Abstract P117: Does Mammalian Target of Rapamycin Complex-2 Regulate Protein Degradation Pathways in the Heart?

Background: Mammalian target of rapamycin (mTOR) occurs in the cell in two distinct multiprotein complexes called mTOR complex 1 (mTORC1) and mTORC2, which contain raptor and rictor, respectively. We have recently demonstrated that mTORC1 activity is required for the hypertrophic response to aortic constriction and for the normal cardiac homeostasis. Moreover, we showed that raptor deletion causes Akt hyperphosphorylation and lower gene expression of Atrogin-1 and MuRF1, two muscle specific E3 enzymes part of the proteasomal degradation pathway. These results suggested that, as a counter-regulatory response to mTORC1 inactivation, mTORC2 reduces protein degradation via phosphorylation of Akt at Ser473. It has previously been shown that the phosphorylation state of Akt regulates Atrogin-1 and MuRF1 gene expression at the transcriptional level via FoxO. In the present study, we have tested whether mTORC2 inactivation induces the ubiquitin-proteasomal degradation pathway. Methods and results: In 10 week-old male mice, transgenic for MerCreMer driven by the α-MHC promoter and homozygous for floxed rictor, deletion of the rictor gene was induced by tamoxifen. Protein and RNA extracts were analyzed at three weeks after tamoxifen by Western blotting and qPCR, respectively. The rictor gene was efficiently ablated from the heart as its protein levels were reduced. Phosphorylation of Akt and PKC-α, direct targets of mTORC2, was abolished, identifying these signaling molecules as downstream targets of mTORC2 in the heart. However, the reduced Akt phosphorylation was not associated with any changes in the mRNA levels of Atrogin-1, MuRF1, and MuRF3. Conclusion: Our study suggests that mTORC2-induced phosphorylation of Akt is not required for the maintenance of low expression levels of these genes. Further studies are ongoing to identify the factors that modulate Atrogin-1 and MuRF1 gene transcription in the heart.