Hypothalamic leptin and insulin signaling pathways

Obesity research is now focusing on dysruption of neuronal pathways involved in energy homeostasis and feeding as one of the main causes of obesity and associated diseases. Insulin and leptin are two key hormones controlling satiety in the hypothalamus (1):

People new to nutrition who find information on low carbohydrate dieting seem to get confused by the fact that insulin stimulates satiety. After all, isn't insulin the evil fat promoting hormone that makes us ultimately fat? Discussing this issue is beyond of this post and interested readers are referred to this review (2). Stephan Guyenet has several posts about satiety and food reward. 

Low carbohydrate diets are known for decreasing plasma insulin levels as well as leptin. Leptin secretion seems to be correlated with insulin secretion, so carbohydrates are the main stimulus for increasing leptin levels (3,4,5). One might think that a very low carbohydrate ketogenic diet (VLCKD) would result in appetite stimulation and low satiety. This leads us to a study published by Park et al (6). 

They used two rodent models: 90% pancreatectomized diabetic (Px) and sham-operated non-diabetic rats (Sham). Rats were infused CSF or 50ug/h of beta hydroxybutyrate (bOHB) (Sigma) into the lateral ventricle for 3h. The hyperinsulinemic euglycemic clamp was performed with a continuous infusion of bOHB. This was the short-term part of the study.

Figure A shows glucose infusion rates and glucose uptake and figure B shows hepatic glucose output at baseline and clamped steady-state. Whole-body glucose infusion rates required to maintain normoglycemia were higher in the bOHB-Px rats, but nearly the same in bOHB-Sham rats, compared to CSF. Glucose uptake was basically the same in all groups, while hepatic glucose output during the clamp was reduced in bOHB-Px rats compared to CSF, but not in Sham rats. 

The long-term study involved Pax rats which were infused CSF or 12ug/h of bOHB into the lateral ventricle for 4 weeks. bOHB levels in Pax-bOHB rats were significately higher compared with CSF, while other metabolic parameters (body weight, epididymal fat pads, food intake, serum leptin, serum insulin) were not significantly different between groups (p<0.05). 

Glucose tolerance was assessed using an OGTT. bOHB rats had lower 30-60min serum glucose levels and lower glucose AUC than the CSF group (A). During the first part of the test (0-40min) insulin AUC was the same in both groups while during the second part (50-120min) was slightly lower for the bOHB group (B).

During the hyperglycemic clamp, peak serum glucose levels were lower on the bOHB group than on the CSF group (since serum glucose increased about 5.5mM from basal levels and these were lower in bOHB rats). Infusion of bOHB increased insulin secretion at 2 min compared to CSF, without differences in second-phase secretion. Insulin AUC was equal in both groups, and glucose infusion rates were higher for bOHB than CSF. Insulin sensitivity at the hyperglycemic clamp state was improved in bOHB rats but not in CSF rats. 

Whole body glucose disposal rates were lower in the bOHB group than in the CSF group without being differences in glucose uptake. As with glucose levels after an overnight-fast, hepatic glucose output was lower in bOHB rats. Because basal serum glucose levels were determined by hepatic glucose production and glucose utilization, and glucose uptake was similar between groups, the differences in glucose infusion rates and glucose uptake could be accounted by insulin-stimulated supression of hepatic glucose production. This reflects increased hepatic insulin sensitivity. As expected, hepatic glucose output during the hyperinsulinemic clamp was lower in bOHB rats, suggesting that ketone infusion was associated with improved hepatic insulin action. 

Now we get to the interesting part: insulin and leptin signaling. Results in the hypothalamus are shown below:

Two samples were used for each group for the immunoblotting assay, and values shown are the mean. As can be seen in the figure, infusion of bOHB potentiated tyrosine phosphorylation of IRS2 and serine phosphorylation of Akt. Expression of GLUT2 and glucokinase were increased. This suggests improved glucose sensing in the hypothalamus.

STAT3 phosphorylation was potentiated more in the bOHB group than in CSF, without differences in AMPK phosphorylation.

Confirming the metabolic parameters evaluated earlier and results in the hypothalamus, insulin signaling was also potentiated in the liver:

Tyrosine phosphorylation of IRS2 and phosphorylation of Akt were increased in the bOHB group. Consistent with these results, PEPCK expression was reduced and GLUT2 and glucokinase expressions were potentiated, as well as phosphorylation of AMPK. 

Summing up

Central infusion of bOHB increased insulin and leptin signaling both in the hypothalamus and the liver, increasing leptin and insulin sensitivity. I think this has huge implications in the treatment of obesity. Insulin and leptin are not deleterious per se. The problems arise when plasma levels reach high enough to start causing metabolic complications and resistance by key tissues to these hormones. 

Additional information

Research on the effects of beta hydroxybutyrate and specifics on food intake regulation with ketogenic diets is scarce. Laeger et al. (7) have reviewed most information available, from which we can get the following conclusions:

- bOHB uptake by the brain is mediated by two forms: difussion and a carrier mediated system by monocarboxylate transporters (MCT) and the sodium coupled MCT 1. 

- Brain bOHB concentration rises concomitantly with increasing plasma ketone body concentrations. The permeability of the BBB for bOHB increases with starvation and high-fat diets, and is reduced with age. 

- Studies examining the effects of bOHB on satiety and the hypothalamus have been done both by central infusion and peripheral infusion.

- bOHB-mediated appetite supression is dependent on the rat strain. In those sensitive, central infusion suppresses food intake and reduces body weight. Conversely, obesity resistant S 5B/P1 rats usually have higher blood ketone concentrations and a higher transport of bOHB across the BBB, leading to chronically higher brain bOHB levels compared with other strains. These rats do not show the same effects as other strains to bOHB infusion. 

- In other rat strains (like Sprague-Dawley), central infusion of bOHB results in loss of bodyweight without affecting blood glucose levels or glycogen content. Other studies have shown that after an initial increase in food intake, there is a long-term supression with an intraventricular infusion of bOHB. Postprandial intermeal interval was also prolonged. 

- Subcutaneous injection of bOHB has shown to reduce food intake under normal physiological conditions. This is also seen in insulin treated hypoglycemic rats receveing an intravenous bOHB infusion.

- In African pygmy goats, hypophagia appears to be related to the amount of bOHB injected intraperitoneally. This leads to a decrease in meal frequency and prolongation of latency to eat without diminishing the meal size. 

- In sham-vagotomized rats, subcutaneous injection of bOHB reduces food intake, but vagotomy of the common hepatic branch eliminated the hypophagic effect. It appears that because most of the common hepatic branch vagal afferent fibers originate in the proximal small intestine and the liver parenchyma barely contain vagal afferents, oxidation of bOHB in enterocytes is signaled to the brain where it is sensed by neurons to affect eating behaviour. This contrasts with earlier hypotheses that speculated that stimulated hepatic bOHB metabolism was recognized by vagal terminals. 

- Attributing reduced food intake to bOHB in studies with ketogenic diets is often difficult because it might include increased amounts of protein, which stimulates satiety.

Park S, Kim da S, & Daily JW (2011). Central infusion of ketone bodies modulates body weight and hepatic insulin sensitivity by modifying hypothalamic leptin and insulin signaling pathways in type 2 diabetic rats. Brain research, 1401, 95-103 PMID: 21652033