PDK1 orchestrates early NK cell development through induction of E4BP4 expression and maintenance of IL-15 responsiveness.
Bottom Line: It remains largely unknown which signal is required to induce E4BP4 expression and what effects it has during NK cell differentiation.Thus, we identify a role for PDK1 signaling as a key mediator in regulating E4BP4 expression during early NK cell development.Our findings underscore the importance of IL-15 self-responsiveness through a positive feedback loop that involves PDK1-mTOR-E4BP4-CD122 signaling.
Affiliation: School of Medicine, and Center of Animal Facility, Tsinghua University, Beijing 100086, China.Show MeSH
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Mentions: PDK1 is a master molecule linking the PI3K pathway with mTOR activation via Akt. To explore whether mTOR-dependent PDK1 signaling is involved in the early NK cell development promoted by IL-15, we stimulated PDK1−/− splenocyte with recombinant IL-15–IL-15R complexes and detected the expression of nutritional receptors and mTOR signaling in CD3−CD122highNK1.1+ cells, which could eliminate the developmental discrepancy in CD122 levels between the two genotypes. We found that after overnight IL-15 stimulation, PDK1-sufficient NK cells displayed a two-to fivefold increase in CD71 and CD98, two nutritional receptors, whereas PDK1−/− NK cells nearly lost the ability to up-regulate these receptors (Fig. 5 A), and these cells consistently displayed impaired activation of AKT-mTOR1 signaling upon IL-15 exposure (Fig. 5 B). To further exclude the possibility that the impaired metabolic activation by IL-15 is caused by the variation in CD122 levels between the genotypes, several pharmacological inhibitors were chosen. Blockage of PI3K–PDK1–mTOR activation largely prevented the IL-15–triggered up-regulation of CD71 and CD98 on bone marrow and splenic NK cells (Fig. 5 C and not depicted), further demonstrating that IL-15 is able to trigger NK cell metabolic activation, which requires mTOR-dependent PI3K signaling. To directly test mTOR signaling in NK cell development, wild-type mice were injected with Torin1 to suppress mTOR activity. As expected, the proproliferative role of IL-15 was notably diminished by mTOR inhibition (Fig. 5 D). Together, these data demonstrate that PDK1 signaling is required for NK cell metabolic activation and proliferation downstream of the IL-15 receptor via activating mTOR.
Affiliation: School of Medicine, and Center of Animal Facility, Tsinghua University, Beijing 100086, China.