Autosomal dominant
isolated growth hormone deficiency type II (
IGHD II) is mainly caused by splice site mutations of the GH-1 gene, leading to deletion of
amino acids 32-71 of the
human growth hormone (hGH). The severe hGH deficit in
IGHD II suggests a dominant negative effect of the partially deleted del(32-71)-hGH on the production, storage or secretion of normal wild-type (wt)-hGH in somatotrophic cells of the pituitary. To shed more light on the cellular and molecular basis of
IGHD II, we established and analysed diverse clones of the rat somatotrophic cell line GH(4)C(1) stably expressing either wt-hGH, del(32-71)-hGH, or both
proteins concomitantly. The cellular morphology of all transfected GH(4)C(1) cell clones showed moderate differences to untransfected GH(4)C(1) cells. On the molecular level, both
cDNA-constructs induced transcription but, under normal culture conditions, only wt-hGH
protein was found to be synthesised and secreted in readily detectable amounts. By contrast, only after inhibition of proteasomes did high amounts of del(32-71)-hGH show up. The solubility of del(32-71)-hGH in nondenaturing
buffer was poor compared to wt-hGH, hinting at molecular aggregation, and several
epitopes recognised by monoclonal hGH
antibodies were not present on del(32-71)-hGH, confirming the assumption that del(32-71)-hGH must be severely misfolded. Expression of both
proteins in Escherichia coli mirrored the findings from the GH(4)C(1) cell clones in terms of solubility and immunological reactivity. The results of the present study indicate that, in
IGHD II, somatotrophs continuously have to remove misfolded del(32-71)-hGH via the proteasomal degradation pathway, suggesting a mechanism that may result in chronic cellular stress.