Cytoplasmic localization of p21 protects trophoblast giant cells from DNA damage induced apoptosis.

Christelle De Renty,Zakir Ullah,Melvin L Depamphilis,Robert Oshima

doi:10.1371/journal.pone.0097434

Abstract

Proliferating trophoblast stem cells (TSCs) can differentiate into nonproliferating but viable trophoblast giant cells (TGCs) that are resistant to DNA damage induced apoptosis. Differentiation is associated with selective up-regulation of the Cip/Kip cyclin-dependent kinase inhibitors p57 and p21; expression of p27 remains constant. Previous studies showed that p57 localizes to the nucleus in TGCs where it is essential for endoreplication. Here we show that p27 also remains localized to the nucleus during TSC differentiation where it complements the role of p57. Unexpectedly, p21 localized to the cytoplasm where it was maintained throughout both the G- and S-phases of endocycles, and where it prevented DNA damage induced apoptosis. This unusual status for a Cip/Kip protein was dependent on site-specific phosphorylation of p21 by the Akt1 kinase that is also up-regulated in TGCs. Although cytoplasmic p21 is widespread among cancer cells, among normal cells it has been observed only in monocytes. The fact that it also occurs in TGCs reveals that p57 and p21 serve nonredundant functions, and suggests that the role of p21 in suppressing apoptosis is restricted to terminally differentiated cells.

Highlights

Genes that regulate cell proliferation and differentiation during mammalian development are often deregulated in human cancer, thereby permitting either unrestrained cell proliferation or increased survivability
Given that monocytes migrate into tissues where they differentiate further into macrophages, we considered the possibility that cytoplasmic localization of p21 is restricted to terminally differentiated cells that no longer proliferate
Based on RT-PCR and Western immuno-blotting analyses, previous studies have shown that all three Cip/Kip genes are transcribed in both trophoblast stem cells (TSCs) and trophoblast giant cells (TGCs), but the p21 and p57 proteins are expressed primarily, if not exclusively, in TGCs [30,31,33]

Summary

Introduction

Genes that regulate cell proliferation and differentiation during mammalian development are often deregulated in human cancer, thereby permitting either unrestrained cell proliferation or increased survivability. One example is the phosphatidylinositol kinase (PI3K) signal transduction pathway that influences cell proliferation, survival, metabolism, and metastasis [1]. Akt/PKB is activated by growth factors and promotes cell survival by preventing apoptosis [3]. The p21 protein is one of the three Cip/Kip cyclin-dependent kinase (CDK) inhibitors that commonly localize to the nucleus where they regulate cell proliferation and differentiation [9]. Akt1-dependent cytoplasmic localization of p21 occurs in a variety of cancers where it promotes tumorigenesis by inhibiting proteins essential for apoptosis [5,6,7,10,11,12,13,14]

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