IκB Kinases Serve as a Target of CD28 Signaling

Edward W Harhaj,Shao-Cong Sun

doi:10.1074/jbc.273.39.25185

Edward W Harhaj, Shao-Cong Sun

Open Access

https://doi.org/10.1074/jbc.273.39.25185

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Abstract

Optimal T cell activation and interleukin-2 production requires a second signal in addition to antigen-mediated T cell receptor (TCR) signaling. The CD28 molecule has been demonstrated to act as an effective costimulatory molecule upon binding by B7.1 or B7.2 present on antigen-presenting cells. The CD28 signal acts in concert with the TCR signal to significantly augment activation of the NF-kappaB family of transcription factors. The interleukin-2 gene is regulated by NF-kappaB among other transcription factors, in part, via a CD28 responsive element (CD28RE) present in the IL-2 promoter. Enhanced activation of NF-kappaB by CD28 is mediated by rapid phosphorylation and proteasome-mediated degradation of the NF-kappaB inhibitory proteins IkappaB alpha and IkappaB beta, which allows for accelerated nuclear expression of the liberated NF-kappaB. Herein, we provide evidence that the catalytic activities of two recently identified IkappaB kinases, IKKalpha and IKKbeta, are significantly elevated when T cells are stimulated through CD28 in addition to mitogen treatment. Catalytically inactive forms of IKKs are able to block the in vivo phosphorylation of IkappaB alpha induced by mitogen and CD28. Furthermore, CD28-mediated reporter gene transactivation of the CD28RE/AP-1 composite element is consistently attenuated by the IKK mutants. These findings suggest that cellular signaling pathways initiated at the TCR and CD28 converge at or upstream of IKK, resulting in more robust kinase activity and enhanced and prolonged NF-kappaB activation.

Highlights

T cell activation and IL-21 production is critically dependent on the transmission of signals derived from the cell surface to the nucleus in order to modulate changes in gene expression [1]
The IL-2 gene promoter contains an enhancer known as the CD28 responsive element (CD28RE) which functions as an integrator of transcription factors activated through the T cell receptor (TCR) and CD28 and is essential for IL-2 transcription mediated through CD28 [6]
The T cell auxiliary molecule, CD28, which provides a costimulatory signal for T cell activation and IL-2 production, is a potent inducer of NF-␬B [23, 39]

Summary

Introduction

T cell activation and IL-21 production is critically dependent on the transmission of signals derived from the cell surface to the nucleus in order to modulate changes in gene expression [1]. The phosphorylated I␬Bs are ubiquitinated and targeted to the proteasome for proteolytic degradation [15] Signals such as TNF-␣ or mitogens such as PMA, which selectively induce the degradation of only I␬B␣, are associated with the transient activation of NF-␬B since the I␬B␣ gene is positively regulated by NF-␬B factors (16 –19). We and others have previously demonstrated that ligation of CD28 initiates a potent costimulatory signal leading to the rapid and persistent degradation of I␬B␤ and enhanced degradation of I␬B␣ [20, 23] It is not known if CD28 is mediating enhanced I␬B kinase activity. We report here that CD28 potentiates the kinase activity of IKK␣ and IKK␤, which are only weakly activated by mitogen or TCR signals alone

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