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XIAP

X-linked inhibitor of apoptosis, E3 ubiquitin protein ligase;XIAP;X-linked inhibitor of apoptosis, E3 ubiquitin protein ligase

E3 ubiquitin-protein ligase XIAP|IAP-like protein|X-linked IAP|baculoviral IAP repeat-containing protein 4|inhibitor of apoptosis protein 3

Xq25

protein-coding

PURPOSE: Neuroblastoma is a common pediatric neoplasm with variable histopathological features that carry an inherent risk of developing distant metastases, in particular bone marrow metastasis. Nestin, X-linked inhibitor of apoptosis (XIAP) and vascular growth factors (VEGF) are biomarkers that are implicated in the tumorigenesis of various cancers. We studied the expression of these biomarkers in neuroblastoma, in relation to bone marrow (BM) metastasis and other histologic parameters. METHODS: Patients with neuroblastoma included seven with BM metastasis and 12 with non-metastatic tumors. Slides from the primary tumors were immunostained with antibodies against nestin, XIAP, VEGF-A, VEGF-B, VEGF-D, VEGF-R1, and VEGF-R2. Immunostaining results were evaluated by two pathologists, graded and statistically correlated with the risk of developing BM metastasis. RESULTS: Nestin was expressed in 16/19 cases with no significant difference between patients with BM metastasis and those without BM metastasis. XIAP was identified in 18/19 tumor cases; the staining density was significantly lower in patients with bone marrow metastasis and those with unfavorable histology. VEGF-R1, VEGF-R2, and VEGF-B were expressed while VEGF-A and VEGF-D were not. Significantly, higher expression of VEGF-B was noted in patients with BM metastasis. CONCLUSION: expression of VEGF-B and XIAP in neuroblastoma may play a role in the development of bone marrow metastasis. Given the limited number of patients in this study, a larger cohort is needed to validate these findings.

A pair of isogenic colon carcinoma cells, SW480 and 620, was used to investigate the mechanisms of acquired tumour necrosis factor-related apoptosis-inducing ligand (TRAIL)-resistance during tumour progression. Whereas primary tumour SW480 cells are sensitive to TRAIL-induced apoptosis, metastatic SW620 cells are resistant. The apoptotic signalling activated by TRAIL in SW480 cells is a type II pathway. We show that in SW620 cells, although caspase-8 is recruited and activated at the death-inducing-signalling complex and Bid is cleaved, this does not lead to caspase-9 activation. Comparison of Bcl-2, Bcl-xL and Mcl-1 levels in both cell lines showed no difference. In SW620 cells transfected with a tBid-GFP construct, tBid-GFP was correctly localized to the mitochondria. Thus, the resistance of SW620 cells is at the level of the mitochondria that can withstand large amounts of tBid. Although caspase-3 was directly cleaved by caspase-8 in SW620 cells to yield the p20 fragment, no further autocatalytic maturation into the p17 fragment was observed. We show that, in contrast to SW480 cells, the SW620 cell line expresses high amounts of X-linked inhibitor of apoptosis (XIAP). Downregulation of XIAP with bortezomib or small-interfering RNA was sufficient to restore the sensitivity of SW620 cells to TRAIL-induced apoptosis in the absence of SMAC/Diablo or cytochrome c release from the mitochondria. Thus, SW620 cells have developed a dual resistance to TRAIL-induced apoptosis: a block at the level of the mitochondria and, after a conversion to a type I pathway, an increased expression of XIAP which inhibits this pathway.

The Rho GDP dissociation inhibitor (RhoGDI) can bind to small GTPases and keep them in a biologically inactive state in cytoplasm, through which it affects actin polymerization and cell motility. However, mechanisms underlying how RhoGDI regulates Rho GTPase complex formation/membrane extraction/GTPase dissociation remain largely unexplored. Our previous studies reported that X-linked inhibitor of apoptosis protein (XIAP) interacted with RhoGDI via its RING domain and negatively modulated RhoGDI SUMOylation and HCT116 cancer cell migration. Here, we identified that RhoGDI SUMOylation specifically occurred at Lys-138, which was inhibited by XIAP domain. We further demonstrated that RhoGDI SUMOylation at Lys-138 was crucial for inhibiting actin polymerization and cytoskeleton formation as well as cancer cell motility. Moreover, SUMO-RhoGDI had a much higher binding affinity to small Rho GTPase compared with the un-SUMOylated form of RhoGDI. Taken together, our study demonstrated a novel modification of RhoGDI, SUMOylation at Lys-138, which played a key role in regulating Rho GTPase activation in cancer cells. The physiological regulation of RhoGDI SUMOylation by the RING domain of XIAP may account for modulation of cancer cell invasion and metastasis by XIAP.

Hepatocellular carcinoma (HCC) is one of the most common malignant tumors worldwide. Despite significantly improved diagnosis and treatment in recent years, the long-term therapeutic effect is compromised by the frequent recurrence and metastasis, of which the molecular mechanisms are not fully understood. Our initial studies in established HCC cell lines with different metastatic capabilities indicated a correlation of metastasis with the resistance to apoptosis and therefore the ability to survive in stressed conditions. Subsequent investigation revealed that increased expression of X-linked inhibitor-of-apoptosis protein (XIAP) was correlated with the resistance to apoptosis and enhanced invasiveness in vitro, which could contribute to increased metastatic foci in vivo. Furthermore, we found that nearly 90% of clinical samples from advanced HCC patients expressed high levels of XIAP. Patients with XIAP-positive tumors had a significantly increased risk of relapse, which resulted from metastasis after total liver resection and orthotopic liver transplantation. Indeed, XIAP expression could be an independent prognostic factor for predicting disease-free survival rate and overall survival rate of these patients. XIAP expression was also highly correlated with advanced cases that exceeded the Milan criteria and could be a prognostic factor for disease-free survival in these patients as well. CONCLUSION: Our studies have shown an important molecule in controlling HCC metastasis, defined a biomarker that can be used to predict HCC recurrence and patient survival after treatment, and suggest that XIAP can be a molecular target subject to intervention to reduce metastasis and recurrence.

The Rho GDP dissociation inhibitor (RhoGDI) can bind to small GTPases and keep them in a biologically inactive state in cytoplasm, through which it affects actin polymerization and cell motility. However, mechanisms underlying how RhoGDI regulates Rho GTPase complex formation/membrane extraction/GTPase dissociation remain largely unexplored. Our previous studies reported that X-linked inhibitor of apoptosis protein (XIAP) interacted with RhoGDI via its RING domain and negatively modulated RhoGDI SUMOylation and HCT116 cancer cell migration. Here, we identified that RhoGDI SUMOylation specifically occurred at Lys-138, which was inhibited by XIAP domain. We further demonstrated that RhoGDI SUMOylation at Lys-138 was crucial for inhibiting actin polymerization and cytoskeleton formation as well as cancer cell motility. Moreover, SUMO-RhoGDI had a much higher binding affinity to small Rho GTPase compared with the un-SUMOylated form of RhoGDI. Taken together, our study demonstrated a novel modification of RhoGDI, SUMOylation at Lys-138, which played a key role in regulating Rho GTPase activation in cancer cells. The physiological regulation of RhoGDI SUMOylation by the RING domain of XIAP may account for modulation of cancer cell invasion and metastasis by XIAP.