Biology of leptin in the pig

The recently discovered protein, leptin, which is secreted by fat cells in response to changes in body weight or energy, has been implicated in regulation of feed intake, energy expenditure and the neuroendocrine axis in rodents and humans. Leptin was first identified as the gene product found deficient in the obese ob/ob mouse. Administration of leptin to ob/ob mice led to improved reproduction as well as reduced feed intake and weight loss. The porcine leptin receptor has been cloned and is a member of the class 1 cytokine family of receptors. Leptin has been implicated in the regulation of immune function and the anorexia associated with disease. The leptin receptor is localized in the brain and pituitary of the pig. The leptin response to acute inflammation is uncoupled from anorexia and is differentially regulated among swine genotypes. In vitro studies demonstrated that the leptin gene is expressed by porcine preadipocytes and leptin gene expression is highly dependent on dexamethasone induced preadipocyte differentiation. Hormonally driven preadipocyte recruitment and subsequent fat cell size may regulate leptin gene expression in the pig. Expression of CCAAT-enhancer binding proteinalpha (C/EBPalpha) mediates insulin dependent preadipocyte leptin gene expression during lipid accretion. In contrast, insulin independent leptin gene expression may be maintained by C/EBPalpha auto-activation and phosphorylation/dephosphorylation. Adipogenic hormones may increase adipose tissue leptin gene expression in the fetus indirectly by inducing preadipocyte recruitment and subsequent differentiation. Central administration of leptin to pigs suppressed feed intake and stimulated growth hormone (GH) secretion. Serum leptin concentrations increased with age and estradiol-induced leptin mRNA expression in fat was age and weight dependent in prepuberal gilts. This occurred at the time of expected puberty in intact contemporaries and was associated with greater LH secretion. Further work demonstrated that leptin acts directly on pituitary cells to enhance LH and GH secretion, and brain tissue to stimulate gonadotropin releasing hormone secretion. Thus, development of nutritional schemes and (or) gene therapy to manipulate leptin secretion will lead to practical methods of controlling appetite, growth and reproduction in farm animals, thereby increasing efficiency of lean meat production.