Increased Expression of TGF-β1 by 4-hexylresorcinol Is Mediated by Endoplasmic Reticulum and Mitochondrial Stress in Human Umbilical Endothelial Vein Cells

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Abstract

In our previous study, 4-hexylresorcinol (4HR) increased the expression level of vascular endothelial growth factor in human umbilical vein endothelial cells (HUVECs) via the transforming growth factor-β1 (TGF-β1)-mediated pathway. Endoplasmic reticulum (ER) and mitochondrial stress is a positive regulator of cellular differentiation. As TGF-β1 is a master regulator for cellular differentiation, 4HR treatment may increase TGF-β1 expression via ER stress. In this study, HUVECs were treated using 4HR (1–100 μM) for 24 h. The 4HR treatment increased ER stress-associated markers and mitochondrial stress. Increased TGF-β1 expression by 4HR administration was alleviated by tauroursodeoxycholate (ER stress inhibitor) treatment. Combining these activities with the elevated acetylation level of histone 3 (H3) by 4HR treatment, TGF-β1 expression was increased in HUVECs. Overall, 4HR increased TGF-β1 expression through upregulation of the stress response of ER as well as H3 acetylation in HUVECs.

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ER stress induces cleavage of membrane-bound ATF6 by the same proteases that process SREBPs.

Jin Ye, Robert Rawson, Ryutaro Komuro … (2000)

ATF6 is a membrane-bound transcription factor that activates genes in the endoplasmic reticulum (ER) stress response. When unfolded proteins accumulate in the ER, ATF6 is cleaved to release its cytoplasmic domain, which enters the nucleus. Here, we show that ATF6 is processed by Site-1 protease (S1P) and Site-2 protease (S2P), the enzymes that process SREBPs in response to cholesterol deprivation. ATF6 processing was blocked completely in cells lacking S2P and partially in cells lacking S1P. ATF6 processing required the RxxL and asparagine/proline motifs, known requirements for S1P and S2P processing, respectively. Cells lacking S2P failed to induce GRP78, an ATF6 target, in response to ER stress. ATF6 processing did not require SCAP, which is essential for SREBP processing. We conclude that S1P and S2P are required for the ER stress response as well as for lipid synthesis.

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Transcriptional induction of mammalian ER quality control proteins is mediated by single or combined action of ATF6alpha and XBP1.

Keisuke Yamamoto, Takashi Sato, Toshie Matsui … (2007)

Metazoans express three unfolded protein response transducers (IRE1, PERK, and ATF6) ubiquitously to cope with endoplasmic reticulum (ER) stress. ATF6 is an ER membrane-bound transcription factor activated by ER stress-induced proteolysis and has been duplicated in mammals. Here, we generated ATF6alpha- and ATF6beta-knockout mice, which developed normally, and then found that their double knockout caused embryonic lethality. Analysis of mouse embryonic fibroblasts (MEFs) deficient in ATF6alpha or ATF6beta revealed that ATF6alpha is solely responsible for transcriptional induction of ER chaperones and that ATF6alpha heterodimerizes with XBP1 for the induction of ER-associated degradation components. ATF6alpha(-/-) MEFs are sensitive to ER stress. Unaltered responses observed in ATF6beta(-/-) MEFs indicate that ATF6beta is not a negative regulator of ATF6alpha. These results demonstrate that ATF6alpha functions as a critical regulator of ER quality control proteins in mammalian cells, in marked contrast to worm and fly cells in which IRE1 is responsible.

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Protein translation and folding are coupled by an endoplasmic-reticulum-resident kinase.

H P Harding, Y. Zhang, D Ron (1999)

Protein synthesis and the folding of the newly synthesized proteins into the correct three-dimensional structure are coupled in cellular compartments of the exocytosis pathway by a process that modulates the phosphorylation level of eukaryotic initiation factor-2alpha (eIF2alpha) in response to a stress signal from the endoplasmic reticulum (ER). Activation of this process leads to reduced rates of initiation of protein translation during ER stress. Here we describe the cloning of perk, a gene encoding a type I transmembrane ER-resident protein. PERK has a lumenal domain that is similar to the ER-stress-sensing lumenal domain of the ER-resident kinase Ire1, and a cytoplasmic portion that contains a protein-kinase domain most similar to that of the known eIF2alpha kinases, PKR and HRI. ER stress increases PERK's protein-kinase activity and PERK phosphorylates eIF2alpha on serine residue 51, inhibiting translation of messenger RNA into protein. These properties implicate PERK in a signalling pathway that attenuates protein translation in response to ER stress.