympathetic nervous system to maintain VLDL-TG secretion at a postprandial level.

Type 3 Deiodinase Is Highly Expressed in Infiltrating Neutrophilic Granulocytes in Response to Acute Bacterial Infection

BACKGROUND Macrophages and polymorphonuclear cells (PMNs) play an important role in the first line of defense against bacteria by infiltrating the infected organ in order to clear the harmful pathogen. Our earlier studies showed that granulocytes express type 3 deiodinase (D3) when activated during a turpentine-induced abscess. We hypothesized that D3 expression by granulocytes may also occur during bacterial infection. METHODS In order to test this hypothesis, we used the following experimental infection models: peritonitis induced by Escherichia coli and acute pneumonia induced by Streptococcus pneumoniae. RESULTS E. coli-induced peritonitis was characterized by infiltration in the liver by inflammatory cells with abundant immunocytochemical D3 expression while no staining was present in hepatocytes of infected or control mice. Acute pneumonia induced by S. pneumoniae resulted in inflamed lungs characterized by numerous infiltrating granulocytes expressing D3 while no D3 staining was present in lung sections without an infiltrate. Serum thyroid hormones were negatively correlated to bacterial outgrowth in both lung and spleen, and thus to the severity of illness. CONCLUSION Infiltrating granulocytes during acute bacterial infection express D3. Our work supports the hypothesis that D3 plays an important role during chemical and bacterial inflammation. Whether the resulting decreased local bioavailability of thyroid hormones or rather the increased local availability of iodide is an important element of the innate immune response remains to be studied.

Impaired bacterial clearance in type 3 deiodinase deficient mice infected with Streptococcus pneumoniae

The activation of type 3 deiodinase (D3) has been postulated to play a role in the reduction of thyroid hormone levels during illness. Using a mouse model of acute bacterial infection, we have recently demonstrated marked D3 immunostaining in neutrophils infiltrating infected organs. These observations suggest a possible additional role for this enzyme in the innate immune response. To further assess the role of D3 in the response to acute bacterial infection, we used null D3 [D3 knockout (D3KO)] and wild type (WT) mice and infected them with Streptococcus pneumoniae. Marked reductions in serum thyroid hormone levels were observed both in D3KO and WT mice. Infection resulted also in a decrease in liver D1 activity in WT, but not in infected D3KO mice. Upon infection, pulmonary neutrophilic influx (measured by myeloperoxidase levels) and IL-6 and TNF concentrations increased equally in D3KO and WT mice, and histological examination of infected mice showed similar pulmonary inflammation in both strains. However, D3KO animals demonstrated significantly higher bacterial load in blood, lung, and spleen compared with WT mice. We conclude that 1) D3 is not required to generate the systemic manifestations of the nonthyroidal illness syndrome in this model; 2) the lack of D3 does not affect the extent of pulmonary inflammation; and 3) bacterial outgrowth in blood, spleen, and lung of D3KO mice is significantly higher than in WT mice. Our results suggest a protective role for D3 in the defense against acute bacterial infection, probably by reinforcing the microbial killing capacity of neutrophils.

Influence of respiratory syncytial virus infection on cytokine and inflammatory responses in allergic mice

Background Th2 lymphocyte responses are associated with inflammation and disease during allergic responses. Exposure to particular environmental factors during the expression of allergy could result in more pronounced Th2‐like immune responses and more severe disease. One factor might be a respiratory virus infection.

Skeletal muscle deiodinase type 2 regulation during illness in mice

We have previously shown that skeletal muscle deiodinase type 2 (D2) mRNA (listed as Dio2 in MGI Database) is upregulated in an animal model of acute illness. However, human studies on the expression of muscle D2 during illness report conflicting data. Therefore, we evaluated the expression of skeletal muscle D2 and D2-regulating factors in two mouse models of illness that differ in timing and severity of illness: 1) turpentine-induced inflammation, and 2) Streptococcus pneumoniae infection. During turpentine-induced inflammation, D2 mRNA and activity increased compared to pair-fed controls, most prominently at day 1 and 2, whereas after S. pneumoniae infection D2 mRNA decreased. We evaluated the association of D2 expression with serum thyroid hormones, (de-)ubiquitinating enzymes ubiquitin-specific peptidase 33 and WD repeat and SOCS box-containing 1 (Wsb1), cytokine expression and activation of inflammatory pathways and cAMP pathway. During chronic inflammation the increased muscle D2 expression is associated with the activation of the cAMP pathway. The normalization of D2 5 days after turpentine injection coincides with increased Wsb1 and tumor necrosis factor alpha expression. Muscle interleukin-1beta (Il1b) expression correlated with decreased D2 mRNA expression after S. pneumoniae infection. In conclusion, muscle D2 expression is differentially regulated during illness, probably related to differences in the inflammatory response and type of pathology. D2 mRNA and activity increases in skeletal muscle during the acute phase of chronic inflammation compared to pair-fed controls probably due to activation of the cAMP pathway. In contrast, muscle D2 mRNA decreases 48 h after a severe bacterial infection, which is associated with local Il1b mRNA expression and might also be due to diminished food-intake.

Thyroid Hormone Receptor α Modulates Lipopolysaccharide-Induced Changes in Peripheral Thyroid Hormone Metabolism

Acute inflammation is characterized by low serum T(3) and T(4) levels accompanied by changes in liver type 1 deiodinase (D1), liver D3, muscle D2, and muscle D3 expression. It is unknown at present whether thyroid hormone receptor alpha (TRalpha) plays a role in altered peripheral thyroid hormone metabolism during acute illness in vivo. We induced acute illness in TRalpha-deficient (TRalpha(0/0)) mice by administration of a sublethal dose of LPS. Compared with wild-type, TRalpha(0/0) mice have lower basal serum T(4) and lower liver D1 activity and muscle D3 mRNA expression, whereas liver D3 activity is higher. These changes are gender specific. The inflammatory response to LPS was similar in WT and TRalpha(0/0) mice. The decrease in serum thyroid hormones and liver D1 was attenuated in TRalpha(0/0) mice, whereas the LPS induced fall in liver D3 mRNA was more pronounced in TRalpha(0/0) mice. Muscle D2 mRNA increased similarly in both strains, whereas muscle D3 mRNA decreased less pronounced in TRalpha(0/0) mice. We conclude that alterations in peripheral thyroid hormone metabolism induced by LPS administration are partly regulated via TRalpha.

Lacking thyroid hormone receptor β gene does not influence alterations in peripheral thyroid hormone metabolism during acute illness

The downregulation of liver deiodinase type 1 (D1) is supposed to be one of the mechanisms behind the decrease in serum tri-iodothyronine (T3) observed during non-thyroidal illness (NTI). Liver D1 mRNA expression is positively regulated by T3, mainly via the thyroid hormone receptor (TR)beta1. One might thus expect that lacking the TRbeta gene would result in diminished downregulation of liver D1 expression and a smaller decrease in serum T3 during illness. In this study, we used TRbeta-/- mice to evaluate the role of TRbeta in lipopolysaccharide (LPS, a bacterial endotoxin)-induced changes in thyroid hormone metabolism. Our results show that the LPS-induced serum T3 and thyroxine and liver D1 decrease takes place despite the absence of TRbeta. Furthermore, we observed basal differences in liver D1 mRNA and activity between TRbeta-/- and wild-type mice and TRbeta-/- males and females, which did not result in differences in serum T3. Serum T3 decreased rapidly after LPS administration, followed by decreased liver D1, indicating that the contribution of liver D1 during NTI may be limited with respect to decreased serum T3 levels. Muscle D2 mRNA did not compensate for the low basal liver D1 observed in TRbeta-/- mice and increased in response to LPS in TRbeta-/- and WT mice. Other (TRbeta independent) mechanisms like decreased thyroidal secretion and decreased binding to thyroid hormone-binding proteins probably play a role in the early decrease in serum T3 observed in this study.

Interleukin-1 modulates endogenous thyroid hormone receptor gene transcription in liver cells

One of the main characteristics of nonthyroidal illness (NTI) is a decrease in serum tri-iodothyronine, partly caused by a decrease in liver deiodinase type 1 (D1) mRNA and activity. Proinflammatory cytokines have been associated with NTI in view of their capability to decrease]:)I and thyroid hormone receptor (TR)beta 1 mRNA expression in hepatoma cells. Proinflammatory cytokine induction leads to activation of the inflammatory pathways nuclear factor (NF)kappa B and activator protein (AP)-1. The proinflammatory cytokine interleukin decreases thyroid hormone receptor (TR)beta 1 mRNA in an NF kappa B-dependent way. The aim of this Study was to unravel the effects of IL-1 beta on endogenous TR alpha gene expression in an animal model and in a liver cell line. The TR alpha gene product is alternatively spliced in TR alpha 1 and TR alpha 2, TR alpha 2 is capable of inhibiting TR alpha 1-induced gene transcription. We showed that both TR alpha 1 and TR alpha 2 mRNA decreased not only after lipopolysaccharide administration in liver of mice, but also after IL-1 beta stimulation of hepatoma cells (HepG2). Using the NF kappa B inhibitor sulfasalazine and the AP-1 inhibitor SP600125, it became clear that the IL-1 beta-induced decrease in TR alpha mRNA expression in HepG2 cells can only be abolished by simultaneous inhibition of NF kappa B and AP-1. The IL-1 beta-induced TR alpha 1 and TR alpha 2 mRNA decrease in HepG2 cells is the result of decreased TR alpha gene promoter activity, as evident from actinomycin D experiments. Cycloheximide experiments showed that the decreased promoter activity is independent of dc novo protein synthesis and therefore most likely due to posttranslational modifications such as phosphorylation or subcellular relocalization