Smad7 Knockdown Restores Aryl Hydrocarbon Receptor-mediated Protective Signals in the Gut

BACKGROUND AND AIM

In Crohn's disease [CD], the pathological process is driven by an excessive immune response that is poorly counterbalanced by regulatory mechanisms. One such a mechanism involves aryl hydrocarbon receptor [AhR], a transcription factor that delivers protective signals in the gut. Expression of AhR is reduced in CD lamina propria mononuclear cells [LPMC] even though factors accounting for such a defect remain unknown. Since CD LPMC express elevated levels of Smad7, an inhibitor of transforming growth factor beta 1 [TGF-β1] activity, and TGF-β1 regulates AhR in other systems, we examined the link between AhR and Smad7 in the gut.

METHODS

AhR and interleukin [IL]-22 were evaluated in normal LPMC stimulated with TGF-β1 and 6-formylindolo[3,2-b]carbazole [Ficz], an activator of AhR, and in CD LPMC incubated with a Smad7 antisense oligonucleotide and then stimulated with Ficz and TGF-β1. AhR and IL-22 expression was evaluated in LPMC of Smad7-transgenic mice. Finally, we evaluated the protective effect of Ficz on colitis in RAG1 mice injected with naïve or Smad7-overexpressing T cells.

RESULTS

In normal LPMC, TGF-β1 induced AhR and this event was associated with increased production of IL-22 following stimulation with Ficz. Treatment of CD LPMC with Smad7 antisense oligonucleotide enabled TGF-β1 to enhance AhR expression. Consistently, AhR expression and Ficz-induced IL-22 production were markedly reduced in T cells of Smad7-transgenic mice. In RAG1 mice, Ficz ameliorated colitis induced by wild type T cells but did not affect colitis induced by transfer of Smad7-overexpressing T cells.

CONCLUSIONS

The inverse correlation between Smad7 and AhR expression helps to propagate inflammatory signals in the gut.

Partial loss of Smad7 function impairs bone remodeling, osteogenesis and enhances osteoclastogenesis in mice

Smad7 is well demonstrated as a negative regulator of TGF-β signaling. Its alteration in expression often results in diseases such as cancer and fibrosis. However, the exact role of Smad7 in regulating bone remodeling during mammalian development has not been properly delineated. In this study we performed experiments to clarify the involvement of Smad7 in regulating osteogenesis and osteoclastogenesis both invivo and invitro. Genetically engineered Smad7(ΔE1) (KO) mice were used, whereby partial functional of Smad7 is lost by deleting exon I of the Smad7 gene and the truncated proteins cause a hypomorphic allele. Analysis with μCT imagery and bone histomorphometry showed that the KO mice had lower TbN, TbTh, higher TbSp in the metaphysic region of the femurs at 6, 12, 24weeks from birth, as well as decreased MAR and increased osteoclast surface compared with the WT mice. In vitro BM-MSC multi-lineage differentiation evaluation showed that the KO group had reduced osteogenic potential, fewer mineralized nodules, lower ALP activity, and reduced gene expression of Col1A1, Runx2 and OCN. The adipogenic potential was elevated in the KO group with more formation of lipid droplets, and increased gene expression of Adipsin and C/EBPα. The osteoclastogenic potential of KO mice BMMs was elevate, with emergence of more osteoclasts, larger resorptive areas, and increased gene expression of TRAP and CTR. Our results indicate that partial loss of Smad7 function in mice leads to compromised bone formation and enhanced bone resorption. Thus, Smad7 is acknowledged as a novel key regulator between osteogenesis and osteoclastogenesis.

Deletion of exon I of SMAD7 in mice results in altered B cell responses

The members of the TGF-beta superfamily, i.e., TGF-beta isoforms, activins, and bone morphogenetic proteins, regulate growth, differentiation, and apoptosis, both during embryonic development and during postnatal life. Smad7 is induced by the TGF-beta superfamily members and negatively modulates their signaling, thus acting in a negative, autocrine feedback manner. In addition, Smad7 is induced by other stimuli. Thus, it can fine-tune and integrate TGF-beta signaling with other signaling pathways. To investigate the functional role(s) of Smad7 in vivo, we generated mice deficient in exon I of Smad7, leading to a partial loss of Smad7 function. Mutant animals are viable, but significantly smaller on the outbred CD-1 mouse strain background. Mutant B cells showed an overactive TGF-beta signaling measured as increase of phosphorylated Smad2-positive B cells compared with B cells from wild-type mice. In agreement with this expected increase in TGF-beta signaling, several changes in B cell responses were observed. Mutant B cells exhibited increased Ig class switch recombination to IgA, significantly enhanced spontaneous apoptosis in B cells, and a markedly reduced proliferative response to LPS stimulation. Interestingly, LPS treatment reverted the apoptotic phenotype in the mutant cells. Taken together, the observed phenotype highlights a prominent role for Smad7 in development and in regulating the immune system's response to TGF-beta.