Leptin gene expression increases with age independent of increasing adiposity in rats

Humans and rats tend to gain weight as they age. Leptin is one regulator of food intake and energy expenditure. To determine if the increase in adiposity with age is related to altered leptin gene expression, we assessed adiposity levels, leptin mRNA levels in epididymal and inguinal white adipose tissue (EWAT and IWAT), and uncoupling protein (UCPI) mRNA levels in interscapular brown adipose tissue (IBAT) from F344 x BN rats ages 3, 12, 18, 24, and 30 months (n = 8/age). Levels of adiposity determined by the adiposity index and the Lee index increased between ages 3 and 24 months, with a decrease at age 30 months. There mere parallel increases with age in body weight, EWAT, and IWAT depot size up to age 24 months, followed by a non-significant decrease at age 30 months. Daily food intake was unchanged with age. In EWAT, leptin mRNA per microgram of RNA was unchanged with age, whereas in IWAT, it increased up to 24 months, then declined at 30 months. Total leptin mRNA levels in both IWAT and EWAT depots increased with age, peaking at age 24 months, and were correlated with adiposity. Serum leptin levels increased with age, also peaking at age 24 months, and were correlated with total leptin mRNA in WAT pads and adiposity. The rate of increase in serum leptin was greater than the increase in adiposity with age, suggesting contributions from both the increase in leptin expression per unit of WAT and the increase in WAT depot size. In addition, UCPI mRNA levels in IBAT did not change with age. These data suggest that adiposity increases with age and cannot be attributed to increased food intake, impaired leptin gene expression, or decreased UCP1 mRNA level in IBAT. Furthermore, leptin gene expression in IWAT increases with age independent of increasing adiposity.