CONTRIBUTION OF mTORC1 AND GSK-3 TO THE REGULATION OF RIBOSOME BIOGENESIS IN RAT POSTURAL MUSCLE UNDER SIMULATED MICROGRAVITY

The investigation was aimed to reveal a potential role of mechanistic target of rapamycin (mTORC1) and glycogen synthase kinase (GSK-3) in the regulation of ribosome biogenesis and protein synthesis in rat soleus muscle during simulated gravitational unloading. Wistar rats were subject to 7-day tail-suspension (hindlimb unloading) and treated with rapamycin (mTORC1 inhibitor) or AR-A014418 (GSK-3 inhibitor). Protein synthesis was measured with SUnSET. Phosphorylation of glycogen synthase-1 (GS-1) and ribosomal protein S6 (rpS6) was measured with the use of Western-blotting. Levels of 28S and 18S rRNA were determined using agarose gel electrophoresis. Expression of 47S pre-rRNA and c-Myc was assessed with RT-PCR. Gravitational unloading led to an increase in phosphorylation of GS-1 (a marker of GSK-3 activity) and inhibited phosphorylation of rpS6 (a marker of mTORC1 activity), expression of the ribosome biogenesis markers (total RNA РНК, 28S and 18S rRNA, 47S pre-rRNA, с-Myc) and protein synthesis in soleus muscle. GSK-3 inhibition prevented an increase in GS-1 phosphorylation and, partly, a reduction in the markers of both ribosomal biogenesis and protein synthesis. The mTORC1 inhibitor contributed to the phospho-rpS6 reduction but did not substantially influence the parameters of translational capacity and protein synthesis. To sum up, unlike mTORC1, an elevated GSK-3 activity can contribute to the inhibition of ribosome biogenesis and protein synthesis in rat soleus muscle under 7-day gravitational unloading.