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Submitted on March 21, 2006
Accepted on June 26, 2006
Departments of Pediatrics, Internal Medicine, and Molecular and Integrative Physiology, University of Michigan, Ann Arbor, MI, USA, and Istituto di Biomedicina e, Immunologia Molecolare CNR, Palermo, Italy
* To whom correspondence should be addressed. E-mail: rammenon{at}umich.edu.
The expression and function of the growth hormone (GH) receptor is critical for the actions of pituitary GH in the intact animal. The role of systemic factors in the reduced expression of the GH receptor and consequent GH insensitivity in pathological states such as sepsis, malnutrition, and poorly-controlled diabetes mellitus (DM) is unclear. In the current study we demonstrate that saturated (palmitic and myristic; 50 µM) fatty acids (FA) inhibit activity of the promoter of the major (L2) transcript of the GH receptor gene; unsaturated (oleic and linoleic) FA (200 µM) do not alter activity of the promoter. Comparable effects with palmitic acid and the non-metabolizable analog bromo-palmitic acid, and failure of triacsin C to abrogate palmitic acids effects on GH receptor expression indicate that this effect is due to direct action(s) of FA. Palmitic acid, but not the unsaturated FA linoleic acid, decreased steady state levels of endogenous L2 mRNA and GHR protein in 3T3-L1 preadipocytes. The effect of FA was localized to two cis-elements located approximately 600 bp apart on the L2 promoter. EMSA and ChIP assays established that both these cis-elements bind the Krüppel-type zinc finger transcription factor, ZBP-89. Ectopic expression of ZBP-89 amplified the inhibitory effect of FA on L2 promoter activity and on steady state levels of endogenous L2 mRNA in 3T3-L1 preadipocytes. Mutational analyses of the two ZBP-89 binding sites revealed that both the sites are essential for palmitic acid's inhibitory effect on the L2 promoter and for the enhancing effect of ZBP-89 on palmitic acid-induced inhibition of the L2 promoter. Our results establish a molecular basis for FA-induced inhibition of GH receptor gene expression in the pathogenesis of acquired GH insensitivity in pathological states such as poorly-controlled DM and small for gestational age (SGA).
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