Identification of the Major Phosphorylation Site in Bcl-xL Induced by Microtubule Inhibitors and Analysis of Its Functional Significance

Meenakshi Upreti,Elena N Galitovskaya,Rong Chu,Alan J Tackett,David T Terrano,Susana Granell,Timothy C Chambers

doi:10.1074/jbc.m805019200

Meenakshi Upreti, Elena N Galitovskaya + Show 5 more

Open Access

https://doi.org/10.1074/jbc.m805019200

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Abstract

Vinblastine and other microtubule inhibitors used as antimitotic cancer drugs characteristically promote the phosphorylation of the key anti-apoptotic protein, Bcl-xL. However, putative sites of phosphorylation have been inferred based on potential recognition by JNK, and no direct biochemical analysis has been performed. In this study we used protein purification and mass spectrometry to identify Ser-62 as a single major site in vivo. Site-directed mutagenesis confirmed Ser-62 to be the site of Bcl-xL phosphorylation induced by several microtubule inhibitors tested. Vinblastine-treated cells overexpressing a Ser-62 --> Ala mutant showed highly significantly reduced apoptosis compared with cells expressing wild-type Bcl-xL. Co-immunoprecipitation revealed that phosphorylation caused wild-type Bcl-xL to release bound Bax, whereas phospho-defective Bcl-xL retained the ability to bind Bax. In contrast, phospho-mimic (Ser-62 --> Asp) Bcl-xL exhibited a reduced capacity to bind Bax. Functional tests were performed by transiently co-transfecting Bax in the context of different Bcl-xL mutants. Co-expression of wild-type or phospho-defective Bcl-xL counteracted the adverse effects of Bax expression on cell viability, whereas phospho-mimic Bcl-xL failed to provide the same level of protection against Bax. These studies suggest that Bcl-xL phosphorylation induced by microtubule inhibitors plays a key pro-apoptotic role at least in part by disabling the ability of Bcl-xL to bind Bax.

Highlights

Bcl-2 proteins function in a hierarchical fashion, and different models have been proposed to explain their complex interrelationships [1,2,3]
Whereas several reports suggest that phosphorylation is catalyzed by c-Jun NH2-terminal kinase (JNK),3 in one study phosphorylation was reported to occur on serine [16], and in another study phosphorylation was reported to occur on threonine residues [14]
In a carefully designed study using synchronized cells examined over an extended time-course, we showed that when cells are treated with both a JNK inhibitor and a microtubule inhibitor, the JNK inhibitor slows the normal progression to mitotic arrest, and events such as Bcl-xL/Bcl-2 phosphorylation are delayed by many hours [17]

Summary

Introduction

Bcl-2 proteins function in a hierarchical fashion, and different models have been proposed to explain their complex interrelationships [1,2,3]. Expression of phospho-defective forms of Bcl-2 renders some cell types more resistant to microtubule inhibitors, suggesting that phosphorylation antagonizes Bcl-2 anti-apo-

Results

Conclusion