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Role of Glucosylceramide in Lung Endothelial Cell Fate and Emphysema.

RATIONALE
The loss of pulmonary endothelial cells in emphysema is associated with increased lung ceramide. Ceramide perturbations may cause adaptive alterations in other bioactive sphingolipids, with pathogenic implications. We reported a negative correlation between emphysema and circulating glycosphingolipids (GSLs). Glucosylceramide (GlcCer), the initial GSL synthesized from ceramide by glucosylceramide synthase (GCS), is required for embryonic survival, but its role in the lung is unknown.


OBJECTIVES
To determine if cigarette smoking (CS) alters lung GlcCer and to elucidate the role of GCS in lung endothelial cell fate.


METHODS
GlcCer was measured by tandem mass spectrometry in bronchoalveolar lavage fluid of CS- or elastase- exposed mice, and GCS was detected by western blotting in chronic obstructive pulmonary disease (COPD) lungs and CS-extract exposed primary human lung microvascular endothelial cells (HLMVECs). The role of GlcCer and GCS on mTOR signaling, autophagy, lysosomal function, and cell death were studied in HLMVECs, with or without CS exposure.


MEASUREMENTS AND MAIN RESULTS
Mice exposed to chronic CS or to elastase and patients with COPD exhibited significantly decreased lung GlcCer and GCS. In mice, lung GlcCer levels were negatively correlated with airspace size. GCS inhibition in HLMVEC increased lysosomal pH, suppressed mTOR signaling, and triggered autophagy with impaired lysosomal degradation and apoptosis, recapitulating CS effects. In turn, increasing GlcCer by GCS overexpression in HLMVEC improved autophagic flux and attenuated CS-induced apoptosis.


CONCLUSIONS
Decreased GSL production in response to CS may be involved in emphysema pathogenesis, associated with autophagy with impaired lysosomal degradation and lung endothelial cell apoptosis.