29. Hsp25 Regulates the Expression of p21 ^{Waf1/Cip1/Sdi1} through Multiple Mechanisms

Park Sang-Hee, Toshiyasu Hirama, Naoyuki Anzai, Yoshiko Kawase, Misao Hachiya, Hisayoshi Kondo, Saori Kawamura, Yasunari Takada, Daisaku Takai and Makoto Akashi

Keywords: Hsp25, p21, translational and post-translational regulation

Exposure of cells to external stresses leads to the induction or activation of certain proteins. Expression of heat shock proteins (Hsps) is induced in response to these stresses. Hsps are known to have molecular chaperone activities; but recent studies have shown that Hsps have a variety of functions such as triggering proliferation, differentiation, and apoptosis of cells. Previously, we found that overexpression of a 25 kDa Hsp (Hsp25) induced the expression of the cell cycle inhibitory protein p21 (WAF1/Cip1/Sdi1) in murine fibroblastoid L929 cells. However, the mechanisms for the induction of p21 by Hsp25 are unknown. In the present study, we have investigated the mechanisms of the regulation of p21 expression by Hsp25 in these cells. The introduction of Hsp25 cDNA stimulated the accumulation of p21 transcripts through transcriptional but not posttranscriptional regulation in these cells. We also found that overexpression of Hsp25 markedly increased the translational rate of p21 and stabilized the protein. Studies using proteasome inhibitors and Western blot analysis for ubiquitination of p21 have demonstrated that the stabilization of p21 is regulated through a ubiquitin-independent pathway. However, no direct association of Hsp25 with p21 was observed. These findings suggest that Hsp25 induces p21 expression through multiple mechanisms and that transcriptional, translational, and post-translational regulations are important in the regulation of p21.