Selective TSC1 deletion in smooth muscle activates mTOR signaling and induces pulmonary hypertension

Abstract

Objective: Constitutive activation of the mammalian target of rapamycin (mTOR) mTORC1 and mTORC2 is associated with pulmonary hypertension (PH) and sustained growth of pulmonary-artery (PA) smooth muscle cells (SMCs). We investigated whether selective mTORC1 activation in SMCs induced by deleting the negative mTORC1 regulator tuberous sclerosis complex 1 gene (TSC1) was sufficient to produce PH in mice.

Approach and Results: Mice expressing Cre recombinase under SM22 promoter control were crossed with TSC1^LoxP/LoxP mice to generate SM22-TSC1^-/- mice. At 8 weeks of age, SM22-TSC1^-/- mice exhibited PH with marked increases in distal pulmonary-artery muscularization and Ki67-positive PA-SMC counts, without systemic hypertension or cardiac dysfunction. Marked activation of the mTORC1 substrates S6K and 4EBP and of the mTORC2 substrates p-Akt^Ser473 and GSK3 was found in lungs and pulmonary vessels from SM22-TSC1^-/- mice compared to controls. Treatment with 5 mg/Kg rapamycin for 3 weeks to inhibit mTORC1 and mTORC2 fully reversed PH in SM22-TSC1^-/- mice. In chronically hypoxic mice and SM22-5HTT+ mice exhibiting PH associated with mTORC1 and mTORC2 activation, PH was maximally attenuated by low-dose rapamycin associated with selective mTORC1 inhibition. Cultured PA-SMCs from SM22-TSC1^-/-, SM22-5HTT+, and chronically hypoxic mice exhibited similar sustained growth-rate enhancement and constitutive mTORC1 and mTORC2 activation; both effects were abolished by rapamycin. Deletion of the downstream mTORC1 effectors S6K1/2 in mice also activated mTOR signaling and induced PH. Conclusion: Activation of mTORC1 signaling leads to increased PA-SMC proliferation and subsequent PH development.