Acute excess caloric intake negatively affects the rhythmic expression of the human growth hormone gene and its association with the circadian‐linked transcription factor Bmal1

Abstract

Rhythmicity of biological functions is fundamental for optimal adaptations to environmental cues. Growth hormone (GH) is a major metabolic homeostatic factor that is secreted with a circadian pattern, but whether it is produced rhythmically is unknown. We used transgenic (TG) mice containing the intact human GH gene locus (human GH/CS‐TG) to investigate the rhythmicity of human GH gene (hGH‐N) expression. Human GH RNA expression oscillates over a 24‐hour cycle. Moreover, we have previously reported that excess caloric intake is associated with a decrease in human GH synthesis with reduced hGH‐N promoter function. Sequence analysis of the hGH‐N promoter revealed the presence of an enhancer motif (E‐box) and as such a putative binding site for circadian transcriptional machinery (Bmal1 and Clock). Results from electrophoretic mobility shift assay support participation of Bmal1/Clock and the hGH‐N E‐box DNA element in a common complex. Transfection studies combined with site‐directed mutagenesis revealed that this E‐box DNA element contributes to basal hGH‐N promoter activity. Furthermore, Bmal1/Clock were able to transactivate the hGH‐N promoter. The ability of Bmal1 to associate with the hGH‐N promoter region containing the E‐box element was confirmed in the human GH/CS‐TG mouse pituitary in situ by chromatin immunoprecipitation assay. Occupancy was reduced in mice fed a high fat diet during the light cycle. This decrease was not seen, however, during the dark cycle. Initial assessment of the mechanism suggests an impact of excess caloric intake on the chromatin and/or DNA structure. In summary, the presence of an E‐box DNA element as well as association of Bmal1 with the hGH‐N promoter region in situ supports the notion that hGH‐N is a cyclic gene. Acute excess caloric intake for three days was sufficient to reprogram the circadian rhythm mediated by chromatin remodeling of the hGH‐N promoter region and interfer with recruitment of Bmal1 to the E‐box element. Further investigation of the rhythmic expression of human GH may provide insight into the changes in GH availability under pathophysiological conditions such as metabolic changes associated with overeating and obesity.Support or Funding InformationFunded by the Canadian Institutes of Health Research