Abstract
X-box binding protein 1 (XBP1) is the transcription factor induced in response to endoplasmic reticulum (ER) stress, which is provoked by the overproduction of unfolded or misfolded proteins, leading to the induction of chaperones, ER-associated degradation factors, or other factors. Growth hormone (GH) is known to have many physiological roles, including the induction of XBP1 gene expression. In this study, we have examined the effect of GH on the XBP1 gene expression, using the liver of hypophysectomized rats.The intravenous administration of GH increased XBP1 mRNA (2.5 Kb and 2.0 Kb) transiently and dose-dependently. The XBP1 mRNA was increased in the adipose tissues, while the increase of 2.5 Kb mRNA was liver-specific. The active form of XBP1 is derived from spliced molecule of the mRNA, and RT-PCR revealed that both unspliced and spliced mRNAs were increased by GH administration. GH administration induced the mRNA of GADD45A, one of the factors activated in response to ER stress, but not the mRNA of CHOP, inducer of apoptosis.These results indicate the possibility that GH has a facilitatory role of unfolded protein response to relieve liver cells from ER stress. [J Physiol Sci. 2008;58 Suppl:S139]
