Full Activation of Kinase Protein Kinase B by Phosphoinositide-Dependent Protein Kinase-1 and Mammalian Target of Rapamycin Complex 2 Is Required for Early Natural Killer Cell Development and Survival.

Junming He,Meixiang Yang,Jun Zhao,Xinlei Hou,Yuhe Quan,Zhongjun Dong

doi:10.3389/fimmu.2020.617404

Junming He, Meixiang Yang + Show 4 more

Open Access

https://doi.org/10.3389/fimmu.2020.617404

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Abstract

The role of PI3K-mTOR pathway in regulating NK cell development has been widely reported. However, it remains unclear whether NK cell development depends on the protein kinase B (PKB), which links PI3K and mTOR, perhaps due to the potential redundancy of PKB. PKB has two phosphorylation sites, threonine 308 (T308) and serine 473 (S473), which can be phosphorylated by phosphoinositide-dependent protein kinase-1 (PDK1) and mTORC2, respectively. In this study, we established a mouse model in which PKB was inactivated through the deletion of PDK1 and Rictor, a key component of mTORC2, respectively. We found that the single deletion of PDK1 or Rictor could lead to a significant defect in NK cell development, while combined deletion of PDK1 and Rictor severely hindered NK cell development at the early stage. Notably, ectopic expression of myristoylated PKB significantly rescued this defect. In terms of mechanism, in PDK1/Rictor-deficient NK cells, E4BP4, a transcription factor for NK cell development, was less expressed, and the exogenous supply of E4BP4 could alleviate the developmental defect of NK cell in these mice. Besides, overexpression of Bcl-2 also helped the survival of PDK1/Rictor-deficient NK cells, suggesting an anti-apoptotic role of PKB in NK cells. In summary, complete phosphorylation of PKB at T308 and S473 by PDK1 and mTORC2 is necessary for optimal NK cell development, and PKB regulates NK cell development by promoting E4BP4 expression and preventing cell apoptosis.

Highlights

Natural killer (NK) cells are important innate lymphocytes capable of mediating both cytotoxicity and cytokine production in response to tumor cells or virus-infected cells, as well as rejecting major histocompatibility complex class I (MHC-I)-mismatched allogeneic bone marrow [1, 2]
We demonstrated that complete phosphorylation of protein kinase B (PKB) at threonine 308 (T308) and serine 473 (S473) by phosphoinositidedependent kinase 1 (PDK1) and mTOR complex 2 (mTORC2) is necessary for optimal NK cell development
We previously showed that PDK1 deletion at the NK progenitor (NKp) stage caused a reduction of NK cell number in PDK1fl/fl/ CD122Cre/+ (PDK1KO) mice

Summary

Introduction

Natural killer (NK) cells are important innate lymphocytes capable of mediating both cytotoxicity and cytokine production in response to tumor cells or virus-infected cells, as well as rejecting major histocompatibility complex class I (MHC-I)-mismatched allogeneic bone marrow [1, 2]. Mice simultaneously lacking the PI3K subunits P110g and d exhibit a severe impairment in early NK cells development and function [6, 7].We previously demonstrated that phosphoinositidedependent kinase 1 (PDK1), a kinase connecting PI3K and mTOR, is essential for NK cell development by inducing transcription factor E4BP4 and maintaining IL-15 responsiveness [8]. Using gene-targeting technique based on Ncr1-Cre mice, recent studies have shown the deletion of mTORC2 at the terminal stage of NK cells did not affect the transcriptional regulation of NK cells, but it can inhibit the function of NK cells by inhibiting mTORC1 [9, 11]

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