PHAPI/pp32 Suppresses Tumorigenesis by Stimulating Apoptosis

Wei Pan,Li S Da Graca,Yufang Shao,Qian Yin,Hao Wu,Xuejun Jiang

doi:10.1074/jbc.m805801200

Wei Pan, Li S Da Graca + Show 4 more

Open Access

https://doi.org/10.1074/jbc.m805801200

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Abstract

PHAPI/pp32 is a tumor suppressor whose expression is altered in various human cancers. Although PHAPI possesses multiple biochemical activities, the molecular basis for its tumor-suppressive function has remained obscure. Recently we identified PHAPI as an apoptotic enhancer that stimulates apoptosome-mediated caspase activation. In this study, we defined the structural requirement for its activity to stimulate caspase activation using a series of truncation mutants of PHAPI. Further, utilizing these mutants, we provide evidence to support the model that the apoptotic activity of PHAPI is required for its tumor-suppressive capability. Consistently, pp32R1, a close homolog of PHAPI and yet an oncoprotein, is not able to stimulate caspase activation. A highly discrete region between these two proteins localizes to an essential caspase activation motif of PHAPI. Additionally, PHAPI is predominantly a nuclear protein, and it can translocate to the cytoplasm during apoptosis. Disruption of the nuclear localization signal of PHAPI caused a modest decrease of its tumor-suppressive function, indicating that nuclear localization of PHAPI contributes to, but is not essential for, tumor suppression.

Highlights

The multiple biochemical activities of PHAPI suggest that this protein is involved in diverse biological events
The apoptosis-accelerating activity of PHAPI provides a plausible explanation for its tumor-suppressive role, which is consistent with recent studies showing that in human breast cancer and nonsmall cell lung cancer, PHAPI expression level correlates with the sensitivity of cancer cells to Apaf-1-mediated apoptosis [28, 33]
To directly examine this possibility, we determined the structural requirement for the apoptotic activity of PHAPI and provided strong evidence to support the model that the apoptotic activity of PHAPI is required for its tumor-suppressive function

Summary

Introduction

The multiple biochemical activities of PHAPI suggest that this protein is involved in diverse biological events. We measured the molecular mass of purified recombinant PHAPI by light scattering technique coupled with gel-filtration chromatography (Fig. 4B), the result (29.2 kDa) indicates that active PHAPI is a monomeric protein. When fulllength PHAPI or the individual truncation mutants are expressed in these cells, the tumor-suppressive function of these proteins to decrease soft agar colony formation can be detected.

Results

Conclusion