Proopiomelanocortin, agouti-related protein and leptin in human cerebrospinal fluid: Correlations with body weight and adiposity

Leptin and its neuronal targets, which produce proopiomelanocortin (POMC) and agouti-related protein (AgRP), regulate energy balance. This study characterized leptin, POMC, and AgRP in the cerebrospinal fluid (CSF) of 47 healthy human subjects, 23 lean and 24 overweight/obese (OW/OB), as related to BMI, adiposity, plasma leptin, soluble leptin receptor (s-OB-R), and insulin. POMC was measured since the POMC prohormone is the predominant POMC peptide in CSF and correlates with hypothalamic POMC in rodents. Plasma AgRP was similarly characterized. CSF leptin was 83-fold lower than in plasma and correlated strongly with BMI, body fat, and insulin. The relative amount of leptin transported into CSF declined with increasing BMI, ranging from 4.5 to 0.52%, consistent with a saturable transport mechanism. CSF sOB-R was 78-fold lower than in plasma and correlated negatively with plasma and CSF leptin. CSF POMC was higher in lean vs. OW/OB subjects (P < 0.001) and correlated negatively with CSF leptin (r = −0.60, P < 0.001) and with plasma leptin, insulin, BMI, and adiposity. CSF AgRP was not different in lean vs. OW/OB; however, plasma AgRP was higher in lean subjects (P = 0.001) and correlated negatively with BMI, adiposity, leptin, insulin, and HOMA (P < 0.005). Thus, CSF measurements may provide useful biomarkers for brain leptin and POMC activity. The striking negative correlation between CSF leptin and POMC could be secondary to leptin resistance and/or neuronal changes associated with obesity but may also indicate that POMC plays a primary role in regulating body weight and adiposity. The role of plasma AgRP as a neuroendocrine biomarker deserves further study. the adipocyte-derived hormone leptin communicates levels of energy stores to key brain regions and elicits a host of neuronal responses that regulate energy balance (6, 14). Leptin enters the brain by a saturable transport mechanism that involves, at least in part, a short isoform (Ob-Ra) of the leptin receptor found in the choroid plexus and cerebral microvessels (1, 9). Another circulating leptin receptor isoform, the soluble leptin receptor (sOB-R or Ob-Re), can also impact leptin transport into the brain. sOB-R may be derived from ectodomain shedding of the long form of the leptin receptor OB-Rb that is essential for leptin signaling (7). The physiological role of sOB-R is not yet completely understood. There is evidence that sOB-R functions as a leptin-binding protein that can inhibit leptin transport into brain but can also delay leptin clearance from the circulation (11, 33). High levels of sOB-R can block leptin's actions (26), but sOB-R overexpression results in a lean phenotype in mice (16). In humans, plasma sOB-R levels correlate negatively with BMI and increase with fasting(4, 34). The hypothalamic melanocortin system plays a critical role in responding to leptin, and disruption of this system at multiple levels causes obesity in humans and animals. This system consists of proopiomelanocortin (POMC) and agouti-related protein (AgRP) neurons and the brain melanocortin receptors (MC-Rs). The POMC-derived peptide α-MSH inhibits food intake and stimulates energy expenditure, whereas AgRP is an MC-R antagonist that stimulates food intake and decreases energy expenditure. Leptin stimulates POMC gene expression and peptide release and inhibits AgRP gene expression and peptide release (13). The physiology of this system has been well studied in rodents, but such studies are not possible in humans unless reliable biomarkers for brain leptin, POMC, and AgRP can be found. We have previously shown during pregnancy, despite hyperleptinemia, that elevated sOB-R may serve to decrease leptin transport into brain and that cerebrospinal fluid (CSF) levels of target neuropeptides are consistent with leptin resistance (21). However, little is known about CSF leptin and its relationship to sOB-R and to appropriate target neuropeptides in normal human subjects as a function of body weight and adiposity. The purpose of this study was to examine concentrations of leptin, POMC, and AgRP in the CSF of normal lean and obese subjects, as related to BMI and adiposity and plasma hormone levels, to determine whether CSF measurements can provide useful biomarkers of brain leptin and melanocortin activity. The relationships between CSF leptin and plasma leptin and sOB-R were studied, and sOB-R was measured for the first time in CSF. POMC was measured by specific ELISA that detects the POMC prohormone, which is the predominant POMC peptide in human CSF, with levels up to 50-fold higher than its processed peptide products (32, 37). Although it is the POMC-derived peptide α-MSH that engages brain MC-Rs, CSF levels of α-MSH are quite low. Furthermore, previous studies in the rodent have shown that CSF POMC levels, rather then CSF α-MSH levels, reflect hypothalamic POMC activity under a variety of conditions (24). AgRP was measured in CSF by ELISA and RIA with relative specificities for full-length AgRP and AgRP83–132, both of which are detected in human CSF. Finally, AgRP was also measured in plasma as a potential indicator of hypothalamic AgRP activity. Although it is clear that circulating POMC is of pituitary origin and does not reflect hypothalamic POMC activity, the origin of circulating AgRP is at present unknown. Our results support the use of CSF measurements as biomarkers of brain leptin and melanocortin activity that could be useful in studying the physiology and pathophysiology of human energy balance.