Stretch-induced alveolar type II cell apoptosis: role of endogenous bradykinin and PI3K-Akt signaling

Apoptosis of alveolar type II (ATII) cells in response to high-amplitude mechanical stretch represents an important mechanism of ventilation-induced lung injury. Previously, it was demonstrated in an in vitro model that stretch-induced ATII cell apoptosis was prevented by angiotensin-converting enzyme (ACE) inhibitors. This study investigates the mechanism by which ACE inhibitors prevent stretch-induced apoptosis and elucidates the role of bradykinin as an endogenous anti-apoptotic factor. Rat ATII cells cultured on flexible membranes were subjected to cyclic stretch (40 cycles/min; 30% increase in surface area) and compared with static controls. Angiotensinogen, the bradykinin precursor T-kininogen, and bradykinin receptor expression were measured by RT-PCR; Angiotensin II and phosphoinositol 3 OH-kinase (PI3K) activity (as phospho-Akt) were measured by enzyme-linked immunosorbent assay; and Bcl-2 and Bcl-X(L) were measured by Western blot. Stretch did not influence angiotensinogen expression or induce angiotensin II generation. The angiotensin II receptor antagonist saralasin did not prevent stretch-induced apoptosis, whereas ACE inhibitors did. Stretch reduced ATII cell bradykinin release (T-kininogen expression and bradykinin supernatant concentration), and subsequently led to reduced PI3K activity and decreased concentrations of the anti-apoptotic proteins Bcl-2/Bcl-X(L). Bradykinin substitution or addition of keratinocyte or hepatocyte growth factor prevented stretch-induced decrease in PI3K activity and Bcl-2/Bcl-X(L) and reduced stretch-induced apoptosis. Mechanical stretch impairs a constitutively expressed, autocrine anti-apoptotic ATII cell survival signal involving bradykinin-mediated stimulation of the PI3K-Akt-Bcl-2/Bcl-X(L) pathway. Restoration of this pathway prevents stretch-induced apoptosis. This may be beneficial when mechanical ventilation cannot completely avoid alveolar overdistension to maintain oxygenation.