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Boosting Right Ventricular (RV) Autophagy with Rapamycin is Insufficient to Attenuate RV Dysfunction, Despite Favorable Effects on RV Afterload

Anthony Cucci

Jordan Wood

Marjorie Albrecht

Amanda Fisher

Mary Van Demark

Robert Presson

Kara Goss

Andrea Frump

Todd Cook

Mary Beth Brown


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Conference: 2014 International PHA Conference and Scientific Sessions

Release Date: 06.21.2014

Presentation Type: Abstracts

File Download: 2014 Conference Abstracts - Anthony Cucci

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Background: We have previously demonstrated an impaired autophagic flux in the myocardium of severe experimental right ventricular failure (RVF), which correlated with the severity of RV dysfunction. Since recent studies in pulmonary hypertension models suggested favorable outcomes with the autophagy inducer rapamycin, we investigated the effects of boosting autophagy on an already dysfunctional RV in a model of maladaptive RV remodeling.

Methods: RVF was induced in male Sprague-Dawley rats by injection of Su5416 (20 mg/kg) and subsequent exposure to hypobaric hypoxia (0.5 atm; 3 weeks), followed by 4 weeks of re-exposure to room air (SuHx).  A group of animals were treated with rapamycin (3mg/kg/d, n=8) or vehicle (n=7) during room air re-exposure.  Untreated normoxia (n=9) or SuHx rats (n=9) served as controls. Hemodynamic, morphologic, and echocardiographic parameters of RV function were measured, complemented by biochemical assessment of cardiomyocyte autophagy, survival, and apoptosis.  p<0.05 was considered statistically significant.

Results: As expected, SuHx-RVs exhibited evidence of severe RV dysfunction on echocardiography including reduced pulmonary arterial acceleration time (PAAT), velocity time integral (VTI), and cardiac output (p<0.05 vs normoxia). SuHx RVs exhibited increased autophagic proteins LC3-II and p62 (p<0.05 vs normoxia), indicating impaired autophagic flux, which was confirmed by lack of increase in LC3-II or p62 after administration of the lysosome inhibitor chloroquine (60 mg/kg 16h prior to sacrifice; n=4).  Rapamycin administration in SuHx rats resulted in increased RV LC3-I to LC3-II conversion and decreased p62 expression, suggestive of initiation of autophagy.  However, rapamycin failed to overcome the impairment in impaired flux (evidenced by further decrease in the autolysosome marker LAMP-2), and its administration was accompanied by increased RV pro-apoptotic signaling (caspase-3 activity) and persistent RV dysfunction.  These changes occurred despite favorable rapamycin effects on RV systolic pressure (RVSP), RV hypertrophy and pulmonary vascular remodeling (p<0.05 vs. normoxia).

Conclusion: Severe experimental RVF is characterized by impaired autophagic flux. Further stimulation of early steps in the RV autophagic cascade with rapamycin was insufficient to overcome the SuHx-induced RV autophagy flux impairment, and actually stimulated pro-apoptotic signaling. Rapamycin treatment was associated with persistent RV dysfunction despite inducing favorable pulmonary vascular hemodynamic changes. Potential disparate effects on RV and pulmonary vascular endpoints need to be considered when developing novel therapeutic interventions for PAH.

Abstract Type: Basic Science

Funding: NIH T32 4685574 (AC), VA Young Investigator Award (AC), Calvin H. English Chair (IP), Gilead PAH Research Scholars Program (TL).