We have published in this journal a study investigating comprehensively the vascular responses in a mouse model of heterozygous endothelial overexpression of the endothelin-1 gene concluding that the latter in combination with obesity aggravates renal arterial dysfunction1. In the course of this study we performed additional measurements leading to intriguing observations that we would like to share with the readership of the journal.
Endothelin-1 contributes to increased vascular reactivity and tone under cardiovascular risk conditions including obesity in both humans and animal models2. However, plasma levels of the potent vasoconstrictor peptide in mice are increased significantly only after eg three weeks of high fat feeding3 but no longer following ten weeks4, and they are the same as in lean animals on standard chow after thirty weeks of diet-induced obesity1. In addition to plasma endothelin-1 levels, pulmonary tissue contents have been determined in mice fed standard chow or high fat diet during such prolonged time using an ELISA kit1 following tissue extraction as described5. Across the organs, the lungs contain the highest levels of the peptide5, and were used to compare endothelin-1 contents between lean and obese animals. By contrast to the similarly low plasma peptide levels in lean and obese wild type mice and littermates with heterozygous endothelial over-expression of the prepro-endothelin-1 gene (TEThet)1 on standard chow or high fat diet, endothelin-1 levels in the lungs were significantly higher (Figure 1A). Surprisingly however, these tissue levels were lower in the lungs of obese than in those of lean animals (Figure 1A) having similar endothelin-1 contents in the lungs as reported5.
Endothelin-1 acts via two distinct receptor subtypes [endothelin-A (ETA) and endothelin-B (ETB)] mediating the characteristic vasoconstrictor response upon activation by the peptide of vascular smooth muscle cells2. By contrast, activation of endothelial ETB-receptors promotes the generation of nitric oxide, which thus leads to inhibition of contraction and facilitated relaxation2. Furthermore, ETB-receptors in the lungs are responsible for the clearance of the peptide especially in rodents6, where endothelial ETB-receptors play a crucial role in this process7. This may help to explain, at least in part, why plasma endothelin-1 is not increased by obesity in mice1.
As in humans, murine obesity is accompanied by augmented plasma cholesterol levels8, which was also evident from our data (Figure 1B). Low-density lipoproteins augment both the gene expression and the protein presence of ETB-receptors in endothelial cells9, and the gene expression of both the ETA and ETB-receptors is increased in tissue of obese compared to lean mice4. In these respects, we found increased ETB-receptor protein presence in the lungs of obese compared to lean animals (Figure 1C) using established antibodies4. Such an up-regulation of the ETB-receptor pathway in the lungs, a tissue of high endothelial cell content, would support the decreased pulmonary tissue endothelin-1 levels observed in obese mice as a likely consequence of an accelerated clearance6. This may occur despite an augmented ETB-receptor mediated increase in vascular tone as seen in some peripheral arteries from obese animals, in particular in those of endothelin-1 gene overexpressing mice1 and contribute to the less than expected vascular and metabolic phenotype. As regards the latter aspect, this may prevent them (or long-term high fat fed wild-type mice in general) from exhibiting elevated circulating endothelin-1 levels as in obese humans10 despite the present hyperinsulinemia1. Moreover, in obese mice, arterial blood pressure could remain in the normotensive range1 due to an accelerated clearance of the peptide despite an increased renal tissue content8.
Taken in conjunction, our results suggest that the activity of ETB-receptors may not only be increased in arteries of obese, especially endothelin-1 gene overexpressing mice1, but also in lung tissue of these animals with diet-induced obesity leading to an augmented clearance of the peptide. Mechanistically this can be explained by the hypercholesterolemia linked to this diet-induced obesity model causing an up-regulation of ETB-receptors in endothelial cells9. Hence, the potent vasoconstrictor peptide endothelin-1 may be cleared more effectively by these receptors6 on pulmonary endothelial cells7 in obese compared to lean animals making the phenotype in the overexpression model less dramatic than expected under the cardiovascular risk condition of obesity.
This work was supported by the Swiss National Science Foundation (grants 138 754 and 143 672 to Oliver BARETELLA), and the Hong Kong Research Grant Council (780410M). The authors thank Boris CHAN for help with blinded assays of endothelin-1; Pan YONG and Dr Dewei YE for advice with Western blot techniques.