Apoptosis-induced release of mature sterol regulatory element-binding proteins activates sterol-responsive genes

It is well established that during the execution of the apoptotic cascade, activated caspase 3 releases sterol regulatory element-binding proteins (SREBP) from the membrane of the endoplasmic reticulum in a proteolytic reaction that is distinct from their normal sterol-dependent activation. However, it is not known whether these transcription factors are capable of activating sterol-responsive genes under such conditions. The construction of SRE expression vectors has permitted characterization of the apoptotic activation of SREBP. Cell lines stably expressing the plasma membrane marker CD32, or GFP, under the control of the SRE promoter were shown to modulate SRE gene expression on the basis of the levels of available sterols. However, during the induction of apoptosis, expression of CD32 and GFP was highly induced, even in the presence of ample sterols. Apoptotic induction of sterol-regulated genes was due to activation of caspase 3 and was impervious to treatment with sphingomyelinase, indicating that activation of SRE genes during apoptosis is sterol independent. Further characterization of this apoptotic response indicated that sterol-regulated genes are activated at an early stage in the apoptotic cascade, preceding the externalization of phosphatidylserine on the plasma membrane of apoptotic cells. These results suggest that activation of sterol-responsive genes early during apoptosis may play a role in the proper execution of this program.