Effect of bombesin receptor subtype-3 and its synthetic agonist on signaling, glucose transport and metabolism in myocytes from patients with obesity and type 2 diabetes

Abstract

Bombesin receptor subtype-3 (BRS-3) is an orphan G-protein-coupled receptor (GPCR) member of the bombesin receptor family. Several studies have suggested an association between obesity, alterations in glucose metabolism, diabetes and the BRS-3 receptor. In this study, we focused on patients simultaneously diagnosed with obesity and type 2 diabetes (OB/T2D). The analysis of BRS-3 expression in the skeletal muscle of these patients revealed a marked decrease in the expression of BRS-3 at the mRNA (23.6±1.3-fold downregulation, p<0.0001) and protein level (49±7% decrease, p<0.05) compared to the normal patients (no obesity and diabetes). Moreover, in cultured primary myocytes from patients with OB/T2D, the synthetic BRS-3 agonist, [D-Try6,β-Ala11,Phe13,Nle14]bombesin6–14, significantly increased the phosphorylation levels of mitogen-activated protein kinase (MAPK), p90RSK1, protein kinase B (PKB) and p70s6K. Specifically, the ligand at 10−11 M induced the maximal phosphorylation of MAPKs (p42, 159±15% of the control; p44, 166±11% of the control; p<0.0001) and p90RSK1 (148±2% of the control, p<0.0001). The basal phosphorylation levels of all kinases were reduced (p<0.05) in the patients with OB/T2D compared to the normal patients. Furthermore, the BRS-3 agonist stimulated glucose transport, which was already detected at 10−12 M (133±9% of the control), reached maximal levels at 10−11 M (160±9%, p<0.0001) and was maintained at up to 10−8 M (overall mean, 153±7%; p<0.007). This effect was less promiment than that attained with 10−8 M insulin (202±9%, p=0.009). The effect of the agonist on glycogen synthase a activity achieved the maximum effect at 10−11 M (165±16% of the control; p<0.0001), which did not differ from that observed with higher concentrations of the agonist. These results suggest that muscle cells isolated from patients with OB/T2D have extremely high sensitivity to the synthetic ligand, and the effects are particularly observed on MAPK and p90RSK1 phosphorylation, as well as glucose uptake. Moreover, our data indicate that BRS-3 may prove to be useful as a potential therapeutic target for the treatment of patients with OB/T2D.

Bombesin receptor subtype 3 as a potential target for obesity and diabetes

“…Furthermore, in patients simultaneously diagnosed with obesity and type 2 diabetes, much lower BRS-3 mRNA levels in skeletal muscle are found, than that previously observed in patients affected by either T2D or obesity, which suggest a potential synergy in the negative impact of these two conditions on BRS—receptor expression[63]. In skeletal muscle myocytes from normals, patients with obesity or with type 2 diabetes mellitus, BRS-3-receptor activation results in stimulation of glucose transport, MAP kinase, p90SRSKI, protein kinase B, PI3K, p70s6 kinase, glycogen synthetase α activity and glycogen synthesis[59,63,64]. In addition the myocytes from patients with the two altered metabolic states alone or together, had increased sensitivity to BRS-3 stimulated glucose uptake and these intracellular cascades (Table 3) [59,63,64].…”

“…Recently detailed studies have been reported of the comparative signaling cascades involved in BRS-3 activation in myocytes of diabetic, obese and normal subjects[59,63,64], and this will be discussed in a later section on role of BRS-3 in obesity, energy metabolism and glucose homeostasis.…”

“…To attempt to better define the mechanisms of the alterations in glucose and metabolic homeostasis that occur in BRS-3-KO mice, a number of studies have been reported investigating the role of BRS-3 in normal cellular/tissue function and cells/tissues from animals or human subjects with pathological conditions such as diabetes and obesity (Table 3)[31,59,63,64,113]. These investigations include the role of BRS-3 in islet/insulin responses, as well as responses of important insulin target tissues such as adipose tissue and skeletal muscle myocytes in normals as well as disease states[31,59,63,64,113].…”

“…These investigations include the role of BRS-3 in islet/insulin responses, as well as responses of important insulin target tissues such as adipose tissue and skeletal muscle myocytes in normals as well as disease states[31,59,63,64,113]. …”

“…The authors concluded that these results demonstrate the important role of BRS-3 in islet regulation and glucose homeostasis, with BRS-3 stimulation directly promoting glucose-stimulated insulin-release, suggesting it may have an important potential role in the treatment of obesity as well as in tratment of diabetes mellitus[31]. These suggestions are supported by studies demonstrating that activation of BRS-3 has important effects on both adipose tissue[102](reviewed above) and skeletal muscle (myocytes)(reviewed in next section)[59,63,64]. …”