Repression of c-Jun occurs through various repressor complexes, namely; Mbd3/NuRD [
], Fbl10/Sin3a/HDACs [
], NCoR1/HDAC3 [
] and SMRT/HDACs [
]. The existence of many repressors for a single transcription factor such as c-Jun may be required for its tightly controlled regulation in various physiological conditions. Auto regulation of c-Jun further explains the necessity of this tight regulation. From our in-vivo and in-vitro results, it is evident that WDR13 regulates c-Jun transcription and activates AP1 transcription after activation of JNK. Evidence of physical interaction of WDR13 with c-Jun/NCoR1/HDAC3 (Fig.
), and the increased WDR13 occupancy at c-Jun promoter after activation of JNKs (Fig.
) suggest an important role of this protein in c-Jun transcriptional regulation. The presence of WDR13 in NCoR1/HDAC3 complex and its interaction with c-Jun raise some interesting questions-1) How does WDR13 activate AP1 target genes in the presence of JNK signal? 2) Does this protein have a role in proteosomal degradation of NCoR1/HDAC3 and exchange of co-repressors with co-activators? 3) Does this protein have a role in interaction of JNK and c-Jun? There may be two possibilities; 1) WDR13 may be involved in exchange of co-repressors with co-activators in the presence of JNK signal by proteosomal degradation of co-repressors at AP1 sites or 2) WDR13 may act like an adaptor for c-Jun, which in turn helps in both repression and activation of c-Jun in a signal-dependent manner. At present we do not understand which one of the above possibilities exists. However, interaction of c-Jun and JNK is neither dependent upon JNK catalytic activity nor only on the presence of JNK target sites in c-Jun [
]. It has been suggested that JNK in its inactive form contributes to the repressor function [
]. In another study, understanding role of
gene in the brain tissues, we have observed activation of some of the AP1 target genes in
null mice [
] and in present study higher basal AP1 reporter activity in
cells and amelioration of AP1 reporter activity after JNK activation suggest that WDR13 is possibly working as c-Jun adapter and may be helping c-Jun/JNK/NCoR1/HDAC3 interactions. However, this needs further validation.
Our previous results in pancreatic beta cells showed that WDR13 binds at p21 promoter and regulates its transcription in MIN6 cells. Since the p21 promoter does not contain an AP1 site, it is possible that binding of WDR13 and c-Jun complex at p21 promoter may be indirectly mediated through sp1 site as suggested by Kardassis et al., [
] or by some other nuclear receptors. In agreement with these results, JNK activation, either by UV in MIN6 and HEK293 cells (Additional file
: Figure S2C, E) or by anisomycin in MEFs, did not show activation of p21 reporter after WDR13 over-expression (Additional file
: Figure S2A). These results suggest that regulation of p21 by WDR13 is not through AP1, but through some other mechanism that may be cell type-specific and is not yet understood. We have provided evidence that WDR13 protein is part of the repressor complex NCoR1/HDAC3. In the presence of JNK signal this protein acts as a transcriptional activator of AP1 target genes. The lack of evidence of interactions of WDR13 with either NFκB and/or SMRT in our study showed that WDR13 might be exclusively associated with c-Jun and NCoR complex to regulate AP1 target gene transcription.
The role of c-Jun in cell proliferation and malignancy has been shown in many tissues including liver [
], mammary glands [
] and colon [
]. However, the role of c-Jun in apoptosis has been shown to differ in various cell types. Some studies suggest that phosphorylation at serine 63/73 of c-Jun protects cells from apoptosis [
], whereas others suggest that phosphorylation of c-Jun supports apoptosis [
]. WDR13 activates c-Jun activity in JNK-dependent manner, and the lack of WDR13 attenuates c-Jun phosphorylation and its activity. Our results are in agreement with the protective role of c-Jun phosphorylation in apoptosis. The expression of
in colon along with the well-known function of c-Jun in the development of colorectal tumor encouraged us to analyze the phenotype of
knockout mice. There was an increase in expression of AP1 target genes in
mice in the proximal colon as compared to control mice after AOM/DSS treatment (Fig.
). Interestingly, after DSS treatment,
mice showed more ulceration (Fig.
) than wild-type littermates, which may explain the reduced incidence of colitis-induced colorectal tumor in these mice. Other studies have suggested that the increased apoptosis may lead to increase in ulceration [
]. It is the increased levels of p-JNK in AOM/DSS model that causes induction of AP1 target genes such as
mice as compared to non-treated controls [
]. However, there was no activation of AP1 target genes (Fig.
), and protection from AOM/DSS-induced colorectal tumor in
mice. Various AP1 target genes are regulated by both canonical and non-canonical Wnt signalling due to cross talk among these pathways [
]. In the present study, we did not detect any interaction of WDR13 with β-catenin (Fig.
). These results suggest that the regulation of AP1 target genes by WDR13 is less likely to be through canonical Wnt signalling. However, further experimentation is needed to rule out the involvement of canonical Wnt signalling in the regulation of AP1 target genes in
null mice. Some of the AP1 target genes are also regulated by NFκB transcription factor [
], and inactivation of IKKβ - an activator of NFκB in intestinal epithelial cells - leads to dramatic reduction in tumor number after AOM/DSS treatment [
] due to increase in apoptosis. The reduced expression of
mice may be another factor responsible for the increase in apoptosis. Surprisingly, in spite of the reduced levels of proliferative markers such as
mice after AOM/DSS treatment, there was no decrease in the number of BrdU positive cells in crypts of colon. Since inflammation and tumorigenesis are inter-linked, and in this study we used whole body knockout mice, we cannot rule out the involvement of immune cells in reduction of colitis-induced colorectal cancer in these mice. Consistent with these results the reduced inflammation on high fat diet or in diabetic mouse model background in
mice may contribute to the increased islet mass and increased beta cell proliferation in pancreas along with the reduced levels of p21 [