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Apoptosis, a form of cell death, plays a fundamental role in tissue homeostasis. Dysregulated apoptosis is intimately linked to immunologic diseases and cancer.
One of the crucial events of the mitochondrial pathway of apoptosis is mitochondrial outer membrane permeabilization (MOMP), which is carried out by the proapoptotic proteins BAK and BAX. Once MOMP occurs, cytochrome c is released from the mitochondria to activate the caspase cascade. The electrical potential across the mitochondrial membrane is then lost, and the cell is committed to self-destruction and death.
Before scientists discovered caspase activation as the prevailing pathway for cell apoptosis, a widely held belief was that sphingomyelin metabolites, especially ceramide, had a fundamental role in engaging cell death. However, the underlying molecular mechanisms were poorly defined, and the significance of the ceramide pathway had been largely ignored.
Findings from Douglas R. Green, PhD (Immunology), and his group have now connected these two fundamental processes for the first time. In a report published in the journal Cell, the team showed that sphingomyelin and ceramide metabolites, sphingosine-1-phosphate (S1P) and its breakdown product hexadecenal (Hex), act as cofactors during the activation of BAK and BAX, respectively.
This work was initiated after the researchers observed that despite the central importance of mitochondria in apoptosis, highly purified mitochondrial membranes could not activate BAK or BAX. Such impairment, however, was restored by components from the heterotypic membranes (e.g., endoplasmic reticulum).
Biochemical fractionation studies revealed that the enrichment of two neutral sphinogomyelinases (SMases) in these heterotypic membranes and, more importantly, the addition of the purified SMases to the purified mitochondrial membrane preparations was sufficient to restore mitochondria-dependent activation of BAK and BAX. Through a series of biochemical and functional investigations, the team identified S1P and Hex, the two sphingolipids activated downstream of SMases, as the missing components needed to activate BAK and BAX.
Although the SMase activity resides in the heterotypic membranes, S1P and Hex are most likely generated within mitochondrial membranes. The physical proximity of the endoplasmic reticulum and mitochondrial membranes facilitates their molecular interactions.
This work by Dr. Green and colleagues revealed fundamental mechanisms of apoptosis by identifying the components of sphingolipid metabolism as specific cofactors that engage the mitochondrial pathway of apoptosis. It represents a new perspective on the intricate interaction between metabolic pathways and signal transduction.
Further exploration of these basic cellular events may provide new targets for therapeutic intervention of human immunologic diseases and cancer.