Nuclear Export of Phosphorylated Galectin-3 Regulates Its Antiapoptotic Activity in Response to Chemotherapeutic Drugs

Abstract

Galectin-3 (Gal-3), a member of the β-galactoside binding protein family containing the NWGR antideath motif of the Bcl-2 protein family, is involved in various aspects of cancer progression. Previously, it has been shown that the antiapoptotic activity of Gal-3 is regulated by the phosphorylation at Ser⁶ by casein kinase 1 (CK1). Here we questioned how phosphorylation at Ser⁶ regulates Gal-3 function. We have generated serine-to-alanine (S6A) and serine-to-glutamic acid (S6E) Gal-3 mutants and transfected them into the BT-549 human breast carcinoma cell line, which does not express Gal-3. BT-549 cell clones expressing wild-type (wt) and mutant Gal-3 were exposed to chemotherapeutic anticancer drugs. In response to the apoptotic insults, phosphorylated wt Gal-3 was exported from the nucleus to the cytoplasm and protected the BT-549 cells from drug-induced apoptosis while nonphosphorylated mutant Gal-3 neither was exported from the nucleus nor protected BT-549 cells from drug-induced apoptosis. Furthermore, leptomycin B, a nuclear export inhibitor, increased the cisplatin-induced apoptosis of Gal-3 expressing BT-549 cells. These results suggest that Ser⁶ phosphoryaltion acts as a molecular switch for its cellular translocation from the nucleus to the cytoplasm and, as a result, regulates the antiapoptotic activity of Gal-3.

We thank Victor Hogan and Vivian Powell for editing the manuscript.

This work was supported in part by grant CA46120 from the NIH/NCI (to A.R.), by DOD grant DAMD17-99-1-9442 (to H.-R.C.K.), and by grant NIH/NCI CA 69480 (to R.S.B.).