Novel mTORC1 Booster LAPTM4A Potentiates Pathological Cardiac Hypertrophy

Excessive unnecessary protein accumulation in cardiomyocyte is a leading contributor for pathological cardiac hypertrophy and has been found closely regulated by the mTORC1 (mechanistic target of rapamycin complex 1) signaling and lysosome transmembrane proteins. However, the precise regulatory mechanism stratifying mTORC1 signaling and the specific functions of lysosomal proteins in protein homeostasis of cardiomyocytes remain largely unidentified.

We screened lysosomal genes conserved in mice, rats, and humans. Adenoviral infection of rat cardiomyocytes was used to assess the functional role of LAPTM4A (lysosome-associated protein transmembrane 4A). To evaluate its effects in vivo, adeno-associated virus 9 driven by the cardiac troponin T promoter was used for cardiomyocyte-specific expression. RNA sequencing and mass spectrometry-based proteomics were performed to elucidate the underlying molecular mechanisms. Last, a dual-luciferase reporter assay was used to screen a Food and Drug Administration-approved drug library for compounds that suppress LAPTM4A expression.

Lysosomal transmembrane proteins expressed in cardiomyocytes were screened for their roles in regulating hypertrophy, and LAPTM4A emerged as a potent promoter of cardiomyocyte hypertrophy and prohypertrophic gene expression. Overexpression of LAPTM4A aggravated cardiac remodeling and dysfunction by enhancing mTORC1-p70S6K (70-kDa ribosomal protein S6 kinase)/4EBP1 (eukaryotic translation initiation factor 4E-binding protein 1)-mediated protein synthesis, without affecting lysosomal autophagy, in a NEDD4L (neural precursor cell expressed developmentally downregulated 4-like)-dependent manner. Mechanistically, LAPTM4A directly interacted with NEDD4L, facilitating K63-linked ubiquitination of AKT (protein kinase B [v-akt murine thymoma viral oncogene homolog]) and subsequent activation of mTORC1 signaling. Cardiomyocyte-specific deletion of LAPTM4A significantly attenuated myocardial hypertrophy and fibrosis induced by transverse aortic constriction in mice. Furthermore, a dual-luciferase reporter screen identified magnolol, a Food and Drug Administration-approved compound, as a suppressor of LAPTM4A expression with marked cardioprotective effects in vivo.

Our study identified a novel mTORC1 booster LAPTM4A and verified interrupting the LAPTM4A-mTORC1 axis can significantly inhibit excessive protein synthesis and pathological cardiac hypertrophy, which might represent an attractive therapeutic approach for this disease.