Naja atra Cardiotoxin 3 Elicits Autophagy and Apoptosis in U937 Human Leukemia Cells through the Ca2+/PP2A/AMPK Axis

Abstract

Cardiotoxins (CTXs) are suggested to exert their cytotoxicity through cell membrane damage. Other studies show that penetration of CTXs into cells elicits mitochondrial fragmentation or lysosome disruption, leading to cell death. Considering the role of AMPK-activated protein kinase (AMPK) in mitochondrial biogenesis and lysosomal biogenesis, we aimed to investigate whether the AMPK-mediated pathway modulated Naja atra (Taiwan cobra) CTX3 cytotoxicity in U937 human leukemia cells. Our results showed that CTX3 induced autophagy and apoptosis in U937 cells, whereas autophagic inhibitors suppressed CTX3-induced apoptosis. CTX3 treatment elicited Ca2+-dependent degradation of the protein phosphatase 2A (PP2A) catalytic subunit (PP2Acα) and phosphorylation of AMPKα. Overexpression of PP2Acα mitigated the CTX3-induced AMPKα phosphorylation. CTX3-induced autophagy was via AMPK-mediated suppression of the Akt/mTOR pathway. Removal of Ca2+ or suppression of AMPKα phosphorylation inhibited the CTX3-induced cell death. CTX3 was unable to induce autophagy and apoptosis in U937 cells expressing constitutively active Akt. Met-modified CTX3 retained its membrane-perturbing activity, however, it did not induce AMPK activation and death of U937 cells. These results conclusively indicate that CTX3 induces autophagy and apoptosis in U937 cells via the Ca2+/PP2A/AMPK axis, and suggest that the membrane-perturbing activity of CTX3 is not crucial for the cell death signaling pathway induction.

Status of Asp29 and Asp40 in the Interaction of Naja atra Cardiotoxins with Lipid Bilayers

“…Structural and functional analyses have suggested that the interaction of CTXs with the membrane is mediated through the tips of loops I/II, and that the basic residues flanked by hydrophobic patch at loop I/II regions are involved in the membrane-permeabilizing activity of CTXs [2,4,6,8]. In agreement, previous studies have reported that modification of the conserved Met residues at loop II of CTXs causes a drastic drop in their membrane-damaging activity [16].…”

“…Collectively, these results indicate that the intact Asp29 may functionally modulate the membrane-interacting mode of CTX1, thereby hindering the performance of CTX1 with highly disruptive membrane activity. However, loop I of S-type CTXs has been suggested to be crucial for their binding with zwitterionic phospholipids [4,6]. Gorai et al [6] proposed that loop II of S-type CTXs is the structural region chiefly governing complex formation of proteins with PC.…”

“…Past studies have shown that CTXs damage human red blood cells and cell membranes [4,12]. Since PC, sphingomyelin, and cholesterol (Chol) are the dominant lipid components of mammalian cell membranes [13], lipid vesicles composed of egg-yolk phosphatidylcholine (EYPC), egg-yolk sphingomyelin (EYSM), and/or Chol were created to explore the interaction of CTX1, CTX3, SEM-CTX1, and SEM-CTX3 with membrane bilayers.…”

“…Cardiotoxins (CTXs) are major components of Elapidae snake venom and exhibit a variety of pharmacological activities including hemolysis, cardiotoxicity, disruption of muscular integrity, and cytotoxicity [1][2][3][4]. However, the exact mechanisms underlying the broad spectrum of biological cytotoxicity [1][2][3][4]. However, the exact mechanisms underlying the broad spectrum of biological activities have not been fully elucidated.…”

Tau‐induced upregulation of C/EBPβ‐TRPC1‐SOCE signaling aggravates tauopathies: A vicious cycle in Alzheimer neurodegeneration

“…PP2A inactivation is regarded as the most important molecular event that causes abnormal tau hyperphosphorylation in the AD brains (Iqbal, Liu, & Gong, 2016). PP2A‐C expression can be inhibited by increased intracellular [Ca 2+ ] i through degradation (Chiou et al, 2019) and activated ER stress (Tay et al, 2012). Therefore, the reduction of PP2A‐C in hTau‐overexpressing neurons may be caused by activated ER stress and elevated intraneuronal [Ca 2+ ] i .…”