No abstract

No abstract

Spinal and bulbar muscular atrophy (SBMA) is a neurodegenerative disorder caused by the expansion of a polyglutamine tract in the androgen receptor (AR). The N-terminal fragment of AR containing the expanded polyglutamine tract aggregates in cytoplasm and/or in nucleus and induces cell death. Some chaperones such as Hsp40 and Hsp70 have been identified as important regulators of polyglutamine aggregation and/or cell death in neuronal cells. Recently, Hsp105␣, expressed at especially high levels in mammalian brain, has been shown to suppress apoptosis in neuronal cells and prevent the aggregation of protein caused by heat shock in vitro. However, its role in polyglutamine-mediated cell death and toxicity has not been studied. In the present study, we examined the effects of Hsp105␣ on the aggregation and cell toxicity caused by expansion of the polyglutamine tract using a cellular model of SBMA. The transient expression of truncated ARs (tARs) containing an expanded polyglutamine tract caused aggregates to form in COS-7 and SK-N-SH cells and concomitantly apoptosis in the cells with the nuclear aggregates. When Hsp105␣ was overexpressed with tAR97 in the cells, Hsp105␣ was colocalized to aggregates of tAR97, and the aggregation and cell toxicity caused by expansion of the polyglutamine tract were markedly reduced. Both ␤-sheet and ␣-helix domains, but not the ATPase domain, of Hsp105␣ were necessary to suppress the formation of aggregates in vivo and in vitro. Furthermore, Hsp105␣ was found to localize in nuclear inclusions formed by ARs containing an expanded polyglutamine tract in tissues of patients and transgenic mice with SBMA. These findings suggest that overexpression of Hsp105␣ suppresses cell death caused by expansion of the polyglutamine tract without chaperone activity, and the enhanced expression of the essential domains of Hsp105␣ in brain may provide an effective therapeutic approach for CAG repeat diseases.

Human colorectal tumor cell lines were established which express wildtype p21 or p21 with a mutation at codon 46 (Cys) or 140 (Gly) on IPTG treatment (LacSwitch). The IPTG-induced wildtype p21 bound to CDK2 and PCNA and inhibited CDK activity in the cells and reduced cell growth rate; whereas, both IPTG-induced mutated p21 proteins neither bound to CDK2 nor a ected the CDK activity but did bind to PCNA, and they did not a ect the cell growth rate. Wildtype p21 suppressed apoptosis and enhanced survival of X-ray-irradiated or adriamycintreated cells; but, mutated p21 neither suppressed apoptosis nor a ected cell survival. When cells were treated with mimosine, a p53-independent p21-inducer, or butyrolactone I, a speci®c inhibitor of CDK, cellular endogenous p21 was induced and X-ray or adriamycininduced apoptosis was blocked. These results suggest that CDK-binding or CDK-inhibitory activity of p21 is required to prevent apoptosis, i.e., CDK is required for apoptosis in human tumor cells.

Hsp105␣ is a mammalian member of the HSP105/110 family, a diverged subgroup of the HSP70 family. Hsp105␣ associates with Hsp70/Hsc70 as complexes in vivo and regulates the chaperone activity of Hsp70/ Hsc70 negatively in vitro and in vivo. In this study, we examined the mechanisms by which Hsp105␣ regulates Hsc70 chaperone activity. Using a series of deletion mutants of Hsp105␣ and Hsc70, we found that the interaction between Hsp105␣ and Hsc70 was necessary for the suppression of Hsc70 chaperone activity by Hsp105␣. Furthermore, Hsp105␣ and deletion mutants of Hsp105␣ that interacted with Hsc70 suppressed the ATPase activity of Hsc70, with the concomitant appearance of ATPase activity of Hsp105␣. As the ATPase activity of Hsp70/Hsc70 is essential for the efficient folding of nonnative protein substrates, Hsp105␣ is suggested to regulate the substrate binding cycle of Hsp70/Hsc70 by inhibiting the ATPase activity of Hsp70/Hsc70, thereby functioning as a negative regulator of the Hsp70/Hsc70 chaperone system.