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Non-alcoholic fatty liver disease (NAFLD) affects 25% of the global population and is strongly associated with obesity, type 2 diabetes (T2D)/insulin resistance (IR), and dyslipidemia. All of these conditions are a public health concern and beget socioeconomic problems. Hepatic steatosis in NAFLD results from an imbalance between substrate availability (fatty acids and carbohydrates) and the hepatic capacity to dispose of fats properly. Lifestyle interventions (diet and physical activity) improve NAFLD, but currently no pharmacological treatment is approved for NAFLD. Natural health products and functional foods include potential candidates to aid in the management of metabolic conditions. Food-derived bioactive peptides have effects beyond their nutritive value and can modulate physiological processes promoting health benefits. There are many food sources of bioactive peptides, including the egg, a universally available and consumed source of protein. We aimed to identify specific effects of Peptide 2 diet supplementation on manifestations of the metabolic syndrome including systemic IR, WAT response to insulin and NAFLD markers, compared with the thiazolidinedione rosiglitazone.
Protocol 1: Male Sprague Dawley (SD) rats (n = 48) were fed with high fat diet (HFD) for 6 wks. Then, half of the animals received HFD+4% EWH with the remainder serving as HFD controls for another 6 wks. At the end of week 12, half of the animals received an intraperitoneal injection of insulin (2 IU/kg of body weight (BW)) to stimulate insulin signaling prior to euthanization using CO2. Protocol 2: Male C57BL/6 mice (5 wks old) were housed 4/cage with ad libitum access to food and water. Mice received a low fat diet (LFD, 10% kcal fat) or a high fat diet (HFD, 45% kcal fat) for 6 wks. After that, the HFD animals were divided into 3 groups: HFD only, HFD + Peptide 2 (PEP2) and HFD + rosiglitazone (ROSI) and continued receiving their respective diets for another 8 wks. LFD animals continued receiving LFD for another 8 wks. After a total of 14 wks, mice either received an intraperitoneal injection of insulin (1.5 IU/kg BW) prior to euthanasia or were directly euthanized using CO2, while some mice underwent hyperinsulinemic-euglycemic clamp prior to euthanasia by ketamine. Peptide 2 was administered at 45 mg/Kg BW/day daily mixed in the diet. Mice were weighed weekly. Blood was collected for further analysis. Tissues were collected for histological analysis.
The analysis revealed that 4% EHW treated animals had reduced p-HSL in rWAT after intraperitoneal injection of insulin (p < 0.05) despite no change in phosphorylation on PKA.Total PKA abundance was reduced by 4% EWH treatment. Plasma and WAT adiponectin and resistin concentrations were not different between groups. Peptide 2 supplementation did not influence final BW or BW gain in comparison to HFD. Body composition analysis revealed that only the LFD animals had a lower fat mass % and higher lean mass % than HFD. In vivo tests confirmed that at week 14 HFD animals were glucose intolerant compared to LFD animals, and that the ROSI group had improved glucose tolerance compared to HFD group. Improvement in insulin sensitivity after PEP2 and ROSI treatment was confirmed during the insulin tolerance test.
In conclusion, this study shows for the first time that egg-white derived Peptide 2 diet supplementation improves IR in HFD-induced obese and IR mice, while at the same time preventing further exacerbation of HFD-induced NAFLD features independently of BW. On the other hand, rosiglitazone-treated mice, despite having improved IR, exhibited worse hepatic steatosis if administered together with HFD. Therefore, compared to rosiglitazone, Peptide 2 promotes more beneficial effects on the combined outcomes of insulin resistance, WAT dysfunction and hepatic steatosis