Tryptophan hydroxylase 1 Inhibition Impacts Pulmonary Vascular Remodeling in Two Rat Models of Pulmonary Hypertension

Pulmonary arterial hypertension (PAH) is a progressive disease defined by a chronic elevation in pulmonary arterial pressure with extensive pulmonary vascular remodeling and perivascular inflammation characterized by an accumulation of macrophages, lymphocytes, dendritic cells, and mast cells. Although the exact etiology of the disease is unknown, clinical as well as preclinical data strongly implicate a role for serotonin (5-HT) in the process. Here, we investigated the chronic effects of pharmacological inhibition of tryptophan hydroxylase 1 (TPH1), the rate-limiting enzyme in peripheral 5-HT biosynthesis, in two preclinical models of pulmonary hypertension (PH), the monocrotaline (MCT) rat and the semaxanib (SUGEN, Medinoah, Suzhou, China)-hypoxia rat. In both PH models, ethyl (S)-8-(2-amino-6-((R)-1-(5-chloro-[1,1'-biphenyl]-2-yl)-2,2,2-trifluoroethoxy)pyrimidin-4-yl)-2,8-diazaspiro[4.5]decane-3-carboxylate and ethyl (S)-8-(2-amino-6-((R)-1-(3',4'-dimethyl-3-(3-methyl-1 H-pyrazol-1-yl)-[1,1'-biphenyl]-4-yl)-2,2,2-trifluoroethoxy)pyrimidin-4-yl)-2,8-diazaspiro[4.5]decane-3-carboxylate, novel orally active TPH1 inhibitors with nanomolar in vitro potency, decreased serum, gut, and lung 5-HT levels in a dose-dependent manner and significantly reduced pulmonary arterial pressure, and pulmonary vessel wall thickness and occlusion in male rats. In the MCT rat model, decreases in lung 5-HT significantly correlated with reductions in histamine levels and mast cell number (P < 0.001, r² = 0.88). In contrast, neither ambrisentan nor tadalafil, which are vasodilators approved for the treatment of PAH, reduced mast cell number or 5-HT levels, nor were they as effective in treating the vascular remodeling as were the TPH1 inhibitors. When administered in combination with ambrisentan, the TPH1 inhibitors showed an additive effect on pulmonary vascular remodeling and pressures. These data demonstrate that in addition to reducing vascular remodeling, TPH1 inhibition has the added benefit of reducing the perivascular mast cell accumulation associated with PH.

Optimization of triazole-based TGR5 agonists towards orally available agents

With the challenge of striking the balance of TGR5 potency and clearance, the screening strategy as well as medicinal chemistry strategy are discussed in this article.

Insulin causes hyperthermia by direct inhibition of warm-sensitive neurons

OBJECTIVE

Temperature and nutrient homeostasis are two interdependent components of energy balance regulated by distinct sets of hypothalamic neurons. The objective is to examine the role of the metabolic signal insulin in the control of core body temperature (CBT).

RESEARCH DESIGN AND METHODS

The effect of preoptic area administration of insulin on CBT in mice was measured by radiotelemetry and respiratory exchange ratio. In vivo 2-[(18)F]fluoro-2-deoxyglucose uptake into brown adipose tissue (BAT) was measured in rats after insulin treatment by positron emission tomography combined with X-ray computed tomography imaging. Insulin receptor-positive neurons were identified by retrograde tracing from the raphe pallidus. Insulin was locally applied on hypothalamic slices to determine the direct effects of insulin on intrinsically warm-sensitive neurons by inducing hyperpolarization and reducing firing rates.

RESULTS

Injection of insulin into the preoptic area of the hypothalamus induced a specific and dose-dependent elevation of CBT mediated by stimulation of BAT thermogenesis as shown by imaging and respiratory ratio measurements. Retrograde tracing indicates that insulin receptor-expressing warm-sensitive neurons activate BAT through projection via the raphe pallidus. Insulin applied on hypothalamic slices acted directly on intrinsically warm-sensitive neurons by inducing hyperpolarization and reducing firing rates. The hyperthermic effects of insulin were blocked by pretreatment with antibodies to insulin or with a phosphatidylinositol 3-kinase inhibitor.

CONCLUSIONS

Our findings demonstrate that insulin can directly modulate hypothalamic neurons that regulate thermogenesis and CBT and indicate that insulin plays an important role in coupling metabolism and thermoregulation at the level of anterior hypothalamus.

Metabolic characterization of a mouse deficient in all known leptin receptor isoforms

We have characterized a newly generated mouse model of obesity, a mouse strain deficient in all five previously described leptin receptor isoforms. These transgenic mice, named the db (333)/db (333) mice, were identified from an ENU mutagenesis screen and carry a point mutation in the seventh exon of the db gene encoding the leptin receptor, resulting in a premature stop codon (Y(333)Stop) and gene product that lacks STAT signaling domains. db (333)/db (333) mice have a morbidly obese phenotype, with body weights diverging from wild type as early as 4 weeks of age (P < 0.05). To determine the contribution of the short isoforms of the leptin receptor in this metabolic phenotype, we performed an extensive metabolic characterization of the db (333)/db (333) mouse in relation to the well-characterized db/db mouse lacking only the long form of the leptin receptor. db (333)/db (333) mice have similar endocrine and metabolic parameters as previously described in other leptin receptor transgenic mice including db/db mice that lack only the long isoform of the leptin receptor. However, db (333)/db (333) mice show a subtle trend toward higher body weight and insulin levels, lower oxygen, carbon dioxide production, respiratory exchange ratio (RER), and temperature than db/db mice suggesting the short isoforms may play an additional role in energy homeostasis.