**8.3. IL-22**

IL-22 binds to the class II cytokine receptor family, IL-22R and IL-10Rβ [183]. IL-22 was shown to activate STAT-1, -3 and -5 in H4IIE rat hepatoma cells by inducing the phosphory‐ lation of JAK1 and Tyk2, but not JAK1 [184]. However, H4IIE failed to respond to IL-10 via activation of JAK1 and Tyk2 indicating a distinct signaling pathway for IL-22 versus IL-10. IL-22 also failed to inhibit pro-inflammatory cytokine gene expression by monocytes in re‐ sponse to LPS although IL-22 did blunt the inhibitory effects of IL-4 produced from Th2 cells, a finding distinct from the activity of IL-10.

late with the immunological status of B-cells since, unlike IL-10, IL-22 does apparently not

Suppression of Pro-Inflammatory Cytokines via Targeting of STAT-Responsive Genes

http://dx.doi.org/10.5772/52506

391

The apoptosis-inducing activity of IL-24/mda-7 has made this unique member of the extend‐ ed IL-10 cytokine family a target for cancer therapeutics [193-195] in view of the finding that IL-24/mda-7 could kill cancer cells specifically without affecting the vitality off normal cells or tissues [196]. Receptor binding of IL24/mda-7 to IL-20R activates STAT1 and STAT3 al‐ though additional signaling pathways have been shown to be modulated by cells over-ex‐ pressing IL-24/mda-7 which did not involve JAK/STAT activation [193]. Besides the interest in IL-24/mda-7 as a tumor suppressor cytokine, mda-7/IL-24 has also been implicated in reg‐ ulating some of components of RA and psoriasis immunopathology [197]. However, some of the details of the mechanism(s) by which IL-24/mda-7 could alter pro-inflammatory cyto‐ kine gene expression in RA via JAK/STAT have not been fully elucidated, although epige‐ netic and other transcriptional factor activity beyond activated STAT proteins have been postulated to play critical roles. Thus, it is of interest that Sahoo et al. [198] recently showed that STAT6 and c-Jun binding to the IL-24 promoter locus in Th2 cells caused trans-activa‐ tion of the IL-24 gene. Finding a relationship between the activators of STAT6 and c-Jun that are relevant to RA which leads to IL-24 gene transcription may hold the key to increasing local IL-24/mda-7 levels by Th2 cells. This, in turn, could help overcome the 'apoptosis-resist‐

IL-3 is one of several major cytokines that drive the differentiation of cells of the hemato‐ poietic lineage. The interaction between IL-3 and its cognate receptor activates several sig‐ naling pathways, including, JAK/STAT, PI3K/Akt/mTOR and the Ras/Raf/MAPK pathways [199]. Downstream events that are regulated by IL-3 which are germane to RA and autoim‐ munity, in general, include the findings that depending on the conditions in the microenvir‐ onment, IL-3 can alter cell proliferation, survival or induce cell death by apoptosis [30].

IL-3 was identified as an activator of JAK2 and STAT5 [200] and the expression of the proapoptotic protein, c-myc. This finding provided the initial evidence that cell proliferation and apoptosis was regulated, in part, by activated STAT5. However, a subsequent study by Chaturvedi et al. [201] provided evidence to the contrary in that the interaction of IL-3 with its receptor activated STAT3 via the phosphorylation of tyr701. Moreover, the results of this study [201] also showed that myeloid cell proliferation was regulated by IL-3-activated Src kinase and not by IL-3-actiivated JAK3. This conclusion was based on the following results. Inhibition of c-Src kinase activation using a dominant-negative (dn) Src mutant also blocked STAT3 activation and, this in turn, inhibited proliferation of the 32Dcl3 myeloid cell line in response to IL-3. Moreover, expression of a dn-JAK2 mutation increased apoptosis in 32Dcl3 cells in the absence of IL-3 which also involved the concomitant down-regulation of ERK-2. Taken together these results indicated that Src kinase activation of STAT proteins regulated

regulate the induction of Ig by activated B-cells [192].

**8.4. IL-24/mda-7**

ance' of RA synovium [96].

**8.5. IL-3**

A role for IL-22 in inflammation was inferred from its involvement as an inducer of pancrea‐ titis-associated protein by pancreatic acinar cells [185] and by the elevated serum levels of IL-22 in patients with active Crohn's disease [186]. With regard to activating various signal‐ ing mechanisms, Lejeune et al. [187] showed that IL-22 activated JAK/STAT. However, IL-22 also activated ERK, JNK and p38 kinase indicating that IL-22 could activate all of the 3 major MAPK pathways. Brand et al. [186] then showed that treating intestinal epithelial cells with TNF-α, IL-1β or LPS significantly increased IL-22R1 gene expression without altering IL-10R2 mRNA. IL-22 also activated STAT1/STAT3, Akt, ERK 1/2 and JNK and, most impor‐ tantly IL-22 increased the expression of SOCS3, TNF-α, IL-8 and human-defensin-2 mRNAs. Because IL-22 was shown to activate several disparate signaling pathways it is conjecture that up-regulation of pro-inflammatory gene mRNAs by IL-22 involves 'cross-talk' between all three pathways. Thus, experiments employing specific SMIs added either individually or together to cells in culture will have to be performed to determine the extent to which any or all of these signaling pathways are involved in regulating TNF-α, IL-8 or IL-1β gene expres‐ sion in response to IL-22.

IL-22 is elevated in RA synovial tissue with the lining and sublining layers of RA synovium expressing the highest levels of IL-22R1 [188]. Recently, Leipe et al. [189] showed that about 50% of the RA patients studied had elevated serum IL-22 compared to a group of healthy subjects. The level of serum IL-22 closely correlated with the extent of bone erosions as de‐ termined from radiographic analysis. However serum IL-22 did not correlate with the pres‐ ence or absence of either rheumatoid factor (RF) or anti-cyclic citrullinated peptide antibodies nor was IL-22 associated with disease activity. CD4 T-cells were identified as the main source of IL-22 in these RA patients. However, in another study, de Rocha Jr et al. [190] showed that elevated serum IL-22 did correlate with the Disease Activity Score-28 (DAS-28) and the Clinical Disease Activity Index, a positive titer for RF and the extent to which bone was eroded. The findings from this study [190] agreed with the results from an‐ other recently published study [191] the latter showing that plasma IL-22 was increased in 30 patients with established RA (i.e. mean disease duration of 10.7 years), even in those pa‐ tients receiving immunomodulatory therapy. Thus, any discrepancies between the results of these various clinical studies relative to establishing a relationship between IL-22, RA dis‐ ease activity and RF levels may involve differences in terms of the types and duration of the immunotherapies employed or in the proportion of RA patients who were in the early or late stage of disease. The relationship between IL-22 and the presence of RF could also corre‐ late with the immunological status of B-cells since, unlike IL-10, IL-22 does apparently not regulate the induction of Ig by activated B-cells [192].
