A Mechanism of Release of Calreticulin from Cells During Apoptosis

Abstract

Calreticulin (CRT) is an endoplasmic reticulum (ER) chaperone responsible for glycoprotein folding and Ca 2+ homeostasis. CRT also has extracellular functions, e.g. tumor and apoptotic cell recognition and wound healing, but the mechanism of CRT extracellular release is unknown. Cytosolic localization of CRT is determined by signal peptide and subsequent retrotranslocation of CRT into the cytoplasm. Here, we show that under apoptotic stress conditions, the cytosolic concentration of CRT increases and associates with phosphatidylserine (PS) in a Ca 2+ -dependent manner. PS distribution is regulated by aminophospholipid translocase (APLT), which maintains PS on the cytosolic side of the cell membrane. APLT is sensitive to redox modifications of its SH groups by reactive nitrogen species. During apoptosis, both CRT expression and the concentration of nitric oxide (NO) increase. By using S-nitroso-L-cysteine-ethyl-ester, an intracellular NO donor and inhibitor of APLT, we showed that PS and CRT externalization occurred together in an S-nitrosothiol-dependent and caspase-independent manner. Furthermore, the CRT and PS are relocated as punctate clusters on the cell surface. Thus, CRT induced nitrosylation and its externalization with PS could explain how CRT acts as a bridging molecule during apoptotic cell clearance.

Contribution of RIP3 and MLKL to immunogenic cell death signaling in cancer chemotherapy

“…The exposure of phosphatidylserine and CRT usually require caspase activation, while they could also happen in a caspaseindependent manner. [51][52][53] Our findings confirm and extend the idea that ICD can be stimulated by distinct pathways. Thus, a caspase-8-dependent type 1 ICD induced by anthracyclines has been distinguished from a caspase-8-independent type 2 ICD induced by photodynamic stress.…”

Proteinase 3, the Autoantigen in Granulomatosis with Polyangiitis, Associates with Calreticulin on Apoptotic Neutrophils, Impairs Macrophage Phagocytosis, and Promotes Inflammation

“…CRT has been shown to interact directly with PS in other cell types (15). Under apoptotic stress conditions, cytosolic CRT increased and associated with PS, maintaining PS on the cytosolic side of the cell membrane (12). PR3 thus belongs to a membrane protein complex together with phospholipid scramblase 1 that is assembled at the plasma membrane concomitantly with PS externalization on neutrophil apoptosis.…”

“…Accordingly, decreased CRT membrane expression reduced apoptotic cell uptake by phagocytes (12). One of the main receptors of CRT is the low-density lipoprotein receptor-related protein (LRP), also called CD91, found at the surface of phagocytes (13)(14)(15).…”

“…CRT is not a transmembrane protein but is exposed at the surface of apoptotic cells (11,12,15) and interacts with LRP at the macrophage surface (13), which may serve as a signaling partner (11,33). PR3 directly binds to CRT at the neutrophil cell surface, and PR3-CRT association could decrease CRT access to LRP on macrophages, thus impairing phagocytosis.…”

ROS-induced autophagy in cancer cells assists in evasion from determinants of immunogenic cell death

“…9,37,38,[43][44][45] On the other hand, autophagy is a catabolic process that tends to remove damaged proteins like oxidized proteins and/or damaged organelles and reduce the effects of stress on a cell. 33,41,46 Since carbonylation of amino acid chains is a good indicator of the level of irreversible oxidative damage of proteins and since such proteins are marked for degradation/removal; 33,47 we next investigated the pattern of early accumulation of carbonylated or oxidatively damaged proteins in cells responding to Hyp-PDT.…”