Our results show that the intraperitoneal administration of STZ to rats significantly increased glucose blood levels four days after injection, as well as decreased body weight. In addition, other diabetes-related signs were observed. These results agree with previous observations that have employed this model and that also report loss of body weight [13, 14].
Several reports suggest that this model of type 1 diabetes induced by STZ is adequate to evaluate the properties of leaves or fruits from different plants [15–17]. In the present study, the aqueous extract of C. papaya maintained the body weight of diabetic rats during treatment. Weight loss is a main sign of diabetes but its mechanism is not clear. It could be due to many factors such as loss of appetite, increased muscle waste and loss of tissue proteins [15, 18, 19]. In addition, the administration of the aqueous extract of C. papaya decreased water consumption. Other natural products also produce a decrease in both water intake and urine volume excretion, indicating that these products can regulate water balance and which is known to depend on the kidney function [18, 20]. In this study, however, kidney function biomarkers were not determined.
This study showed that the C. papaya leaf aqueous extract significantly diminished blood glucose levels (p<0.05) in diabetic rats. This hypoglycemic effect is similar to the one reported for other plants [17, 18, 21, 22]. Such effect may be explained in part by either a decrease in the rate of intestinal glucose absorption [23–25] or an increase in peripheral glucose utilization [24, 25]. In this line, some authors have ascertained increased catabolism of glucose due to GLUT4 translocation to the plasma membrane in muscle and brown adipose cells [19, 20], with up-regulation of the uncoupling protein-1 in brown adipose tissue and hepatic gluconeogenesis [25, 26], causing as a result hyperinsulinemia or enhancement of peripheral glucose utilization [19, 27, 28]. Moreover, a possible stimulatory mechanism on the few surviving β-cells has been considered, which could allow the release of more insulin [29–32]. Our results suggest that the aqueous C. papaya leaf extract may act by stimulating the few remaining β-cells with the subsequent release of more insulin, instead of pointing to the regeneration of β-cells of the islets as responsible for the insulin increase (Figure 1e).
Different reports have shown that the islets appear to be preferentially affected in diabetes by destruction of insulin-secreting β-cells [29, 31, 33]. In this respect, the damage to islets in diabetic rats treated with C. papaya extract was reduced. There are reports suggesting that in diabetic rats, the administration of plant extracts can be effective in cell regeneration and restoration of islet size, even producing cell hyperplasia [29, 32, 33]. β-cells have shown a remarkable potential for regeneration at the pre-clinical stage of diabetes which is a key question when addressing type 1 diabetes [29, 30]. In addition, some authors using a pancreatic islet cell line have reported that plant extracts play a role in stimulating insulin secretion in β cells [20, 22, 32, 33]; however, this does not seem to be the case for insulin-producing β-cells deteriorated by STZ.
We also found that insulin levels were diminished in diabetic rats compared to non-diabetic animals. This result is consistent with reports from other authors using STZ to induce type 1 diabetes in which they observed that STZ depletes insulin production by pancreatic β-cells [33, 34]. Several authors suggest that the hypoglycemic or antidiabetic effect of some natural extracts can be attributed to their insulin-trophic effect that enables the reduction of blood glucose levels, liver glycogen content, and serum lipids through the control of serum insulin [20, 30]. Interestingly, our results show that C. papaya extract increased insulin production, with no significant hypoglycemic effect on non-diabetic rats.
Our results demonstrated that triacylglycerol levels decreased in diabetic rats with the administration of C. papaya extract. The hyperlipidemia associated with diabetes may result from an accelerated hepatic triglyceride biosynthesis and the release of VLDL without an increase in its rate of clearance from the blood by lipoprotein lipase, which is dependent on the insulin/glucagon ratio [27, 30]. Furthermore, the increase in lipid profile may be a consequence of increased lipids breakdown and mobilization of free fatty acids from peripheral deposits [30, 32]. This biochemical evidence for the hypotriacylglycerolemic and hypoglycemic actions of C. papaya extract is supported by the improvement in the histological features of fat (Figure 3a) and glycogen (Figure 3b) content in hepatocytes of diabetic rats. The C. papaya extract decreased liver glycogen content in diabetic rats (Figure 3d,f). This diminution was not due to the action mediated by insulin because insulin levels were not increased in diabetic rats treated with C. papaya. It is known that in diabetes the activation of the gluconeogenic enzymes may also be due to the state of insulin deficiency given under that condition; insulin functions as a suppressor of gluconeogenic enzymes [33, 35]. The inhibition of glucose intestinal absorption could have caused the significant reduction in liver glycogen. However, a direct effect of C. papaya extract on the activities of hepatic gluconeogenic enzymes cannot be ignored and need further investigation. Additionally, the consumption of herb extracts in diabetic rats reduced blood glucose levels and increased glucose tolerance not only by promoting insulin sensitivity, but also by reversed activity of hepatic enzymes in diabetic rats near to normal levels, through mechanisms that involve gluconeogenesis [35–37]. Several studies have revealed that increased total lipid, triglycerides and total cholesterol levels in the serum of diabetic rats are also found in liver and kidney [27, 37]. Previous studies have reported that some phytocomponents, particularly saponins and steroids, elicit antihyperlipidemic action by inhibiting intestinal lipid absorption via resin-like action and inhibition of lipase activity [23, 27, 30]. On the other hand, the elevation of serum biomarker enzymes such as ALT, AST and ALP has been observed in diabetic rats indicating impaired liver function that may be due to hepatic damage induced by hyperglycemia [32–35]. In the present study, our results showed that C. papaya treatment produced a decrease in serum aminotranferases in diabetic rats. Liver damage in diabetic rats (Figure 3a and b) was confirmed, as well as improvement in hepatocyte morphology after the C. papaya treatment (Figure 3c to f). Moreover, reduced levels of total and direct bilirubin concentrations were observed in diabetic rats treated with C. papaya. Besides, biochemical evidence indicates that the increment in bilirubin concentration is generated by an enhanced liver function and muscle wasting. Furthermore, diabetes itself can induce injury to the bile ducts and cause muscle damage [22, 23]. Similar results have been reported after the administration of Croton cajura extract in diabetic rats showing a decrease in other biochemical markers (transaminases, nitrogen and antioxidant enzymes in serum). This reduction led to a recovery in the metabolism of the diabetic rats and prevented the development of diabetic complications [27, 32, 37]. Our results suggest that the aqueous extract of C. papaya at low doses (0.75 and 1.5 g/100 mL) regulates bile transit and hepatic function in diabetic rats, but at high doses it can be hepatotoxic (3 g/100 mL). In this respect, there are reports of liver damage due to natural and drug treatments forcing discontinuation of treatment and the urgency of re-evaluating the pharmacokinetics and pharmacodynamics of these compounds [35–37].
In our study, there is evidence of a reduction in NO metabolites in diabetic rats [38, 39]. In addition, the administration of the aqueous extract of C. papaya to diabetic rats increased NO levels. As it is well known, diabetes is characterized by hyperglycemia and hyperlipidemia, two biochemical features associated with inhibition of endothelial nitric oxide synthase (eNOS), leading to diminished NO production, increased formation of reactive oxygen species (ROS), impaired endothelium-dependent relaxation, increased formation of free radicals and lower efficacy of antioxidant systems, which lead to an imbalance between free-radical formation and the protection against them [32, 38–40]. However, the presence of antioxidant molecules regulating NO production generates a diminution in oxidative stress [38–40]. Several studies have reported that medicinal plant extracts have flavonoids, saponines and polyphenols that increase the activity of antioxidative systems [40, 41]. This antioxidant effect of plant extracts decreases the oxidative stress generated by diabetes, resulting in a reduced or delayed progression of the endothelial degeneration, nephropathy and neuropathy [38–41]. In this sense, the antidiabetic effect of C. papaya extract can be due to its content of chemical constituents responsible for antioxidant actions.
Data are preliminary on the hypoglycemic effect of Carica papaya leaves in streptozotocin-induced diabetic rats. This study have some limitations: a sample size with six animals in every group, a short period of study, the diabetes model correspond more to a type 1 diabetes than to type 2 diabetes, moreover the active metabolite in the C. papaya leaves was not identified. Further studies administering the extract for longer periods of time are necessary.
Taken altogether, these results show that the administration of the extract of C. papaya leaf induced a significant reduction in glucose and triacylglycerol plasma concentrations (0.75 and 1.5 g/100 mL). In addition, this extract exhibited an antioxidant action and was not hepatotoxic at low doses (0.75 and 1.5 g/100 mL). The suggested mechanism for C. papaya could be similar to that reported for some sterols which decrease the activity of lipid- and carbohydrate-hydrolyzing enzymes in the small intestine, thus reducing the conversion of disaccharides and triglycerides into absorbable monosaccharides and free fatty acids [23, 31]. Currently, there are few reports on the effect of papaya leaves in experimental diabetes. In recent years, the use of therapeutic phytoproducts has been consistent; however, multicenter, large-scale clinical trials are needed to evaluate their safety and efficacy, as well as their interaction with conventional drugs when administered simultaneously.