To the best of our knowledge, this is the first study investigating the effect of BS on plasma concentrations of ANGPTL3 and ANGPTL4. We included both NGT and T2D obese patients to gauge the effect of surgery over a broader range of insulin/glucose abnormalities associated with obesity and to evaluate the differential effect of RYGB and BPD on ANGPTLs.
The main findings of this study are: a) both BPD and RYGB cause a reduction in ANGPTL4 levels; b) ANGPTL3 increases after BPD but not after RYGB; c) the increase in ANGPTL3 after BPD is significantly associated with the concomitant reduction in FFA. In addition, we found that NGT individuals with morbid obesity have similar levels of ANGPTL3 and ANGPTL4 as obese or non-obese T2D patients, and that insulin sensitivity is directly related to ANGPTL3 and inversely related to ANGPTL4.
As expected, BS effectively reduced body weight in all patients, independently of the surgical technique, this paralleled a near doubling of insulin sensitivity. It is noteworthy that TG reduction was obtained with RYGB but not with BPD, which conversely was more effective in reducing TC and LDL-C. As described in a recent meta-analysis, RYGB is the gold standard surgery technique in morbid obesity, but BPD is more effective for the long-term remission of T2D and for cholesterol reduction 20,21. The latter finding follows from the fact that BPD is a malabsorptive procedure, which causes enhanced bile acid loss; this in turn stimulates bile acid synthesis and an enhanced liver uptake of circulating LDL 22.
Plasma levels of ANGPTL4 were found to be significantly reduced in all patients treated with either RYGB or BPD: The estimated overall size of this reduction was ~12%. As ANGPTL4 is mainly produced by adipose tissue 2,23, its reduction can be easily explained by the marked post-surgical reduction of body fat mass. Insulin is a negative regulator of ANGPTL4 synthesis and secretion through the activation of ChREBP and the inhibition of peroxisome-proliferator receptor α and γ (PPAR-α and PPAR-γ), which are the main inducers of this angiopoietin 2,24. The observed reduction in circulating ANGPTL4 may be, therefore, ascribed, at least in part, to the post-surgical improvement of whole-body insulin sensitivity with the attendant fall in glucose and insulin levels (Table 2 and 3). As ANGPTL4 acts as an inhibitor of LPL activity, its reduction following BS might activate TG hydrolysis, thereby leading to TG lowering; this hypothesis was recently proved by Singh et al.25 that reported, in a mouse model selectively knocked-out for ANGPTL4, a consistent reduction of circulating lipids, increased hepatic clearance of dietary derived chylomicrons and increased liver and adipose tissue insulin sensitivity25. Unfortunately, our study did not include measurements of post-heparin lipase activity (PHLA) before and after surgery so that further studies are needed to clarify this point.
The effect of BS on ANGPTL3 appears to be strictly dependent from surgical procedure employed as its levels were unchanged by RYGB but increased significantly after BPD. Although several studies reported ANGPTL3 to be directly related to BMI (with higher levels in obese people), the positive relation between body fat and ANGPTL3 has not been consistently confirmed 11,12. We did not find a significant correlation between ANGPTL3 and TG levels, which is in line with some previously reported results 26 but not with others 27. This controversial relationship suggests that other factors may affect their mutual regulation. In a previous study, we reported that the relation between plasma levels of ANGPTL3 and TG is not linear 26. In fact, TG levels (and other lipids such as total cholesterol, HDL- and LDL-cholesterol) were strongly correlated with ANGPTL3 only when ANGPTL3 levels are below 60 ng/dL; at higher levels there no longer was a significant association with serum lipids 26. The synthesis and secretion of ANGPTL3 are finely regulated by multiple factors, primarily by liver X receptor (LXR) 4,28. While oxysterols and fatty acids are natural ligands of LXR, the receptor also is activated under conditions of intracellular lipid deprivation and abundant intracellular glucose, thereby acting as a glucose sensor, stimulating lipogenesis and inhibiting gluconeogenesis 29,30.
Insulin has been shown to be an inhibitor of ANGPTL3 expression. Haridas et al. reported a reduction in plasma ANGPTL3 during a euglycemic hyperinsulinemic clamp in healthy subjects, and in culture medium of IHH cells treated with insulin 16. Yilmaz et al. found a direct relationship between HOMA-IR (an indirect index of hepatic insulin resistance) and plasma ANGPTL3 in patients with fatty-liver disease 15. In our patients, the BPD-induced rise in ANGPTL3 was directly related to changes in insulin sensitivity and inversely related to changes in serum FFA. This might be explained by differences in the intracellular downstream signaling of LXR activation 31,32. Thus, the post-surgical doubling of insulin sensitivity may raise intracellular glucose concentrations, and activate LXR thereby stimulating lipogenesis and liver lipid export 30. In BPD patients, the concomitant reduction in cholesterol and bile acids is a double hit for LXR hyperactivation, thereby promoting ANGPTL3 secretion. Consistently, Cinkajzlová et al. found increased levels of serum ANGPTL3 in patients suffering from short-bowel syndrome (SBS) in comparison with healthy controls 12. Notably, BPD patients can be considered as analog to SBS patients in terms of nutrients absorption and loss.
FFA were found to be negative drivers of ANGPTL3 in the BPD group. Although medium-chain FFA are LXR agonists 33, only C-10 and C-12 FFA are able to activate LXR as do oxysterols or glucose 33. Serum FFA decrease is associated with increased adipose tissue insulin sensitivity, resembling a post-prandial state where insulin inhibits adipose tissue lipolysis 34,35.
In interpreting these results, several limitations must be considered. The number of patients included in the study is small and the assignment of patients to RYGB or BPD was not randomized (due to partially different indications and clinical experience with the two procedures). More importantly, the difference of BMI among RYGB and BPD patients is wide. Finally, plasma levels of ANGPTL8, another angiopoietin-like protein involved in the activation of ANGPTL3, were not investigated.
In summary, BS showed the expected effect in improving the metabolic status of obese patients with or without T2D. While circulating ANGPTL4 was uniformly decreased by surgery, ANGPTL3 remained stable in RYGB patients but increased in BPD patients. This suggests that the chronic nutrient loss and malabsorption associated with BPD may be counterbalanced by the increased production of ANGPTL3 in the liver. It is possible that the decrease in ANGPTL4 (expressed mostly in WAT) might promote the uptake and storage of TG-derived lipids into WAT, whereas the increase in ANGPTL3 (expressed mostly in liver) might potentially slow down lipid uptake into liver.
In this regard, it is worth mentioning that our results highlighted an important relation between FFA, bile acids metabolism and insulin sensitivity with ANGPTL3, thus opening a new view on the complex regulation of ANGPTL3 metabolism, further in vitro and in vivo work is necessary to detail the molecular mechanism underlying ANGPTL3 regulation in extreme metabolic conditions.