TGF-β1 and IFN-γ stimulate mouse macrophages to express BAFF via different signaling pathways

Abstract

B cell-activating factor belonging to the TNF family (BAFF) is primarily expressed by macrophages and dendritic cells and stimulates the proliferation, differentiation, and survival of B cells and their Ig production. In the present study, we examined the pathways by which TGF-beta1 and IFN-gamma induce BAFF expression to see if TGF-beta1 and IFN-gamma regulate B cell differentiation via macrophages. We found that TGF-beta1 stimulated mouse macrophages to express BAFF and that a typical TGF-beta signaling pathway was involved. Thus, Smad3 and Smad4 promoted BAFF promoter activity, and Smad7 inhibited it, and the BAFF promoter was shown to contain three Smad-binding elements. Importantly, TGF-beta1 enhanced the expression of membrane-bound and soluble forms of BAFF. IFN-gamma further augmented TGF-beta1-induced BAFF expression. IFN-gamma caused phosphorylation of CREB, and overexpression of CREB increased IFN-gamma-induced BAFF promoter activity. Furthermore, H89, a protein kinase A (PKA) inhibitor, abrogated the promoter activity. Neither Stat1alpha (a well-known transducing molecule of IFN-gamma) nor AG490 (a JAK inhibitor) affected BAFF expression in response to IFN-gamma. Taken together, these results demonstrate that TGF-beta1 and IFN-gamma up-regulate BAFF expression through independent mechanisms, i.e., mainly Smad3/4 and PKA/CREB, respectively.

TGF-β1 Stimulates Mouse Macrophages to Express APRIL through Smad and p38MAPK/CREB Pathways

“…Between the two pathways, we do not distinguish at the moment which pathway is more relevant to TGFβ1-induced APRIL expression as often demonstrated by others (Hanafusa et al, 1999;Kamaraju and Roberts, 2005;Patel et al, 2010). We have previously shown that TGF-β1 induces the expression of BAFF, another TNF family member, mainly through Smad3/4 signaling (Kim et al, 2008). Thus, the mechanisms underlying APRIL and BAFF induction by TGF-β1 are slightly different, though both molecules are involved in Ig synthesis by B lymphocytes.…”

“…Ig isotype switching is a critical event in B-cell differentiation, and TGF-β1 is a specific switching factor for the IgA and IgG2b mouse isotype (Kim and Kagnoff, 1990;McIntyre et al, 1993). Since we have previously demonstrated that TGF-β1 stimulates mouse macrophages to express BAFF (Kim et al, 2008), it was worthwhile to investigate if TGF-β1 can modulate macrophages to express another member of the same family, APRIL. Indeed, we found that TGF-β1 induced APRIL transcription in the mouse macrophage cell line, P388D1 as shown in Fig.…”

“…We note that the present report does not address this issue and that it is essential to clone the promoter region to elucidate whether this is the case. Nevertheless, we and others have been unable thus far to clone the APRIL promoter region, although we were successful in the construction of a mouse BAFF promoter reporter (Kim et al, 2008). One of the probable obstacles in determining the APRIL promoter region is the intrinsic complexity of the gene structure and its expression pattern: another TNF family gene, TWEAK, is closely linked to the APRIL gene and the two are often expressed together as a TWE-PRIL fusion protein (Bossen and Schneider, 2006).…”

“…Nonetheless, there is accumulating evidence that macrophages affect B cell proliferation and differentiation (Craxton et al, 2003;Fagarasan and Honjo, 2000;Litinskiy et al, 2002). We have shown that APRIL, as well as BAFF, are produced by TGFβ1-stimulated mouse macrophages (Kim et al, 2008). BAFF and APRIL can activate B cells to express activation-induced deaminase (AID), which is a critical enzyme for Ig class switch recombination (Castigli et al, 2005;Litinskiy et al, 2002;Yamada et al, 2005).…”

“…In addition, we have previously shown that TGF-β1 and IFN-γ stimulate mouse macrophages to express BAFF (Kim et al, 2008). Therefore, we explored the effects of TGF-β1 on APRIL expression in mouse macrophages to see if TGF-β1 indirectly regulates B cell differentiation by influencing macrophages.…”