Cigarette smoke extract induces apoptosis of rat alveolar Type II cells via the PLTP/TGF-β1/Smad2 pathway

• CS exposure causes a lung injury and enhances PLTP and active caspase-3 expression in rat lung.
• CSE decreases the proliferation of RLE-6TN and induces the apoptosis in a time and dose dependent manner.
• CSE induces PLTP accumulation and TGF-β1/Samd2 pathway, caspase-3 activation in rat alveolar Type II cells.
• PLTP is an upstream signal molecule of TGF-β1/Smad2 pathway, through which CS induces apoptosis of lung epithelial cells.
• TGF-β1 and cleaved caspase-3 production and Smad2 phosphorylation induced by CSE are partly inhibited by PLTP siRNA.

Apoptosis of alveolar epithelial cells has been implicated in the pathogenesis of acute lung injury. Phospholipid transfer protein (PLTP) may play a role in apoptosis. In the present study, the effect of the novel function of PLTP in cigarette smoke extract (CSE)-induced apoptosis of alveolar epithelial cells and the possible mechanism were examined. Male Wistar rats were exposed to air and cigarette smoke (n = 10/exposure) for 6 h/day on 3 consecutive days, then the lungs were sectioned and examined. To investigate effects on alveolar epithelial cells, rat alveolar epithelial cells (RLE-6TN) were treated with different concentrations of CSE for various times. siRNA for PLTP was transfected into cells and an inhibitor of the transforming growth factor-β1 (TGF-β1) type I receptor was administered prior to CSE exposure. Apoptosis was measured, and mRNA expression of PLTP and TGF-β1 and protein levels of PLTP, TGF-β1, p-Smad2 and cleaved caspase-3 were analyzed. The results showed that apoptosis, as well as expression of PLTP, TGF-β1, p-Smad2 and cleaved caspase-3 were all significantly increased after CSE stimulation (P < 0.05). Furthermore, the expression of TGF-β1, p-Smad2 and cleaved caspase-3 induced by CSE could be partly abrogated by knockdown of PLTP. The expression of PLTP showed no significant change as a result of TGF-β1 receptor inhibition, while cleaved caspase-3 showed a remarkable reduction. PLTP may act as an upstream signal molecule of the TGF-β1/Smad2 pathway and is likely to be involved in CSE-induced apoptosis of alveolar epithelial cells.