Obesity is associated with leptin resistance as evidenced by hyperleptinemia. Resistance arises from impaired leptin transport across the blood-brain barrier (BBB), defects in leptin receptor signaling, and blockades in downstream neuronal circuitries. The mediator of this resistance is unknown.
Here, we show that milk, for which fats are 98% triglycerides, immediately inhibited leptin transport as assessed with in vivo, in vitro, and in situ models of the BBB. Fat-free milk and intralipid, a source of vegetable triglycerides, were without effect.
Both starvation and diet-induced obesity elevated triglycerides and decreased the transport of leptin across the BBB, whereas short-term fasting decreased triglycerides and increased transport.

Three of four triglycerides tested intravenously inhibited transport of leptin across the BBB, but their free fatty acid constituents were without effect. Treatment with gemfibrozil, a drug that specifically reduces triglyceride levels, reversed both hypertriglyceridemia and impaired leptin transport.
We conclude that triglycerides are an important cause of leptin resistance as mediated by impaired transport across the BBB and suggest that triglyceride-mediated leptin resistance may have evolved as an anti-anorectic mechanism during starvation.

Decreasing triglycerides may potentiate the anorectic effect of leptin by enhancing leptin transport across the BBB.
Leptin is a 16-kDa protein secreted by fat cells (1) that regulates feeding and energy expenditures by acting at sites primarily within the central nervous system (2–4). Obesity in humans and rodents is almost always associated with a resistance to, rather than a deficiency of, leptin (5–7).

Resistance is associated with impaired transporter, receptor, postreceptor, and downstream neuronal circuitry functions in animal models of obesity (9–13).
Leptin is transported across the blood-brain barrier (BBB) by a saturable transporter (8), and impaired transport can be acquired, may precede receptor/postreceptor defects, worsens with increasing obesity, and is to some extent reversible (14–16).

The relation between cerebrospinal fluid and serum levels of leptin in obese humans (17,18) suggests that defective BBB transport accounts for more of the overall resistance to leptin than the receptor/postreceptor defects (19).
The obesity-related defect in leptin BBB transport has two aspects (10). First, circulating substances cause an immediate impairment.

Leptin itself, which is elevated in obesity, is likely one of these circulating substances. Second, an unidentified mechanism impairs transport in obese mice even when BBB transport is assessed by brain perfusion, a method that removes the immediate effects of blood-borne substances.
Fasting or leptin administration can partially reverse these defects in leptin transport (16).

Starvation, like obesity, is accompanied by a decreased BBB transport rate of exogenous leptin (20). Whereas it is difficult to explain the evolutionary advantage of decreased leptin transport in obesity, an advantage is obvious in starvation.
Decreasing the amount of the anorectic protein reaching the central nervous system should enhance the drive for seeking food.

The mechanism of the starvation-induced impairment in transport is unknown but cannot be caused by leptin itself because its levels decrease with fasting (21).
Here, we postulate that triglycerides may underlie the impairment in BBB transport in both obesity and starvation.

Triglycerides are decreased with fasting but are elevated with starvation and tend to be elevated with obesity.
Supporting this hypothesis is the observation that mice with impaired triglyceride synthesis are protected against development of both diet-induced obesity and obesity-induced leptin resistance (22).

Thus, hypertriglyceridemia could explain impaired transport of leptin across the BBB in both starvation and obesity.
Obesity is associated with leptin resistance caused by impaired leptin transport across the BBB, defects in leptin-receptor signaling, and blockades in downstream neuronal circuitries.
The inability of obese mice to respond to peripherally administered leptin while responding to centrally administered leptin is likely caused by a defect in leptin transport across the BBB. It is unclear what causes defective transport of leptin in either obesity or starvation.

Because serum triglycerides are elevated in both starvation and obesity, we postulated that triglycerides inhibit leptin transport across the BBB.
Here, we showed that starvation-induced inhibition of leptin transport was caused by a circulating factor; that the fat component of milk (which is 98% triglycerides) as well as specific triglycerides could induce inhibition of leptin transport across the BBB in vivo, in situ, and in vitro; that the FFAs comprising those triglycerides were ineffectual

That manipulation of triglyceride levels with diet or fasting in normal or obese mice had an inverse effect on leptin transport; and that reduction of triglycerides by pharmacological intervention reversed the impairment in leptin transport.
Taken together, these findings show that triglycerides directly inhibit the transport of leptin across the BBB and so could be a major cause of leptin resistance at the BBB.

We fasted mice for 48 h to determine whether starvation impairs the transport of intravenously administered I-Lep. We confirmed that short-term fasting decreased serum triglyceride levels, whereas 48 h of fasting (starvation) increased them.

The level of reduction in the brain uptake of intravenous administered I-Lep was almost identical to the results found by others (20).
That group further showed that longer fasts progressively impaired leptin transport across the BBB to the point of total inhibition after 5 days of starvation. This inhibition of leptin transport and the accompanying decrease in levels of leptin in the blood (21) are likely adaptive because they would reduce the anorectic signal in starvation.
In contrast to intravenous injection, brain perfusion found no difference in leptin uptake between starved and fed mice.
Because the intravenous and brain perfusion methods usually give similar results except when a circulating factor has an acutely reversible effect on transport (31–33), the results show that starvation inhibits leptin transport by releasing a blood-borne factor.
We more directly tested triglycerides by injecting bovine whole milk or intralipid into the peritoneal cavity.
The fat in milk is 98% triglycerides (34), whereas the fat in intralipid is a soybean oil-based source of triglycerides containing the essential FFAs linolenic and linoleic acid, purified egg phospholipids, and glycerol.
Milk increased serum triglyceride and leptin levels by ∼40% and produced an immediate long-lasting impairment in leptin transport across the BBB.
Serum triglycerides showed a time-dependent decline during the course of the study in both milk- and vehicle-injected animals, probably related to diurnal rhythm.
However, at those times when leptin transport was inhibited, serum triglycerides in milk-injected animals were higher than the vehicle-injected animals’ highest value (baseline).
The increase in serum leptin levels was likely produced by the mouse because pasteurization significantly reduces leptin levels in milk (35), and our immunoassay was species specific.
The increase in serum leptin from ∼4.5 to 6.5 ng/ml during the period of leptin inhibition is likely too low to explain the inhibition in leptin transport.
Previous work shows that this would result in only about a 10% decrease in the leptin transport rate (33).
Additionally, leptin levels were highest at 6 h, a time when transport was no longer significantly inhibited. In fact, the 6-h serum leptin level for vehicle-treated mice was a little higher than the serum leptin level in milk-treated mice at 2 h, the time of greatest inhibition in leptin transport.
Milk also inhibited leptin transport in the in situ brain perfusion model and in the in vitro brain monolayer model of the BBB, conditions where leptin secretion could not occur.
In comparison to milk, intralipid was without effect, suggesting that plant triglycerides and essential FFAs do not inhibit leptin transport.
Milk given intravenously was immediately effective at less than one-tenth the intraperitoneal dose.
The immediacy of effect after intravenous injection suggests that triglycerides rather than a degradation product (e.g., FFAs) affected transport.
Nonfat milk, which contains the same concentration of proteins and phospholipids as whole milk and has only the triglycerides removed (34), was without effect.
These results show that inhibition was not caused by leptin remaining in pasteurized milk (35,36).
They also show that animal-derived triglycerides impair leptin transport across the BBB, but not essential FFAs, plant-derived triglycerides, or milk proteins.
We directly tested the ability of triglycerides to inhibit leptin transport across the BBB. Three of four commercially available triglycerides (triolein, DPOG, and DSOG) inhibited uptake of I-Lep when injected intravenously at a dose that equaled the total triglyceride content of milk (Fig. 6A).
A dose-response curve suggests that, at least in the case of triolein, lower doses are also effective. DMOG, the triglyceride that did not inhibit leptin transport, illustrates that the sn-1 position is important for the inhibitory effect.
Myristate, as a medium-chain FFA, is only produced in by mammary alveolar cells; therefore, triglycerides containing it may not reflect diet or obesity (37).
Additionally, it would not be expected to circulate in significant amounts in blood. These results suggest that leptin transport will be inhibited by triglycerides endogenous to blood.

We ruled out the possibility that FFAs hydrolyzed from the triglycerides were inhibiting leptin transport. The FFAs (palmitate, stearate, and oleate) that could be hydrolyzed from the triglycerides were without effect at doses that would have produced blood levels higher than those seen in starvation.
We also tested oleate at the same dose as triolein, but found it was without effect. Because the molecular weight of triolein is only ∼75% fatty acid with the remainder comprised of the glycerol backbone, we tested oleate at a molarity at least 30% higher than could be achieved with total hydrolyzation of triolein. This shows that the triglycerides themselves and not the FFAs derived from them are responsible for inhibiting leptin transport.
We tested the pathophysiological relevance of hypertriglyceridemia by studying the effects of dietary-induced obesity on the relation between triglycerides and I-Lep uptake.

We also tested the ability of a 16-h fast to affect the relation between triglycerides and I-Lep uptake in these groups of mice. As Fig. 7 illustrates, diet-induced obesity increased triglycerides and reduced I-Lep uptake by the brain. In both lean and diet-induced obese mice, fasting reduced triglycerides and increased I-Lep uptake by the brain.
Gemfibrozil is selective for reduction of serum triglyceride levels and is used clinically for the treatment of hypertriglyceridemia.
Short-term administration of gemfibrozil reduced triglyceride levels to <100 mg/dl in four of six mice (Fig. 8). These responders had a statistically significant increase in leptin transport in comparison to mice fed vehicle (P < 0.05).
This showed that reduction of triglyceride levels by pharmacological treatment could enhance leptin transport across the BBB.
These results show that serum triglycerides have a rapid and immediate effect on the transport of leptin. As such, they explain the inhibition in leptin transport seen with starvation.
They also likely contribute to the inhibition seen with obesity. Triglycerides could produce their effect on leptin transport by binding leptin in the circulation or by acting directly on the leptin transporter.

Other BBB transporters are known to be regulated by uncompetitive and noncompetitive mechanisms (38,39), and leptin transport is altered by α1-adrenergic agonists, glucose, and insulin (40,41). It may be that the leptin transporter possesses a regulatory site controlled by triglycerides.
The ability of triglycerides to inhibit leptin transport into the brain completes a negative feedback loop between leptin action and triglycerides. Leptin promotes triglyceride hydrolysis and FFA oxidation and inhibits FFA synthesis (42,43,44), therefore decreasing triglyceride levels.
The importance of leptin in reducing triglyceride levels is dramatically illustrated in patients with lipodystrophy and lipoatrophy. These patients have little or no fat mass and, as a result, have little or no leptin.
They also have very severe hypertriglyceridemia that is reversed by treatment with leptin (45). The ability of triglycerides to induce leptin resistance would counter the leptin-induced shift toward use of triglycerides as an energy source and so help to conserve fat stores.
This would make evolutionary sense because hypertriglyceridemia has probably more often represented starvation than obesity.
Healthy baboons living in the wild have fat stores and serum leptin levels that are a fraction of those seen in Western humans and laboratory animals (46), but when supplied with abundant calories, develop a condition resembling the metabolic syndrome X (47), including the development of hyperlipidemia (48).
These studies in wild baboons are consistent with the hypothesis that ancestral levels of leptin were much lower than those seen in Western civilization and that starvation was a more probable threat than obesity.

The usefulness of leptin resistance in obesity is less clear than its obvious utility in starvation. Starvation-induced hypertriglyceridemia may have been so dominant an evolutionary pressure that leptin resistance induced by obesity-related hypertriglyceridemia was never selected against.
Alternatively, it may be that the anorectic effect of leptin must be overridden to maintain an adequate intake of water-soluble vitamins, minerals, electrolytes, and other substances less efficiently stored than fat.
Our results also provide a mechanism to explain previous findings of why mice unable to synthesize triglycerides are more sensitive to leptin (22).
Mice lacking acyl coenzyme A:diacylglycerol acyltransferase 1, a critical enzyme needed to synthesize triglycerides, are more sensitive to infusions of leptin.
Without this enzyme, obesity did not develop in a strain of mice normally resistant to leptin but did in a strain that is leptin deficient.
This shows that leptin is critical to the mechanism by which lack of triglycerides protects from diet-induced obesity. The results presented here provide one mechanism by which lowering triglycerides can increase leptin sensitivity.
In conclusion, these studies show that serum triglycerides impair the ability of the BBB to transport leptin.
Triglycerides are likely a major cause of the leptin resistance seen in both starvation and obesity (9,10,16,20). Lowering triglycerides may be therapeutically useful in enhancing the effects of leptin on weight loss.

This study was supported by VA Merit Review R01 N541863 and RO1 AA12743.
The authors thank Dr. Harold M. Farrell, Jr., Agricultural Research Service, U.S. Department of Agriculture, for help in determining the fat content of milk.

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