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Clusterin as a novel Bcl-2 Homology 3 (BH3)-only protein mediating apoptosis
:Nuclear clusterin by ethanol treatment induced neuronal cell death
Nayoung Kim, Gu Seob Roh, Hyun Joon Kim, Sang Soo Kang, Gyeong Jae Cho , Wan Sung Choi
Department of Anatomy and Neurobiology, Institute of Health Sciences, College of Medicine,
Gyeongsang National University, 92 Chilam-dong, Jinju, Gyeongnam 660-751, Korea
Clusterin, a heterdoimeric glycoprotein expressed in many types of cells and tissues, is
known to increase in response to a variety of cytotoxic stimuli and in neurodegenerative
diseases; however, its exact role in cells is still unclear. Ethanol is known to trigger apoptotic
neurodegeneration, and ethanol administration to immature rats during the period of brain
development induces cell death. Nonetheless, the effect of ethanol on clusterin expression
and function is not known. In the present study, clusterin expression and its role in ethanolinduced cell death were investigated. After ethanol administration, clusterin was
overexpressed in most parts of the brain, especially in the cortex and the laterodorsal
thalamic nucleus. Clusterin was detected in cell nuclei and colocalized with TUNEL-positive
cells. In addition, iso-forms of clusterin that have been linked to cell death were significantly
increased in ethanol-treated rat brain. Clusterin siRNA partially inhibited ethanol-induced cell
death, and transfection of a clusterin cDNA lacking a leader peptide enhanced both the
nuclear localization of clusterin and cell death. Together, these results suggest that the
ethanol-induced nuclear overexpression of clusterin does not protect cells, but rather leads
to cell death.
Clusterin (Clu) is ubiquitously expressed and implicated in diverse, yet contrasting, cellular
processes such as apoptosis and anti-apoptosis1,2. Whereas Clu is known to inhibit the proapoptotic function of Bax in some cellular contexts3, the molecular mechanism by which Clu
can also promote apoptosis in other contexts is currently unknown. In this study, we found
that the nuclear form of Clu localized in the mitochondria and interacted directly with antiapoptotic Bcl-XL. Using computational and biochemical analyses, we discovered a
conserved BH3 motif on Clu, which is responsible for its interaction with Bcl-XL. Because
BH3-dependent association of nClu with Bcl-XL interferes with the interaction of Bcl-XL with
Bax, overexpression of nClu allowed Bax activation and elicited the subsequent
mitochondrial apoptotic pathway. The mechanism of BH3-mediated Bcl-XL interaction may
explain how Clu mediates pro-apoptotic function in specific cellular contexts. Our results
reveal a novel molecular function for Clu and may provide the molecular mechanism for its
reported apoptotic effects, expanding our knowledge of mitochondria-mediated cell death.