Molekularni mehanizmi uključeni u nastanak stresa endoplazmatskog retikuluma - šta znamo do sada

Abstract

Endoplasmic reticulum (ER) is an intracellular organelle involved in protein synthesis and folding. When the balance between cell needs for proteins and ER capacity to fold proteins is disrupted, nonfunctional, unfolded, or misfolded proteins accumulate in ER lumen, leading to endoplasmic reticulum stress (ER stress). One of the ways cell uses to overcome ER stress is unfolded protein response (UPR) activation. UPR is initiated by the activation of three ER transmembrane proteins. These proteins are IRE-1a (inositol requiring enzyme-1a), PERK (protein kinase RNA-like endoplasmic reticulum kinase) and ATF6 (activating transcription factor 6) and they are activated when ER chaperone, GRP78 (glucose-regulates protein 78) releases their intraluminal domains. Activation of these transmembrane sensors starts mechanisms that should restore ER function. If ER function is not restored and balance is not achieved, apoptosis is induced in order to maintain cell homeostasis. Activated IRE-1a leads to XBP-1 (X-box binding protein-1) mRNA splicing and activates MAP kinases and inflam-matory pathways that involve nuclear factor cB (NFcB). Activated ATF 6 (ATF6f) functions as a transcriptional factor and increases gene expression for XBP-1, while PERK activation leads to phosphorylation and inactivation of eukaryotic initiation factor 2 (eIF2a) which further leads to decreased protein synthesis. Additionally, eIF2a phosphorylation leads to selective synthesis of ATF4, a transcriptional factor that in irreversibly damaged cells induces cell death activation by C/EBP homologous protein (CHOP) transcription. It is known that ER stress and UPR have a role in different diseases pathogenesis such as diabetes, inflammation, tumors and neurodegenerative diseases. Knowing signaling pathways of UPR and mechanisms by which UPR is involved in diseases pathogenesis can be very significant in targeted therapeutic approaches development.