From this study, quercetin appears to be the major phenolics present in APPE. From previous studies, quercetin has been proven to inhibit tissue oxidative damage via its augmentation effect on oxidative status. Moreover, it has been established to promote insulin release by enhancing the regeneration of pancreatic islets 21. In addition, plants phenolics has been indicated to inhibit the action of amylase in the digestion of carbohydrate in diabetes while others have been proven to lower the risk of complications arising from type 2 diabetes 22. The two principal enzymes involved in the breakdown of dietary carbohydrates, α-glucosidase and α-amylase, have been shown to be under the influence of flavonoids, phenolic acid and tannins which inhibit their actions 21.
Alloxan exhibits a selective destruction of islet Langerhans β-cells of the pancreas which usually result in massive reduction in insulin production culminating in hyperglycemia and the subsequent derangement in energy metabolism 23, 24 as vividly observed in this study. In compliance with previous studies, the present study showed that alloxan intoxication in rats led to reduced body weight while leading to enhanced food consumption. Though data on food intake is not reported in this study alloxan-induced rats were observed to consume more food than non-diabetic control rats. The usual decrease in body weight in diabetic state could be attributed to reduce cellular uptake and utilization of blood glucose which as well impact the rate of lipid dispensation and gluconeogenesis 22. The improvement in body weight in alloxan-induced diabetic rats administered APPE could have been a resultant anabolic effect of its phenolic phytochemicals which are capable of abrogating the physiological consequences of both alloxan- and hyperglycemia-induced oxidative stress through their antioxidant activity. Dietary plant polyphenols have been reported to control carbohydrate and lipid metabolism thus improving hyperglycemia, dyslipidemia and insulin resistance through improved β-cell function 25.
The hallmark of diabetic management is the restoration of blood glucose level to normal. As observed in the present study, APPE at 200 mg/kg body weight proofs to be an effective hypoglycemic agent in decreasing blood glucose concentrations to normal value in alloxan-induced diabetic rats. Hence, APPE could be attributed to produce or potentiate a beneficial role on carbohydrate utilization in alloxan-induced diabetic rats.
Data obtained in this study showed significantly higher level of glycosylated haemoglobin in diabetic but untreated rats (DC) compared to both normal control and diabetic groups treated with APPE. This shows that haemoglobin becomes glycosylated in a condition of high blood glucose concentration. The reduction in the level of glycated hemoglobin in the APPE-treated diabetic rats clearly suggests that APPE treatment reduces protein glycosylation in diabetic rats. The inhibitory action of APPE on protein glycosylation could be attributed to the antioxidative effects of polyphenolics on Millard reaction. Several antioxidants including ascorbic acid and β-carotene have been demonstrated to inhibit protein glycosylation both in vivo and in vitro 26, 27. Dietary antioxidants have also been reported to be capable of scavenging free radicals produced by protein glycosylation 28.
Oxidative stress has been postulated to be capable of activating several pathological changes in almost every type of kidney cells including endothelial cells, mesangial cells, tubular cells etc. This process in turn results in the development of diabetic neuropathy. It has also been pointed out that fibrosis which appears to be the most prominent feature of diabetic neuropathy is coordinately linked to inflammation 29. In accordance with previous postulations, serum level of pro- inflammatory cytokines were observed to be elevated in alloxan-induced diabetic rats in this study 30, 31. Data provided in this study revealed that serum pro-inflammatory molecules including IL-1β, IL-6 and TNF-α were upregulated in alloxan-induced diabetic rats. On the contrary, there was a corresponding reduction in serum concentrations of anti-inflammatory cytokines, IL-10.
Recently, the concentrations of these pro-inflammatory molecules were observed to increase correlate positively with the progression of neuropathy 32. These observations give an indication of a possible direct link between these inflammatory markers and glomerular damage. According to Akdis et al. 33. IL-10 produced from CD4+ Th2 cells otherwise known as cytokine synthesis inhibitory factor, inhibits the synthesis of pro-inflammatory cytokines including IL-4 and IL-5 (from Th2 cells) and the production of IL-2 and IFN-γ by Th1 cells. Thus, the anti-inflammatory role of IL-10 involves its inhibitory actions on both adaptive and innate immune cells 34. It is viewed that the synthesis and secretion of these fibrogenic cytokines in the local microenvironment may be responsible for the observed damage in renal architecture 35. In addition, the potentials of these molecules in recruiting circulating while blood cells and their migration into the kidney tissue have also be pinpointed as possible risk factor in diabetic neuropathy.
The histological examinations of the intestine, heart, liver and kidney revealed intact architecture in all the tissues across the various treatment groups except in the kidney where varying degree of interface hepatitis and glomerulosclerosis was observed in the alloxan-induced diabetic rats. However, APPE at 100 and 200 mg/kg body weight abrogated these abnormalities and restored the kidney architecture. These findings further reinforced the claim that diabetic neuropathy is the most prominent feature of diabetic complications.
Based on findings from this study, it is posited that the concerted effects of sustained hyperglycemia (as a consequence of alloxan-destruction of pancreatic beta cells) and the eventual generation of keto-aldehydes leading to the production of the highly potent free radical, H2O2 as well as glycosylation of blood protein in hemoglobin could have been responsible for the stimulation, recruitment and release of pro-inflammatory cytokines (IL-1β, IL-6 and TNF-α) thus leading to the observed kidney damage. Pro-inflammatory cytokines such as TNF-α and IL-1β have been reported to trigger the NF-κB pathway thus mediating the phosphorylation of serine molecules in insulin receptor substrate 36. The phosphorylation of insulin receptor substrate is thought to potentiate insulin resistance 37. However, these negative effects were remarkably abrogated by avocado peel polyphenolic extract especially at high doses (especially at 200 mg/kg body weight) via its inhibitory effects on carbohydrate hydrolyzing enzymes 22 as well as its antioxidant activity in preventing the oxidation of glucose to ketoaldehydes and generation of reactive glycated proteins.