Mechanical forces and TGFβ1 reduce podocyte adhesion through α3β1 integrin downregulation

Methods. Conditionally immortalized murine podocytes were exposed to mechanical stretch and/or TGF beta 1 for 48 h. Podocyte adhesion, apoptosis and alpha 3 beta 1 integrin expression were assessed. Results. Stretch and TGF beta 1 significantly reduced podocyte adhesion and alpha 3 beta 1 integrin expression, events paralleled by increased apoptosis. Blockade of beta 1 integrin, with a specific antibody, demonstrated a reduced podocyte adhesion indicating that beta 1 integrin downregulation was required for the loss of podocyte adhesion. This was linked to an increase in podocyte apoptosis. The role of apoptosis in podocyte adhesion was further investigated using caspase-3 inhibitors. Podocyte apoptosis inhibition did not affect stretch- and TGF beta 1-mediated integrin downregulation and the loss of podocyte adhesion, suggesting that alpha 3 beta 1 integrin downregulation is sufficient to alter cell adhesion. Although stretch significantly increased podocyte TGF beta type I, II and III receptors but not podocyte TGF beta 1 secretion, the combination of stretch and TGF beta 1 did not show any additive or synergistic effects on podocyte adhesion and alpha 3 beta 1 integrin expression. Conclusions. These results suggest that downregulation of alpha 3 beta 1 integrin expression, by mechanical forces or TGF beta 1, is per se sufficient to reduce podocyte adhesion. Apoptosis may represent a parallel important determinant of the podocyte loss from the GBM.