SLC4A7 sodium bicarbonate co-transporter controls mitochondrial apoptosis in ischaemic coronary endothelial cells

Aims: Bicarbonate transport has been shown to participate in Apoptosis under ischaemic stress. However, the precise transporting mechanisms involved in ischaemic Apoptosis are unknown and were thus the aim of the present study.

Methods and results: Rat coronary endothelial cells (EC) were exposed to simulated in vitro ischaemia for 2 h, and Apoptosis was subsequently determined by chromatin staining and Caspase-3 activity analysis. By examining the expression of bicarbonate transporters (BT) in EC by Reverse Transcriptase polymerase chain reaction and western blotting, a marked expression of the electroneutral sodium bicarbonate co-transporter (SLC4A7) was defined. To analyse the potential role of this transporter during Apoptosis, a selective inhibitor (S0859, Sanofi-Aventis) was applied. Treatment with S0859 significantly increased Caspase-3 activity and elevated the number of apoptotic EC. These results were comparable with an unselective inhibition of all BT due to withdrawal of bicarbonate in the anoxic medium. Knockdown of SLC4A7 in EC by transfecting appropriate siRNA similarly increased Apoptosis of EC under simulated ischaemia. The initial characterization of the participating mechanisms of SLC4A7-dependent Apoptosis revealed an activation of the mitochondrial pathway of Apoptosis, i.e. cleavage of caspase-9 and binding of Bax to mitochondria. In contrast, no activation of the endoplasmic reticulum-dependent pathway (caspase-12 cleavage) or the extrinsic apoptotic pathway (Caspase-8 cleavage) was found. Finally, a mitochondrial localization of SLC4A7 was demonstrated.

Conclusion: The electroneutral sodium bicarbonate co-transporter SLC4A7 localizes in mitochondria and suppresses the ischaemia-induced activation of the mitochondrial pathway of Apoptosis in coronary EC.