Identification of entero-endocrine mechanisms in the improvement of glucose tolerance after foregut bypass surgery for type 2 diabetes in Zucker diabetic fatty rats

Abstract

배경 및 목적: 대사수술의 항당뇨 효과가, 영양분의 빠른 소장 원위부 자극에 의한 인크레틴(incretin) 분비 증가(후장 가설) 때문인지, 또는 영양분의 소장 근위부 우회에 의한 가상의 ‘당뇨병 유발 인자(항인크 레틴, anti-incretin)’ 분비 회피(전장 가설)로 인한 것인지 명확치 않다. 전장우회술의 혈당 조절기전을 이해하는 것은 대사수술을 최적화하고 새로운 치료 방향 설정에 기여할 수 있을 것이다. 이 연구는 전장우 회술 (foregut bypass; duodenojejunal bypass; DJB)의 1) 췌장 베타세포 및 인슐린 신호 전달에 대한 영향과, 2) 내당능 개선을 유도하는 장 내분 비-췌도 호르몬 기전을 밝히려 하였다. 대상 및 방법: 실험 1) 당뇨비만 쥐(Zucker diabetic fatty rat; ZDF)을 DJB, 짝지워 먹이기(pair-fed; DJBPF) 및 임의 섭식(ad libitum fed; AL) 군으로 무작위 배정하였다. 경구혈당부하검사와 혈장 인슐린 농도 측정을 수 술 후 16주까지 정기적으로 시행하였다. 세 군간의 췌도 구조 변화를 조직형태학분석을 통해 비교하였다. 내장지방 세포 내 인산화-Akt/Akt 비를 측정하여 인슐린 신호 전달을 비교하였다. 실험 2) ZDF rat에서 DJB 와 위절제술(sleeve gastrectomy; SG) 및 조합 수술(SG + DJB) 후 내 당능 개선과 호르몬 변화를 비교하였다. 결과: DJB 후 체중과 음식 섭취량 변화는 없었다. 수술 후 내당능은 비교군과 대조적으로 DJB 2주 후 현저히 개선되었(p<0.05). 혈장 인슐 린 농도는 DJB 군에서만 16주간 수술전과 차이없이 유지되었고, 비교 군에서는 지속적으로 저하되었다(p<0.01). 췌도는 DJB군에서 비교군 보다 높은 비율의 확장투사형태(islets with expanding projections)를 보였 고(P<0.01), 수술 전 동일 연령 ZDF rat에서의 비율과 같았으며, 베타세 포 분획은 비교 군들보다 3배 이상 넓었다(p<0.01). DJB군은 세포핵내 PDX1발현도가 3배 이상 높았고(p<0.01), 섬유화도가 낮은 경향을 보 였다. 내장지방 세포 내의 Akt 인산화비는 DJB 군에서 유의하게 높았 다 (p < 0.05). SG 군의 체중과 섭취량은 유의하게 감소하였다. 내당능 개선 정도는 DJB 군에서 가장 현저하였고(p < 0.05), SG 군은 AL 군과 유의한 차이 를 보이지 않았다. 인크레틴(glucagon-like peptide-1, glucose-dependent insulinotropic peptide, peptide YY) 분비 반응은 SG 군에서 만 유의하게 증가하였고, DJB 군에서는 변화되지 않았다. 경구 당부하 글루카곤 억 제 반응은 DJB 군에서만 정상 수준으로 회복되었다. 결론: 전장우회술은 당뇨쥐에서 체중 감소와 무관하게 내당능을 개선 하였고, 혈장 인슐린 농도 감소를 장기간 방지였으며, 베타세포의 형 태학적 적응 결과를 유지하였고, 인슐린 신호 전달을 촉진하였다. 전장우회술의 내당능 개선은 인크레틴 분비 반응과 무관하며, 고혈당 유발의 병리적 기전 일부를 정상화 시켰고, 이는 전장 가설에 합치한 다. 향후 전장의 항인크레틴 증명 연구가 필요하다.

Background and objective: It is not clear whether the anti-diabetic effect of metabolic surgery is due to the expedited delivery of nutrients to the distal intestine and consequent enhancement of glucagon-like peptide-1 (GLP-1) (incretins) secretion (hindgut hypothesis) or the exclusion of the duodenum and proximal jejunum and consequent reduction of “putative diabetogenic factors” (foregut hypothesis). Understanding how the bypassed foregut contributes to glycemic regulation could help optimize the metabolic surgical procedure and identify critical targets for novel therapeutic interventions. We aimed to characterize how foregut bypass surgery alone (duodenojejunal bypass [DJB] surgery) affects glucose tolerance and beta-cell functions and investigate the distinct effect of foregut bypass surgery on entero-insular hormonal regulation of glucose homeostasis. We also aimed to draw a rationale for further research to discover a diabetogenic anti-incretin factor in the proximal intestine. We hypothesize that (1) the long-term beneficial effect of foregut bypass surgery on glycemic control is primarily associated with beta-cell function and (2) the post-bypass glucose tolerance improvement is induced by eliminating the anti-incretin activity contributing to the pathophysiology of diabetes. Materials and Methods: In the first set of experiments, Zucker Diabetic Fatty (ZDF) rats were randomly assigned to the DJB, sham-operated DJB pair-fed (sham-DJBPF), and sham-operated ad-libitum fed (sham-AL) groups. Glucose tolerance (GT) and plasma insulin levels were measured during periodically performed oral glucose tolerance test up to 16 weeks postoperatively. Histomorphometric analyses were performed to evaluate the modification of islet architectures. Intracellular insulin signaling in visceral adipocytes was measured by the phosphorylated Akt/Akt ratio using Western blot. In the second set of experiments, the effects of DJB surgery in ZDF rats based on entero-insular hormonal changes were compared to those of gastric resection alone (sleeve gastrectomy [SG]). Other animals underwent a combination procedure (SG + DJB surgery). Outcome measures were weight, food intake (FI), GT, and gut hormones. Results: DJB surgery did not substantially affect weight and FI. GT in DJB animals more significantly improved compared to that in sham-DJBPF and sham-AL animals (p < 0.01), and the value of GT 2 weeks after surgery were significantly better compared to its preoperative value (p < 0.05). Postoperatively, in contrast to the progressive deterioration of GT and the decrease in plasma insulin levels in sham-operated animals (p < 0.01), DJB animals prevented GT aggravation and plasma insulin level declining. The proportion of the islets having expanding projections was significantly higher in DJB animals than in sham-operated animals (p < 0.01). DJB had larger fractional area of beta cells (p < 0.01) and lesser incidence of fibrosis in the islet than sham-operated animals (p = 0.083). DJB animals had significantly higher levels of Akt phosphorylation in the visceral fat tissues than sham-operated animals (0p < 0.05). SG significantly reduced weight gain and food consumption. DJB rats showed weight-independent improvement in GT, which improved less after SG. Furthermore, SG significantly increased insulin, GLP-1, glucose-dependent insulinotropic peptide, and peptide YY responses to oral glucose. In contrast, DJB surgery had no effects on the postprandial levels of these entero-endocrine hormones. DJB surgery restored postprandial glucagon suppression in diabetic rats, whereas SG had no effect in ZDF rats’ glucagon response. The combination procedure had higher GT than SG alone with modestly compromised GLP-1 levels. Conclusion: Foregut bypass surgery improved GT immediately after surgery and prevented the further decrease of plasma insulin levels, which may be attributable to preserving the morphological compensation of islets to insulin resistance. Moreover, this surgery also facilitated insulin signaling in its peripheral target tissues. Foregut bypass surgery did not improve diabetes by increasing incretin levels or enhancing factors that reduce glycemia. It may work by reducing the mechanism promoting hyperglycemia, which would be consistent with the anti-incretin theory. Considering abovementioned results, further elucidation of the molecular mechanism, which may be mediated by putative anti-incretin, is required.