Superinduction of leptin mRNA in mouse hypothalamic neurons.

In marked contrast to several other species, including rats and humans, leptin gene expression is undetectable in mouse brain. This unexpected finding may reflect unique energy regulation pathways in the mouse. We investigated possible mechanisms by which leptin (ob) gene expression is suppressed in mouse brain: (a) the possibility that ob mRNA levels might be detectable in vitro through the superinduction of gene expression following protein synthesis inhibition and (b) whether chromatin modification of the ob gene was responsible for this repression. Experiments were conducted on mouse hypothalamic neurons in vitro. Cells were treated with (a) protein synthesis inhibitors: cycloheximide (CHX; 25 µg/ml); puromycin (50 µg/ml); anisomycin (5 µM); (b) trichostatin A (histone deacetylase inhibitor; 500 nM); and (c) 5-aza-2'-deoxycytidine (DNA methylation inhibitor; 5 µM). Following the incubations, cells were harvested for the preparation of RNA and ob mRNA was detected using real-time reverse transcription PCR. Protein synthesis inhibitors induced a rapid increase in ob mRNA levels in mouse hypothalamic neurons in vitro. For example CHX stimulation of ob mRNA was detectable at 60 min after treatment and reached a maximum between 4 and 6 h. A dose-response analysis, with concentrations of CHX of 1, 2, 10, 25, and 50 μg/ml, indicated that CHX was already effective at 1.0 μg/ml, with a maximal effect by 25 μg/ml. In contrast, incubation with trichostatin A and 5-aza-2'-deoxycytidine had no effect and ob mRNA remained undetectable. These data show that leptin gene expression is superinduced in ob-negative mouse hypothalamic neurons following inhibition of protein synthesis. They confirm that the previously reported absence of leptin mRNA in mouse brain is probably because of an active repressive mechanism, although this may not involve chromatin modification.