Antiapoptotic protein partners fortilin and MCL1 independently protect cells from 5-fluorouracil-induced cytotoxicity

Fortilin, a potent 172-amino acid antiapoptotic polypeptide (Li, F., Zhang, D., and Fujise, K. (2001) J. Biol. Chem. 276, 47542-47549), binds MCL1, a protein of the antiapoptotic Bcl-2 family. The fortilin-MCL1 interaction stabilizes and increases the half-life of fortilin but not necessarily of MCL1 (Zhang, D., Li, F., Weidner, D., Mnjoyan, Z. H., and Fujise, K. (2002) J. Biol. Chem. 277, 37430-37438). It is not known to what extent each protein depends on the other for its apoptotic activity. Here, we present evidence that fortilin and MCL1 are capable of functioning as antiapoptotic proteins independently of each other. Using a robust small interfering RNA (siRNA)-mediated gene silencing system developed in our laboratory, we analyzed the cytoprotective effects of fortilin and MCL1 together and apart in U2OS cell lines exposed to 5-fluorouracil (5-FU) in both monoclonal and polyclonal cell populations. When MCL1 was silenced by MCL1-targeted siRNA, fortilin was still able to protect cells from 5-FU-induced cytotoxicity in a dose-dependent manner. Conversely, when fortilin was silenced by fortilin-targeted siRNA, MCL1 was also able to protect cells from 5-FU-indueed cytotoxicity in a dose-dependent manner. Together, these data clearly suggest that fortilin and MCL1 can exert their cytoprotective activities independently of each other. The silencing of fortilin and MCL1 did not qualitatively change the subcellular localization of MCL1 and fortilin, respectively. The biological significance of fortilin-MCL1 interaction may be that it increases cellular resistance to apoptosis by allowing MCL1, an independently antiapoptotic protein, to stabilize another independently antiapoptotic protein, fortilin.