TGF‐β signaling involves a wide array of signaling molecules and multiple controlling events. Scaffold proteins create a functional proximity of signaling molecules and control the specificity of signal transduction. While many components involved in the TGF‐β pathway have been elucidated, little is known about how those components are coordinated by scaffold proteins. Here, we show that Axin activates TGF‐β signaling by forming a multimeric complex consisting of Smad7 and ubiquitin E3 ligase Arkadia. Axin depends on Arkadia to facilitate TGF‐β signaling, as their small interfering RNAs reciprocally abolished the stimulatory effect on TGF‐β signaling. Specific knockdown of Axin or Arkadia revealed that Axin and Arkadia cooperate with each other in promoting Smad7 ubiquitination. Pulse‐chase experiments further illustrated that Axin significantly decreased the half‐life of Smad7. Axin also induces nuclear export of Smad7. Interestingly, Axin associates with Arkadia and Smad7 independently of TGF‐β signal, in contrast to its transient association with inactive Smad3. However, coexpression of Wnt‐1 reduced Smad7 ubiquitination by downregulating Axin levels, underscoring the importance of Axin as an intrinsic regulator in TGF‐β signaling.

Axin and p53 are tumor suppressors, controlling cell growth, apoptosis, and development. We show that Axin interacts with homeodomain‐interacting protein kinase‐2 (HIPK2), which is linked to UV‐induced p53‐dependent apoptosis by interacting with, and phosphorylating Ser 46 of, p53. In addition to association with p53 via HIPK2, Axin contains a separate domain that directly interacts with p53 at their physiological concentrations. Axin stimulates p53‐dependent reporter transcription in 293 cells, but not in 293T, H1299, or SaOS‐2 cells that are defective in p53 signaling. Axin, but not AxinΔHIPK2, activates HIPK2‐mediated p53 phosphorylation at Ser 46, facilitating p53‐dependent transcriptional activity and apoptosis. Specific knockdown of Axin by siRNA reduced UV‐induced Ser‐46 phosphorylation and apoptosis. Kinase‐dead HIPK2 reduced Axin‐induced p53‐dependent transcriptional activity, indicating that Axin stimulates p53 function through HIPK2 kinase activity. Interestingly, HIPK2ΔAxin that lacks its Axin‐binding region acts as a dominant‐positive form in p53 activation, suggesting that the Axin‐binding region of HIPK2 is a putative autoinhibitory domain. These results show that Axin acts as a tumor suppressor by facilitating p53 function through integration of multiple factors.

Axin is a multifunctional protein, regulating Wnt signaling and the c-Jun N-terminal/stress-activated protein kinase (JNK/SAPK) pathway as well as tumorigenesis. In the present study, we found that Axin interacts with three SUMO-1 (small ubiquitin-related modifier) conjugating enzymes 3 (E3), PIAS1, PIASxbeta, and PIASy. The extreme C-terminal six amino acid residues of Axin are critical for the Axin/E3 interaction as deletion of the six residues (AxinDeltaC6) completely abolished the ability of Axin to interact with E3 enzymes. AxinDeltaC6 also failed to activate JNK, although it was intact in both its interaction with MEKK1 and homodimerization. Consistent with the presence of a doublet of the KV(E/D) sumoylation consensus motif at the C-terminal end (KVEKVD), we found that Axin is heavily sumoylated. Deletion of the C-terminal six amino acids drastically reduced sumoylation, indicating that the C-terminal six amino acids stretch is the main sumoylation site for Axin. Sumoylation-defective mutants failed to activate JNK but effectively destabilized beta-catenin and attenuated LEF1 transcriptional activity. In addition, we show that dominant negative Axin mutants blocked PIAS-mediated JNK activation, in accordance with the requirement of sumoylation for Axin-mediated JNK activation. Taken together, we demonstrate that sumoylation plays a role for Axin to function in the JNK pathway.

Background/Aims. It is known that chronic low-grade inflammation contributes to the initiation and development of both diabetes and diabetic nephropathy (DN), so we designed this study to investigate the role of P2X7R and NLRP3 inflammasome in DN pathogenesis and the antagonistic effects of artificially cultivated Ophiocordyceps sinensis (ACOS). Methods. A rat model of DN caused by high-fat-diet feeding and low-dose streptozotocin injection and a mouse podocyte injury model induced by high-glucose (HG) stimulation were established, and the intervention effects of ACOS on them were observed. The biological parameters of serum and urine and the pathological manifestations of kidney tissue were examined. The expression of mRNA and protein of P2X7R and NLRP3 inflammasome (NLRP3, ASC, and caspase-1) and downstream effectors (IL-1β and IL-18), as well as podocyte-associated molecules, was determined by real-time quantitative PCR and Western blot assay, respectively. Results. The DN rats showed to have developed insulin resistance, elevated fasting blood glucose, increased urinary protein excretion, and serum creatinine level as well as corresponding glomerular pathological alterations including podocyte damages. ACOS significantly antagonized the above changes. The experiments in vivo and in vitro both displayed that the mRNA and protein expression of P2X7R, NLRP3, ASC, caspase1 (procaspase-1 mRNA in the gene level and active caspase-1 subunit P10 in the protein level), IL-1β, and IL-18 was significantly upregulated and the mRNA and protein expression of podocyte-associated molecules was significantly changed (downregulation of nephrin, podocin, and WT-1 expression and upregulation of desmin expression) indicating podocyte injury in the kidney tissue of DN rats and in the HG-stressed mouse podocytes, respectively. ACOS also significantly antagonized all the above changes. Conclusion. Our research work suggests that P2X7R and NLRP3 inflammasome are involved in the pathogenesis of DN, and ACOS can effectively inhibit the high expression of P2X7R and the activation of NLRP3 inflammasome, which may contribute to the therapeutic effects of Ophiocordyceps sinensis.

Axin is a major scaffold protein, interacting with diverse molecules involved in a number of signaling pathways. Axin can undergo dimer/oligomerization via its DIX domain. Here we show that whereas deletion of the DIX domain at the C terminus rendered Axin incapable of forming dimer, a larger deletion of the C-terminal region restored the ability of Axin to form dimers. Detailed analyses revealed that Axin actually contains two separate domains (D and I) in addition to the DIX domain for homodimerization. The D, I, and DIX domains alone can form homodimers. Interestingly, D and I domains strongly interact with each other, suggesting that Axin can form an intramolecular structure through D and I interaction in the absence of DIX. We also found that DIX-DIX homodimeric interaction is weak but that point mutations in the DIX domain abolished Axin homodimerization. We propose a model to suggest that Axin forms homodimeric interactions through three domains, D, I, and DIX. More importantly, lack of DIX-DIX interaction caused by point mutations in the DIX domain or deletion causes Axin to form an intramolecular loop through the D and I domains, disallowing homodimer formation. Ccd1 interacts with Axin D domain yet fails to interact with AxinΔDIX, confirming that D is masked after D-I looping. The Axin mutants that are defective in homodimer formation fail to activate JNK but have no effect on β-catenin signaling. Our findings have thus provided a structural basis of conformational changes in Axin, which may underlie the diversity of Axin functions.

Podocyte injury critically contributes to the pathogenesis of obesity-related glomerulopathy (ORG). Recently, lipid accumulation and inflammatory responses have been found to be involved in podocyte injury. This study is to explore their role and relationship in podocyte injury of ORG. In animal experiments, the ORG mice developed proteinuria, podocyte injury, and hypertriglyceridemia, accompanied with deregulated lipid metabolism, renal ectopic lipid deposition, activation of NOD-like receptor protein 3 (NLRP3) inflammasome, and secretion of IL-1β of the kidney. The expression of adipose differentiation-related protein (ADRP), CD36, sterol regulatory element-binding protein 1 (SREBP-1), and peroxisome proliferator-activated receptor α (PPARα) in renal tissue were increased. In in vitro cell experiments, after cultured podocytes were stimulated with leptin, similar to ORG mice, we found aggravated podocyte injury, formatted lipid droplet, increased expression of ADRP and CD36, activated NLRP3 inflammasome, and released IL-1β. In addition, after blocking CD36 with inhibitor sulfo-N-succinimidyl oleate (SSO) or CD36 siRNA, activation of NLRP3 inflammasome and release of IL-1β are downregulated, and podocyte injury was alleviated. However, after blocking NLRP3 with MCC950, although podocyte injury was alleviated and release of IL-1β was decreased, there was no change in the expression of CD36, ADRP, and intracellular lipid droplets. Taken together, our study suggests that CD36-mediated lipid accumulation and activation of NLRP3 inflammasome may be one of the potential pathogeneses of ORG podocyte injury.