A Truncated Form of p23 Down-regulates Telomerase Activity via Disruption of Hsp90 Function

Abstract

The Hsp90-associated protein p23 modulates Hsp90 activity during the final stages of the chaperone pathway to facilitate maturation of client proteins. Previous reports indicate that p23 cleavage induced by caspases during cell death triggers destabilization of client proteins. However, the specific role of truncated p23 (⌬p23) in this process and the underlying mechanisms remain to be determined. One such client protein, hTERT, is a telomerase catalytic subunit regulated by several chaperone proteins, including Hsp90 and p23. In the present study, we examined the effects of p23 cleavage on hTERT stability and telomerase activity. Our data showed that overexpression of ⌬p23 resulted in a decrease in hTERT levels, and a down-regulation in telomerase activity. Serine phosphorylation of Hsp90 was significantly reduced in cells expressing high levels of ⌬p23 compared with those expressing full-length p23. Mutation analyses revealed that two serine residues (Ser-231 and Ser-263) in Hsp90 are important for activation of telomerase, and down-regulation of telomerase activity by ⌬p23 was associated with inhibition of cell growth and sensitization of cells to cisplatin. Our data aid in determining the mechanism underlying the regulation of telomerase activity by the chaperone complex during caspasedependent cell death.Telomerase is a specialized reverse transcriptase responsible for the maintenance and preservation of telomere ends in germ cells, immortalized cells, and cancer cells (1). hTERT, 3 the reverse transcriptase subunit of telomerase, possesses catalytic activity, whereas the associated RNA component, human telomerase RNA, serves as a template for the synthesis of telomeric sequences (2). Expression analyses of hTERT and human telomerase RNA components in heterologous systems have enhanced our understanding of the biochemical features of telomerase. Human telomerase activity has been reconstituted in a variety of in vitro systems, including yeast, baculovirus, rabbit reticulocyte, wheat germ, and human cell extracts (3-5). In each system, the essential roles of hTERT and human telomerase RNA in active telomerase complexes have been confirmed. Recent studies have identified other proteins associated with the telomerase holoenzyme. For instance, Hsp90 and its co-chaperone, p23, bind hTERT and contribute to telomerase activity (6). The Hsp90 chaperone complex, which includes Hsp90, p23, Hsp70, p60, and Hsp40/ydj, is required for the assembly of human telomerase both in vitro, in a cell-free rabbit reticulocyte lysate system, and in vivo, in human cells.Among the Hsp90 partners, the acidic protein p23 is the smallest and has a relatively simple structure (7,8). p23 is ubiquitously expressed in all eukaryotes, from yeast to humans. Initially discovered as part of the Hsp90 complex with the progesterone receptor (9), p23 has since been identified in complexes containing a variety of Hsp90-associated proteins, including other steroid receptors (10), the heme-regulated kinase HRI (11), Fes tyrosine kinase, heat shock t...

HMGB1 gene knockout in mouse embryonic fibroblasts results in reduced telomerase activity and telomere dysfunction

“…The latter would be compatible with DNA microarrays revealing diminished expression of numerous heat shock proteins (chaperones) in HMGB1 −/− MEFs relative to the parental cells (Krynetskaia et al 2008), including Hsp90 mRNA as revealed by qRT-PCR (data not shown). Hsp90 protein has previously been shown to be an important factor in activation of telomerase (Holt et al 1999;Woo et al 2009). …”

Regulation of molecular chaperones through post-translational modifications: Decrypting the chaperone code

“…The ER-specific isoform GRP94 is likewise phosphorylated by this kinase [97,98], raising again the logistical question of how an ER chaperone could possibly be modified by a cytosolic kinase. Although the exact biological function of these PTMs in human Hsp90 is still unclear, some authors have observed implications in apoptosome formation, arylhydrocarbon receptor-dependent transcriptional repression, BRaf/MKK/ERK signaling and telomerase activation [99][100][101][102]. In another report, Hsp90 was shown to enhance phosphorylation of eukaryotic initiation factor 2α, but only when the chaperone itself had been phosphorylated beforehand by CK2 [103].…”

HSP90AB1: Helping the good and the bad

“…Overexpression of a truncated p23 corresponding to deltap23 leads to a decrease in hTERT levels and to down-regulation of telomerase activity (Woo et al 2009). …”