The Unfolded Protein Response and the Phosphorylations of Activating Transcription Factor 2 in the trans-Activation of il23a Promoter Produced by β-Glucans

Abstract

Current views on the control of IL-23 production focus on theregulation ofil23a, the gene encoding IL-23 p19, by NF- Bincombination with other transcription factors. C/EBP homolo-gous protein (CHOP), X2-Box-binding protein 1 (XBP1), acti-vator protein 1 (AP1), SMAD, CCAAT/enhancer-binding pro-tein (C/EBP ), and cAMP-response element-binding protein(CREB) have been involved in response to LPS, but no data areavailable regarding the mechanism triggered by the fungalmimic and -glucan-containing stimulus zymosan, which pro-duces IL-23 and to a low extent the related cytokine IL-12 p70.Zymosan induced the mobilization of CHOP from the nuclearfractions to phagocytic vesicles. Hypha-formingCandidaalsoinduced the nuclear disappearance of CHOP. Assay of tran-scription factor binding to theil23apromoter showed anincrease of Thr(P)-71–Thr(P)-69-activating transcription fac-tor 2 (ATF2) binding in response to zymosan. PKC and PKA/mitogen- and stress-activated kinase inhibitors down-regulatedThr(P)-71–ATF2 binding to theil23apromoter andil23amRNA expression. Consistent with the current concept ofcomplementary phosphorylations on N-terminal Thr-71 andThr-69 of ATF2 by ERK and p38 MAPK, MEK, and p38 MAPKinhibitors blunted Thr(P)-69–ATF2 binding. Knockdown ofatf2mRNA with siRNA correlated with inhibition ofil23amRNA, but it did not affect the expression ofil12/23bandil10mRNA. These data indicate the following: (i) zymosan decreasesnuclear proapoptotic CHOP, most likely by promoting its accu-mulation in phagocytic vesicles; (ii) zymosan-inducedil23amRNA expression is best explained through coordinated B-and ATF2-dependent transcription; and (iii)il23aexpressionrelies on complementary phosphorylation of ATF2 on Thr-69and Thr-71 dependent on PKC and MAPK activities.