Despite its discovery more than 25 years ago in ram rete testis fluid as the factor responsible for clustering of Sertoli cells [
1], the physiologic significance of clusterin (CLU) expression in a range of other tissues is still unclear. CLU is present in nearly all mammalian cells and in plasma, breast milk, urine, and cerebrospinal fluid [
2]. CLU expression has been implicated as a factor in a variety of biological processes such as spermatogenesis, lipid transport, cell differentiation, cell death, and cell survival, which at least partially accounts for the multiplicity of names given to this protein, including ApoJ (apolipoprotein J) [
3], TRPM-2 (testosterone-repressed prostatic message-2) [
4], SGP-2 (sulfated glycoprotein 2) [
5], XIP8 (ionizing radiation-induced protein 8) [
6], SP 40-40 (serum protein 40,40) [
7], CLI (complement-lysis inhibitor) [
8], gp80, glycoprotein III, and T64 [
2,
9]. CLU is encoded by a single gene located on human chromosome 8 [
10]. There are two isoforms that result from alternate translation start sites. One is a glycosylated, secretable form that is translated from the full-length CLU message using the first AUG initiation codon. This isoform includes a leader sequence directing ER synthesis and secretion as well as sites for glycosylation. It is detected as two bands on immunoblots that represent a 60-kDa uncleaved protein (p-CLU) and a 40-kDa product resulting from cleavage of the leader sequence (m-CLU) [
11]. The second isoform of CLU is translated using the second AUG codon of the CLU mRNA, and the product lacks a leader sequence, is not secreted, and is not glycosylated. This form of CLU can be detected in the nucleus (n-CLU) [
12].
Increased CLU expression is related to neoplastic progression in some cancers. The strongest evidence linking high levels of CLU to carcinogenesis comes from studies of breast and prostate cancer [
13,
14]. Based in part on differential expression levels between prostate cancer and normal prostate tissue as well as on data identifying CLU as an anti-apoptotic (pro-cell survival) factor, clinical trials evaluating antisense RNA specific for CLU (OGX-011) are underway in patients with prostate cancer [
15]. Tumor cell apoptosis is proposed as the therapeutic mechanism for anti-CLU therapy. It is of note, however, that depending on the tumor type studied, CLU expression can have paradoxical anti-tumor effects [
16,
17].
CLU interacts with the pro-apoptotic multi-domain Bcl-2 family protein Bax and Bax–Ku70 protein complexes [
18]. CLU–Bax binding inhibits Bax activation and mitochondrial translocation and cell death [
19]. Recently, we showed that histone deacetylase (HDAC) inhibitor (HDACI)-induced cell death in neuroblastoma (NB) is Bax-dependent and occurs as a result of HDACI-induced disruption of Bax binding to Ku70 protein [
20]. In NB, HDACI treatment leads to increased Ku70 acetylation and, because Ku70-Bax binding is acetylation sensitive, Bax is released from Ku70, resulting in Bax-dependent apoptosis. Since CLU is expressed in NB cells [
17], its binding to Bax and Ku70–Bax raises the possibility that CLU inhibits apoptosis by modulating Ku70 acetylation. Here we test the hypothesis that CLU suppresses Ku70 acetylation, stabilizing the Ku70–Bax complex and preventing apoptosis. We conducted overexpression as well as CLU depletion experiments to test this model. Although CLU prevents apoptosis in NB, our results show that CLU does not affect Ku70 acetylation in NB cells. These results suggest that the anti-apoptotic effects of CLU are independent of modulation of Ku70 acetylation.