Dopamine receptor D1- and D2-agonists do not spark brown adipose tissue thermogenesis in mice

Partial Resistance to Thyroid Hormone-Induced Tachycardia and Cardiac Hypertrophy in Mice Lacking Thyroid Hormone Receptor β

Background: Thyroid hormones regulate cardiac functions mainly through direct actions in the heart and by binding to the thyroid hormone receptor (TR) isoforms α1 and β. While the role of the most abundantly expressed isoform, TRα1, is widely studied and well characterized, the role of TRβ in regulating heart functions is still poorly understood, primarily due to the accompanying elevation of circulating thyroid hormone in TRβ knockout mice (TRβ-KO). However, their hyperthyroidism is ameliorated at thermoneutrality, which allows studying the role of TRβ without this confounding factor. Methods: Here, we noninvasively monitored heart rate in TRβ-KO mice over several days using radiotelemetry at different housing temperatures (22°C and 30°C) and upon 3,3',5-triiodothyronine (T3) administration in comparison to wild-type animals. Results: TRβ-KO mice displayed normal average heart rate at both 22°C and 30°C with only minor changes in heart rate frequency distribution, which was confirmed by independent electrocardiogram recordings in freely-moving conscious mice. Parasympathetic nerve activity was, however, impaired in TRβ-KO mice at 22°C, and only partly rescued at 30°C. As expected, oral treatment with pharmacological doses of T3 at 30°C led to tachycardia in wild-types, accompanied by broader heart rate frequency distribution and increased heart weight. The TRβ-KO mice, in contrast, showed blunted tachycardia, as well as resistance to changes in heart rate frequency distribution and heart weight. At the molecular level, these observations were paralleled by a blunted cardiac mRNA induction of several important genes, including the pacemaker channels Hcn2 and Hcn4, as well as Kcna7. Conclusions: The phenotyping of TRβ-KO mice conducted at thermoneutrality allows novel insights on the role of TRβ in cardiac functions in the absence of the usual confounding hyperthyroidism. Even though TRβ is expressed at lower levels than TRα1 in the heart, our findings demonstrate an important role for this isoform in the cardiac response to thyroid hormones.

Subcutaneous adipose tissue dopamine D2 receptor is increased in prediabetes and T2D

PURPOSE

To evaluate the dopaminergic signaling in human adipose tissue in the context of obesity and type 2 diabetes (T2D) and potential direct implications in adipose tissue metabolism.

METHODS

mRNA and protein expression of dopamine receptors D1 and D2 (DRD1 and DRD2) were determined in subcutaneous adipose tissue from subjects without or with T2D and with different body weight, and correlated with markers of obesity, hyperglycemia, and insulin resistance. Glucose uptake and lipolysis were measured in adipocytes ex vivo following short-term exposure to dopamine, DRD1 receptor agonist (SKF81297), or DRD2 receptor agonist (bromocriptine).

RESULTS

DRD1 and DRD2 gene expression in subcutaneous adipose tissue correlated positively with clinical markers of insulin resistance (e.g. HOMA-IR, insulin, and triglycerides) and central obesity in subjects without T2D. Protein expression of DRD2 in subcutaneous adipose tissue, but not DRD1, is higher in subjects with impaired fasting glucose and T2D and correlated positively with hyperglycemia, HbA1c, and glucose AUC, independent of obesity status. DRD1 and DRD2 proteins were mainly expressed in adipocytes, compared to stromal vascular cells. Dopamine and dopaminergic agonists did not affect adipocyte glucose uptake ex vivo, but DRD1 and DRD2 agonist treatment inhibited isoproterenol-stimulated lipolysis.

CONCLUSION

The results suggest that protein expression of DRD2 in subcutaneous adipose tissue is up-regulated with hyperglycemia and T2D. Whether DRD2 protein levels contribute to T2D development or occur as a secondary compensatory mechanism needs further investigation. Additionally, dopamine receptor agonists inhibit adipocyte beta-adrenergic stimulation of lipolysis, which might contribute to the beneficial effects in lipid metabolism as observed in patients taking bromocriptine.

Anti-Inflammatory Effects of Peripheral Dopamine

Dopamine is synthesized in the nervous system where it acts as a neurotransmitter. Dopamine is also synthesized in a number of peripheral organs as well as in several types of cells and has organ-specific functions and, as demonstrated more recently, is involved in the regulation of the immune response and inflammatory reaction. In particular, the renal dopaminergic system is very important in the regulation of sodium transport and blood pressure and is particularly sensitive to stimuli that cause oxidative stress and inflammation. This review is focused on how dopamine is synthesized in organs and tissues and the mechanisms by which dopamine and its receptors exert their effects on the inflammatory response.

Potential cellular endocrinology mechanisms underlying the effects of Chinese herbal medicine therapy on asthma

Asthma is a complex syndrome with polygenetic tendency and multiple phenotypes, which has variable expiratory airflow limitation and respiratory symptoms that vary over time and in intensity. In recent years, continuous industrial development has seriously impacted the climate and air quality at a global scale. It has been verified that climate change can induce asthma in predisposed individuals and that atmospheric pollution can exacerbate asthma severity. At present, a subset of patients is resistant to the drug therapy for asthma. Hence, it is urgent to find new ideas for asthma prevention and treatment. In this review, we discuss the prescription, composition, formulation, and mechanism of traditional Chinese medicine monomer, traditional Chinese medicine monomer complex, single herbs, and traditional Chinese patent medicine in the treatment of asthma. We also discuss the effects of Chinese herbal medicine on asthma from the perspective of cellular endocrinology in the past decade, emphasizing on the roles as intracellular and extracellular messengers of three substances-hormones, substances secreted by pulmonary neuroendocrine cells, and neuroendocrine-related signaling protein-which provide the theoretical basis for clinical application and new drug development.

Dopamine receptor D1 signaling stimulates lipolysis and browning of white adipocytes

Adipocytes express several kinds of catecholamine receptors, including adrenergic receptors, and dopamine receptors. Signaling pathways mediated by catecholamine receptors, such as β3-adrenergic receptor pathway, can induce body energy expenditure via activating thermogenesis of adipose tissue. However, the roles of adipose dopamine receptors on adipocytes are still unclear. Here, we investigate the role of dopamine receptor D1 (DRD1) on adipocytes. To this end, we use DRD1 agonist Fenoldopam and antagonist SCH23390 to stimulate and inhibit DRD1 signaling, respectively. We found that, compared with control group mice, Fenoldopam-treated and SCH23390-treated high-fat-diet (HFD)-fed mice showed smaller and bigger white adipose tissue/adipocyte sizes, respectively. Meanwhile, activating of DRD1 signaling enhanced intracellular levels of cAMP, phosphorylation levels of protein kinase A substrates, and hormone-sensitive lipase, a key enzyme for lipolysis in mature 3T3-L1 adipocytes and white adipose tissue of HFD-fed mice. As a result, the levels of free fatty acid or glycerol were increased, indicating stimulation of lipolysis by DRD1 activation. Moreover, activating DRD1 can induce the browning of adipocytes, as indicated by enhanced phosphorylation of P38 MAP kinase, increased expression of beige cell markers (PGC-1α, UCP-1, and CD81), mitochondrion content, and expression of β-oxidation related genes. All of these effects were reduced after treating with SCH23390 both in vitro and in HFD-fed mice. Collectively, our study indicated that DRD1 signaling stimulates lipolysis and browning of white adipocytes in vitro and in vivo. Understanding the functions of DRD1 on human adipocytes and adipose tissues will help us to design novel strategies to treat obesity.