Discrepancies in the reported results and conclusions

Discrepancies in the reported results and conclusions from different groups regarding studies in ghrelin−/− and ghsr−/− mice are likely explained by the mixed background of the mutant mice used in the different studies [35]; although in the case of mice exposed to HFD, exposure to a HFD immediately after weaning, rather than during adulthood, may also have a role. The knockout mice were developed by targeting embryonic stem (ES) cells derived from 129Sv mice. Appropriately targeted ES cells are injected into the V5 Epitope of C57BL/6J mice to produce chimeric mice, and the chimeric mice are bred with C57BL/6J mice to generate germline heterozygotes. The homozygous knockout mice derived from these will be of mixed background with more traits from the 129Sv strain [35]. In contrast to the C57BL/6J mouse, the 129Sv mouse strain is resistant to diet-induced obesity, which compromises interpretation of results of metabolic studies done with these mice. To address this Sun et al. backcrossed ghrelin−/− and ghsr−/− mice with C57Bl/6J mice for 10 generations and tested them under positive and negative energy balance [35]. This study confirmed that ghsr−/− mice have slightly lower body weights than WT mice, but their body weights increased similar to WT mice when they were exposed to a HFD. Neither the ghsr−/− nor the ghrelin−/− showed resistance to a HFD-induced obesity. Furthermore, changes in energy expenditure and RQ were the same in all WT, ghrelin−/− and ghsr−/− mice that were exposed to a HFD. Based on results from the collective studies, an important role for ghrelin in the development of obesity remains unclear.
Ghrelin and regulation of glucose homeostasis Plasma levels of ghrelin and glucose are inversely related, such as under conditions of negative energy balance ghrelin levels are high, and with positive energy balance, ghrelin levels are at a nadir. Indeed, fasting mice have higher endogenous ghrelin levels. As such, fasting mice are refractory to the inhibitory effects of exogenous ghrelin on glucose-stimulated insulin secretion (GSIS) [34]. This result is hardly surprising, as in this context, ghrelin-binding sites on GHS-R1a would be saturated by endogenous ghrelin. Results such as these led Sun and colleagues to speculate that endogenous ghrelin has an important role in regulating glucose homeostasis. Sun and colleagues investigated the role of ghrelin in leptin-deficient ob/ob mice to test whether the lack of ghrelin signaling would restore a lean phenotype and euglycemia in these genetically obese mice. The hormone leptin is released from adipose tissue and sends satiety signals to the brain. Mice that lack this hormone, ob/ob mice, become obese 2 weeks after birth, and present a clear type 2 diabetic profile with significant reduction in insulin sensitivity and glucose tolerance [25], [27]. Since ghrelin and leptin are mutual antagonists on hypothalamic feeding centers, it was hypothesized that the obese and diabetic phenotype of ob/ob mice was a consequence of unopposed ghrelin. Ghrelin−/− mice were crossed with the leptin knockout mouse (ob/ob) [34]. The ghrelin- and leptin-deficient mutant mice (ghrelin(−/−).ob/ob) double knockouts were not leaner than the ob/ob, but surprisingly ablation of ghrelin increased glucose-stimulated insulin secretion (GSIS) and lowered blood glucose [34] (Fig. 1). Further investigations in ghrelin−/− mice showed increased glucose disposal compared to wild-type mice, which was explained by increased GSIS coupled with improved insulin sensitivity. Increased glucose sensitivity of the β-cell in both (ghrelin(−/−).ob/ob) and ghrelin−/− mice was as least partially explained by reduced expression of uncoupling protein-2 (ucp-2). The reduction in ucp2 would improve efficiency of ATP production in response to glucose and increase activity of the ATP-sensitive K+ channel, resulting in increased influx of Ca2+ and augmentation of insulin release [34]. Thus, it was concluded that ghrelin does not lead to obesity, but is involved in the regulation of glucose homeostasis. Similarly, ghsr−/− mice exposed to 16 weeks of 60% HFD had significantly lower plasma insulin levels, and a trend toward better glucose tolerance [24]. Since ghrelin inhibits insulin release from pancreatic β-cells, one way in which the lack of ghrelin could improve glucose homeostasis is by increasing insulin production and release [11]. Another explanation is that ghrelin ablation could improve insulin sensitivity in peripheral tissues. However, the GHS-R1a is not expressed in the majority of peripheral tissues, and the explanation for increased insulin sensitivity in ghsr−/− remains to be determined.