Correction: Non-Secreted Clusterin Isoforms Are Translated in Rare Amounts from Distinct Human mRNA Variants and Do Not Affect Bax-Mediated Apoptosis or the NF-κB Signaling Pathway

Abstract

Clusterin, also known as apolipoprotein J, is expressed from a variety of tissues and implicated in pathological disorders such as neurodegenerative diseases, ischemia and cancer. In contrast to secretory clusterin (sCLU), which acts as an extracellular chaperone, the synthesis, subcellular localization and function(s) of intracellular CLU isoforms is currently a matter of intense discussion. By investigating human CLU mRNAs we here unravel mechanisms leading to the synthesis of distinct CLU protein isoforms and analyze their subcellular localization and their impact on apoptosis and on NF-κB-activity. Quantitative PCR-analyses revealed the expression of four different stress-inducible CLU mRNA variants in non-cancer and cancer cell lines. In all cell lines variant 1 represents the most abundant mRNA, whereas all other variants collectively account for no more than 0.34% of total CLU mRNA, even under stressed conditions. Overexpression of CLU cDNAs combined with in vitro mutagenesis revealed distinct translational start sites including a so far uncharacterized non-canonical CUG start codon. We show that all exon 2containing mRNAs encode sCLU and at least three non-glycosylated intracellular isoforms, CLU 1-449 , CLU 21-449 and CLU 34-449 , which all reside in the cytosol of unstressed and stressed HEK-293 cells. The latter is the only form expressed from an alternatively spliced mRNA variant lacking exon 2. Functional analysis revealed that none of these cytosolic CLU forms modulate caspase-mediated intrinsic apoptosis or significantly affects TNF-α-induced NF-κB-activity. Therefore our data challenge some of the current ideas regarding the physiological functions of CLU isoforms in pathologies.

Clusterin/apolipoprotein J, its isoforms and Alzheimer's disease

“…iCLU forms have been suggested to play a role in apoptosis (Yang et al, 2000 ; Debure et al, 2003 ; Zhang et al, 2005 ), DNA repair (Yang et al, 2000 ), transcription (Santilli et al, 2003 ) and microtubule organization (Kang et al, 2005 ). One of the iCLU is produced from Exon 3 and lacks Exon 2 (Leskov et al, 2003 ; Kim et al, 2012 ; Prochnow et al, 2013 ), while another iCLU isoform lacks exon 5 (Kimura and Yamamoto, 1996 ; Kimura et al, 1997 ; Leskov et al, 2003 ). sCLU has been suggested to promote cell survival (Trougakos and Gonos, 2009 ), while iCLU is associated with decreased cell growth and apoptosis (Yang et al, 2000 ; Scaltriti et al, 2004 ; Kim and Choi, 2011 ).…”

HDL Accessory Proteins in Parkinson’s Disease—Focusing on Clusterin (Apolipoprotein J) in Regard to Its Involvement in Pathology and Diagnostics—A Review

“…Clusterin (also termed ApoJ, SPG2, TRPM-2, SP-40,40, and CLI-dependent on the source of its discovery and described characteristics) is a ubiquitous glycoprotein present intra-and extracellularly in many organs (mainly in the CNS, but also in the liver, testis, prostate, pancreas, etc. ), as well as in body fluids (including serum, cerebrospinal fluid, mother's milk, semen, and urine) ( [43] and references therein, [91,92]).…”

“…The major heterodimeric secretory form of CLU (sCLU) is one of the most prominent extracellular chaperones [92,98]. This group of proteins includes proteins involved in the clearance of the organism from cellular debris and misfolded proteins, thus protecting from the formation of toxic protein aggregates.…”

“…sCLU has been demonstrated to form soluble high-molecular-weight (HMW) complexes with such proteins as fibrinogen, albumin, or ceruloplasmin and is seemingly able to remove them from blood plasma in the mode of receptor-mediated endocytosis. In such a way CLU scavenges unwanted proteins from the circulation, delivering the contents for lysosomal degradation in nonprofessional phagocytes (e.g., hepatocytes and endothelial cells [100]) [92,[101][102][103][104].…”

“…Although their functioning is not fully elucidated, they are believed to regulate cell viability, demonstrating proapoptotic (nCLU) or antiapoptotic (psCLU) properties [91]. However, not all studies have confirmed the apoptosis-modulatory effect of intracellular CLU isoforms [92]. Additionally, the profile of CLU expression seems to be tissue-and cell-specific.…”