ИСТИНА

Heat-shock proteins (HSPs) are synthesized in great amounts in skeletal muscle, but HSP90 and HSP70 levels dramatically drop by 70–75% under muscle unloading in parallel with muscle atrophy. Probably, that inability to produce HSPs has an important effect on muscle atrophy during unloading. We assumed that under muscle unloading, HSP90 function, which leads to HSP70 synthesis preclusion, might be changed. We have tested the hypothesis that the HSP90 inducer 17-(allylamino)-17-demethoxygel-danamycin (17-AAG) can prevent HSP70 decrease and attenuate atrophy of unloaded soleus due to modulation of HSP90 function. 17-AAG, which specifically binds to the HSP90 ATP-binding site and disrupts its ATP-dependent function, is known to activate HSP90 nonspecifically by the activation of heat-shock factor 1 (HSF1), thereby causing the activation of the heat-shock or stress-response pathways. During unloading, we administered HSP90 inducer 17-AAG to disrupt ATP-dependent function and to increase the HSP amounts. The aim of the study was to analyze the involvement of HSP 90 into the anabolic and catabolic signalling alterations under conditions of muscle unloading. Male Wistar rats were divided into control (C, n=7), 3-day hindlimb unloading (HS, n=7), and 3-dayHS with HSP90 inducer administration, 17-AAG (60 mg/kg, HS+17-AAG, n=8). The experiment was carried out in accordance with the rules of biomedical ethics (protocol 264, March 5, 2011) certified by the Russian Academy of Sciences Committee on Bioethics. The HS model was used to employ a tail-traction method of noninvasive tail-casting procedure (without anesthesia), as described previously (Morey, E. R.,1979). The rats were sacrificed by intraperitoneal nembutal overdose (75 mg/kg). The relative weight of soleus muscle to body weight (soleus/BW (mg/g)) in HS group was less than those of the C and HS+17-AAG groups (p<0,05). We revealed HSP90, HSP70 mRNA decrease in HS (but not in HS+17-AAG) group vs. C (p<0,05). The unloading resulted in significant of µ-calpain and conjugated ubiquitin (Ub) increasing (proteins as well as mRNA levels) vs. C group whereas 17-AAG administration prevented these alterations (studied by SDS-PAGE and rt-PCR). pFOXO3 protein was decreased only in HS group vs. C, but not in HS+17-AAG. Content of E3-lygases (MuRF-1, MAFbx) mRNA was increased in both suspended groups. We found equal amounts of pAkt in control and HS+17-AAG groups (in parallel to pFOXO3 levels), but pAkt levels in the HS group were diminished. Modulation of HSP90 function by 17-AAG treatment lends support to the suggestion that anabolic signaling and phosphorylation of the FOXO3 transcription factor are controlled by Akt. Soleus atrophy attenuation in the HS+17-AAG group was not associated with phosphorylation of p70S6k, because its level was equal in all three groups of rats. In summary, 17-AAG administration (enhancing HSP90 expression) under HS conditions attenuated soleus atrophy and prevented HSPs mRNA drop, maintained p-Akt and p-FOXO 3 content at the control level. Moreover, the prevention of ubiquitination and μ-calpain expression were increased in the HS+17-AAG-treatment group. Supported by grant RFBR 11-04-00787-a