Nonalcoholic fatty liver disease (NAFLD) is common not only in Western countries where obesity prevalence is rising, but it is also being increasingly recognized in individuals with normal body mass index (BMI). There is little controversy about our incomplete knowledge of the pathogenesis of fatty liver. As the pieces slowly come together, it would be imprudent to rush to judgment about the relative importance or role of any single molecule. Case in point: leptin.
Leptin, an adipocytokine produced primarily in visceral adipocytes, is the gene product of the ob gene, identified in the mid 1990s.1 Leptin receptors are found primarily in the hypothalamus but also in vascular endothelial cells. Leptin's effects on the hypothalamus regulate hunger and energy expenditure. In humans, leptin levels correlate with percentage body fat and are correspondingly higher in women. In the genetically obese mouse model (ob/ob),1 a mutated gene results in little or no leptin production. These obese mice lose weight with leptin replacement, and hence, in that model, leptin is clearly involved in obesity.
The story in humans is different and what role leptin may play remains unclear. Unlike the animal models of obesity, obese humans have elevated leptin levels, indicating resistance to leptin rather than a deficiency.2 This is an important difference to be kept in mind when considering leptin and its receptor in the pathogenesis of NAFLD.
There is a large body of evidence implicating leptin in the scheme of NAFLD overall. It is generally accepted that insulin resistance is almost always present in NAFLD, particularly in nonalcoholic steatohepatitis (NASH).3,4 Serum leptin in multivariate analysis is related to insulin levels, fat mass, lean mass, and correlates strongly with insulin resistance in humans.5,6 In 47 biopsy proven cases of NASH, leptin levels were twice those found in controls matched for BMI, gender, and age. While there was no direct correlation with degree of inflammation or fibrosis, the authors concluded that leptin is likely involved in the pathogenesis of insulin resistance, and hence steatosis.7
There may be hormonal interactions resulting in differences between genders as leptin levels differ in men and women, even after adjusting BMI.8 In young Japanese adults with ultrasound confirmed steatosis and mean BMI of less than 25, higher leptin levels were found in women, with a strong correlation to visceral/subcutaneous fat ratio.9 Gender differences also appear to influence the circulating soluble form of the leptin receptor, which determines the free serum leptin index. In a study of 118 healthy young adults, the leptin receptor levels appeared to be regulated by gender, adiposity, and hormone levels.10 These studies raise further questions about the role of sex hormones, degree and location of adiposity, and the role of circulating soluble leptin receptor in addition to leptin itself. Visceral adiposity appears to be the metabolically active store of fat cells, often quantified by waist-to-hip ratio measurements.
Not all published studies have shown these aforementioned relationships. Chalasani et al11 compared 26 biopsy-proven NASH patients to 20 nonbiopsied controls and found no difference in leptin levels between the two groups. However, no relationship between leptin and insulin resistance was found either, making it difficult to understand if this were truly a representative group.11 The correlation of leptin to subcutaneous fat in that study was also unexpected, since most other studies point to visceral fat as the metabolically active adipocytokine producing fat. In this issue of Journal of Clinical Gastroenterology, Liangpunsakul and Chalasani compared 288 individuals from the NHANES III database with serum leptin levels and unexplained alanine aminotransferase (ALT) abnormalities with 720 matched controls with normal ALT.12 The elevated ALT in the study group was presumed due to NAFLD, since other causes of liver disease were ruled out, including alcohol abuse. Leptin levels did not significantly differ between groups. Since there was no difference in serum leptin levels between the presumptive NAFLD patients with elevated ALT and normal ALT controls, they concluded that leptin was likely uninvolved in the pathogenesis of NAFLD. However, using multivariate analysis, the ALT level was indeed a predictor of serum leptin levels in men, but not women. Again, this gender-specific finding comes into play. This corroborated what was demonstrated by an independent assessment of the same NHANES database by Ruhl and Everhart, showing that leptin levels were indeed associated with elevated ALT levels.13 Thus, leptin may have a gender-specific association with ALT levels.
While this paper is interesting, it raises some concerns about how we deduce pathophysiologic mechanisms from a retrospective analysis of epidemiologic data. The very fact that only 288 patients of the much larger cohort with elevated ALT were assessed suggests a selection bias. Moreover, the NHANES III database has important limitations. The most obvious is that none of the patients has confirmed NAFLD or NASH but rather a presumed diagnosis based on elevated ALT. There is a risk in assuming that all unexplained ALT elevations are due to NAFLD, but more importantly, normal ALT individuals may also have NAFLD, both in children and adults, as other recent studies have shown.14,15 To further demonstrate that elevated ALT may be a poor marker of NAFLD, a study of 32 individuals with biopsy-proven NAFLD revealed that leptin levels were elevated in those with normal ALT and was negatively correlated with elevated ALT.16 Even in the presence of normal ALT, advanced fibrosis may exist in the setting of steatosis or NASH.15 The relationship between aminotransferases and NASH, or presence of fibrosis, has not yet been established. While it is recognized that NAFLD can lead to cirrhosis, the mechanism of how this occurs is not understood. When one considers that leptin appears to be a profibrogenic adipocytokine in the mouse ob/ob model, only more questions are generated about its role in humans.17-19 Leptin, either alone or through its interactions with other cytokines, may not only have a role in steatosis but development of fibrosis.
So where does this leave us? Several animal and human studies with confirmed diagnosis of NAFLD through histologic assessment suggest that serum leptin levels are elevated and hence likely involved in the pathogenesis of NAFLD. At the very least, leptin is associated with hyperinsulinemia, fat mass, visceral adiposity, and in some cases BMI. These are the same risk factors for NAFLD. In this retrospective evaluation of individuals with elevated ALT versus matched controls with similar BMI but normal ALT, leptin levels differed in only the men. While this is notable information, it does little to elucidate the pathogenesis of NAFLD. It does highlight the importance of first making the diagnosis before assigning the study group and the control group.
This then leads to the inevitable discussion of how to diagnose NAFLD. ALT elevations alone in the setting of elevated BMI will not be enough. Individuals with normal ALT may have NAFLD as well. The need to distinguish steatosis from NASH is an important one when formulating mechanisms of pathogenesis. This will limit how the NHANES III database can be used. Although difficult to achieve, the study and control groups of future studies both need confirmation of liver histology.
The role of leptin remains enigmatic in NAFLD. Serum measurements of any peptide cannot establish causality or lack thereof in a cross-sectional analysis. Based on serum levels in this epidemiologic database, leptin cannot be quickly dismissed as a noncontender. Whether or not it is involved in the NAFLD puzzle is not the question: it is. What role it plays and how significant it is, that is the real question. What is clear, however, is that we must standardize how we diagnose NAFLD, dismiss the notion that only those with elevated ALT have NAFLD, and have well-defined study and control groups when prospectively assessing variables thought to be involved in the pathogenesis of NAFLD.
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