Of mice and (wo)men: the obesity (ob) gene, its product, leptin, and obesity

Short-term effects of leptin on skeletal muscle protein metabolism in the rat

We have examined the short-term effects of leptin on protein metabolism in the rat. Indeed, an intravenous leptin administration (100 microg/kg body weight), which resulted in no changes in circulating insulin in the time interval studied, induced a decrease in the incorporation of (14)C-leucine to (14)C-skeletal muscle protein. No changes were observed in relation to muscle protein degradation (either measured in vivo following isotope preloading or in vitro as tyrosine released into the incubation medium) and gene expression associated with the different proteolytic systems (cathepsin B, m-calpain and ubiquitin-proteasome system). The effects of leptin on amino acid incorporation into muscle protein do not seem to be direct because incubation of isolated EDL muscles in the presence of 10 microg/ml of leptin did not modify either the protein incorporation or the oxidation of (14)C-leucine. It may, therefore, be suggested that leptin is able to influence protein synthesis in skeletal muscle through the action of an unknown mediator.

Short-term effects of leptin on lipid metabolism in the rat

In this study, we have examined the short-term effects of leptin on lipid metabolism in the rat. Acute leptin administration induced hypertriglyceridaemia (31% increase in plasma triacylglycerols) which was not associated with changes in lipoprotein lipase activity in white adipose tissue. Surprisingly, leptin administration did not induce any changes in the lipogenic rate in either white adipose tissue or liver. Leptin administration caused a decreased tissue uptake of exogenous 14C-triacylglycerols. These data suggest that leptin induces important changes in lipid uptake in adipose tissue and skeletal muscle which could be responsible for the observed hypertriglyceridaemia.

Development of obesity and insulin resistance in the Israeli sand rat (Psammomys obesus). Does leptin play a role?

The Israeli Sand Rat (Psammomys obesus) is an excellent polygenic model for the study of obesity and diabetes. The metabolic characteristics and the heterogeneous development of these defects, including elevated leptin levels, mimic those found in susceptible human populations. Interestingly, only animals that develop metabolic abnormalities demonstrate hyperleptinemia and, in these animals, leptin administration at the same dose that is effective in ob/ob mice is ineffective in reducing food intake or body weight. Perhaps leptin resistance needs to develop in Israeli Sand Rats to allow the development of obesity and, in fact, leptin resistance may be the "thrifty gene" that predisposes individuals to the development of obesity and subsequent metabolic abnormalities. However, there remain many unanswered questions about the physiological actions of leptin. The widespread tissue location of receptors and the actions of leptin independent of food intake highlight the need for further research aimed at determining the major physiological action of this newly discovered and exciting hormone.

The global epidemiology of non-insulin-dependent diabetes mellitus and the metabolic syndrome

Non-insulin-dependent diabetes mellitus (NIDDM) constitutes about 85% of all cases of diabetes in developed countries and it has now reached epidemic proportions in many developing nations, as well as disadvantaged groups in developed countries, e.g., Mexican- and African-Americans and Australian Aborigines and Torres Strait Islanders. The diagnosis of NIDDM is usually made after the age of 50 years in Europids, but it is seen at much younger age in these high prevalence populations, which also include Pacific Islanders, Native Americans, and migrant Asian Indians and Chinese. There is enormous variation in NIDDM prevalence between populations, and exceptionally high rates have been documented in populations who have changed from a traditional to a modern lifestyle, e.g., American Pima Indians, Micronesians, and other Pacific Islanders, Australian Aborigines, migrant Asian Indians, and Mexican-Americans. Over the next decade, following the initial phase of the NIDDM epidemic, macro- and microvascular complications will emerge as a major threat to future public health throughout the world with huge economic and social costs. The major cause of death in NIDDM is macrovascular disease (coronary artery, peripheral vascular, and cerebrovascular), which accounts for at least two-thirds of NIDDM mortality. A key strategy in reducing macrovascular disease lies in the better understanding of the Deadly Quartet or Metabolic Syndrome. New data suggest that hyperleptinemia rather than hyperinsulinemia may play an important and central role in the genesis of the cardiovascular disease risk factor cluster that constitutes the Metabolic Syndrome.