Immunohistochemical detection of XIAP in melanoma

Targeted Degradation of XIAP is Sufficient and Specific to Induce Apoptosis in MYCN-overexpressing High-risk Neuroblastoma

XIAP, the most potent mammalian inhibitor of apoptosis protein (IAP), critically restricts developmental culling of sympathetic neuronal progenitors, and is correspondingly overexpressed in most MYCN-amplified neuroblastoma tumors. Because apoptosis-related protein in the TGFβ signaling pathway (ARTS) is the only XIAP antagonist that directly binds and degrades XIAP, we evaluated the preclinical effectiveness and tolerability of XIAP antagonism as a novel targeting strategy for neuroblastoma. We found that antagonism of XIAP, but not other IAPs, triggered apoptotic death in neuroblastoma cells. XIAP silencing induced apoptosis while overexpression conferred protection from drug-induced apoptosis. From a screen of IAP inhibitors, first-in-class ARTS mimetic A4 was most effective against high-risk and high XIAP-expressing neuroblastoma cells, and least toxic toward normal liver- and bone marrow-derived cells, compared with pan-IAP antagonists. On target engagement assays and nuclear magnetic resonance spectroscopy, A4 was observed to degrade rather than inhibit XIAP, catalyzing rapid degradation of XIAP through the ubiquitin-proteasome pathway. In MYCN-amplified neuroblastoma patient-derived xenografts, A4 significantly prolonged survival as a single agent, and demonstrated synergism with standard-of-care agents to reduce their effective required doses 3- to 6-fold. Engagement and degradation of XIAP by ARTS mimetics is a novel targeting strategy for neuroblastoma that may be especially effective against MYCN-amplified disease with intrinsically high XIAP expression. First-in-class ARTS mimetic A4 demonstrates preclinical efficacy and warrants further development and study.

SIGNIFICANCE

XIAP degradation is sufficient to kill MYCN-amplified neuroblastoma which overexpresses and relies on XIAP as a brake against cell death, without affecting normal cells.

XIAP promotes melanoma growth by inducing tumour neutrophil infiltration

Elevated expression of the X‐linked inhibitor of apoptosis protein (XIAP) has been frequently reported in malignant melanoma suggesting that XIAP renders apoptosis resistance and thereby supports melanoma progression. Independent of its anti‐apoptotic function, XIAP mediates cellular inflammatory signalling and promotes immunity against bacterial infection. The pro‐inflammatory function of XIAP has not yet been considered in cancer. By providing detailed in vitro analyses, utilising two independent mouse melanoma models and including human melanoma samples, we show here that XIAP is an important mediator of melanoma neutrophil infiltration. Neutrophils represent a major driver of melanoma progression and are increasingly considered as a valuable therapeutic target in solid cancer. Our data reveal that XIAP ubiquitylates RIPK2, involve TAB1/RIPK2 complex and induce the transcriptional up‐regulation and secretion of chemokines such as IL8, that are responsible for intra‐tumour neutrophil accumulation. Alteration of the XIAP‐RIPK2‐TAB1 inflammatory axis or the depletion of neutrophils in mice reduced melanoma growth. Our data shed new light on how XIAP contributes to tumour growth and provides important insights for novel XIAP targeting strategies in cancer.

Immunity to X-linked inhibitor of apoptosis protein (XIAP) in malignant melanoma and check-point blockade

Expression of inhibitors of apoptosis protein (IAP) family members is associated with poor prognosis in cancer patients. Immunity to ML-IAP (livin) and survivin has been well studied in patients with a variety of tumors. XIAP, the most potent inhibitor of apoptosis, is widely expressed in melanoma. To better define its potential role as an immunogenic target, cellular and humoral responses to XIAP were investigated in patients with advanced melanoma. An overlapping peptide library covering the full length of the XIAP protein was used to screen T cell responses of peripheral blood mononuclear cells (PBMC) from stage-IV melanoma patients treated with or without anti-CTLA4 (ipilimumab). The screen identified an array of peptides that predominantly induced CD4⁺ T cell responses. XIAP epitope-specific CD4⁺ T cells revealed proliferative responses to melanoma cells that express XIAP. Humoral responses to XIAP were also explored. Cellular and humoral responses to XIAP were associated with beneficial clinical outcomes after ipilimumab-based treatment, supporting XIAP as a potential therapeutic target.

Key regulators of apoptosis execution as biomarker candidates in melanoma

Resistance to apoptosis is frequently detected in malignant melanoma, a skin cancer with rapidly growing incidence rates. Apoptosis resistance may develop with disease progression and may be associated with the poor responsiveness of metastatic melanoma to apoptosis-inducing treatments, such as genotoxic chemotherapy and radiotherapy. Likewise, the efficacy of novel treatment options (targeted kinase inhibitors and immunotherapeutics) that indirectly lead to cell death may depend on the susceptibility of melanoma to apoptosis. At its core, apoptosis execution is regulated by the interplay between a comparatively small number of pro- and anti-apoptotic proteins, and consequently numerous studies have investigated the potential of these players as biomarker candidates. Here, we provide a comprehensive overview of biomarker discovery studies focusing on key regulators of apoptosis execution, critically review the findings of these studies, and outline strategies that address current limitations and challenges in exploiting regulators of apoptosis execution as prognostic or predictive biomarkers in melanoma.

Loss of T-cadherin (CDH-13) regulates AKT signaling and desensitizes cells to apoptosis in melanoma

An understanding of signaling pathways is a basic requirement for the treatment of melanoma. Currently, kinases are at the center of melanoma therapies. According to our research, additional alternative molecules are equally important for development of melanoma. In this regard, cancer progression is, among other factors, driven by an altered adhesion via cadherins. For instance, the de-regulated expression of the adhesion molecule T-cadherin is found in various cancer types, including melanoma, and influences migration and invasion. T-cadherin is thought to affect cellular function largely through its signaling and not its adhesion properties because the molecule is anchored into the cell membrane by a glycosylphosphatidylinositol (GPI) moiety. However, detailed knowledge about the consequences of the loss of T-cadherin in melanoma is currently lacking. For this reason, we were interested in assessing which signaling pathways are initiated by T-cadherin. The tumor growth of subcutaneously injected T-cadherin-positive melanoma cells was diminished compared with T-cadherin-negative cells in nude mice. The difference in tumor volume was not due to decreased proliferation but rather due to increased apoptosis. After the expression of T-cadherin was induced, we detected V-AKT murine thymoma viral oncogene homolog (AKT) and FoxO3a hypophosphorylation accompanied by the downregulation of the antiapoptotic molecules BCL-2, BCL-x and Clusterin. Furthermore, we detected a diminished transcriptional activity of CREB and AP-1. We demonstrated that T-cadherin functions as a pro-apoptotic tumor suppressor that antagonizes AKT/CREB/AP-1/FoxO3a signaling, whereas NFκB, TCF/LEF and mTOR are not part of the T-cadherin signaling pathway. Notably, we found that the restoration of T-cadherin in melanoma cells causes sensitization to apoptosis induced by CD95/Fas antibody CH-11.

The anti-melanoma activity of dinaciclib, a cyclin-dependent kinase inhibitor, is dependent on p53 signaling

Although cyclin dependent kinase (CDK)-2 is known to be dispensable for the growth of most tumors, it is thought to be important for the proliferation of melanoma cells, where its expression is controlled by the melanocyte-lineage specific transcription factor MITF. Treatment of a panel of melanoma cells with the CDK inhibitor dinaciclib led to a concentration-dependent inhibition of growth under both 2D adherent and 3D organotypic cell culture conditions. Dinaciclib targeted melanoma cell lines regardless of cdk2 or MITF levels. Inhibition of growth was associated with a rapid induction of G2/M cell arrest and apoptosis. Treatment of human melanoma mouse xenografts with dinaciclib led to tumor regression associated with reduced retinoblastoma protein phosphorylation and Bcl-2 expression. Further mechanistic studies revealed that dinaciclib induces p53 expression whilst simultaneously downregulating the expression of the anti-apoptotic factors Mcl-1 and XIAP. To clarify the role of p53 activation in the dinaciclib-induced cell death, we generated melanoma cell lines in which p53 expression was knocked down using a shRNA lentiviral vector. Knockdown of p53 completely abolished the induction of apoptosis seen following dinaciclib treatment as shown by a lack of annexin-V staining and caspase-3 cleavage. Altogether, these data show that dinaciclib induces apoptosis in a large panel of melanoma cell lines through a mechanism requiring p53 expression.

Anti-apoptotic proteins on guard of melanoma cell survival

Apoptosis plays a pivotal role in sustaining proper tissue development and homeostasis. Evading apoptosis by cancer cells is a part of their adaption to microenvironment and therapies. Cellular integrity is predominantly maintained by pro-survival members of Bcl-2 family and IAPs. Melanoma cells are characterized by a labile and stage-dependent phenotype. Pro-survival molecules can protect melanoma cells from apoptosis and mediate other processes, thus enhancing aggressive phenotype. The essential role of Bcl-2, Mcl-1, Bcl-X(L), livin, survivin and XIAP was implicated for melanoma, often in a tumor stage-dependent fashion. In this review, the current knowledge of pro-survival machinery in melanoma is discussed.

Association of expression of XIAP-associated factor 1 (XAF1) with clinicopathologic factors, overall survival, microvessel density and cisplatin-resistance in ovarian cancer

XIAP-associated factor 1 (XAF1) was identified as a novel X-linked inhibitor of apoptosis (XIAP) binding partner that can reverse the anti-apoptotic effect of XIAP. XAF1 levels are greatly decreased in many cancer tissues and cell lines. The aim of this study was to investigate the expression of XAF1 and XIAP in advanced epithelial ovarian cancer and role of XAF1 in cisplatin resistance of ovarian cancer cells. Tissues from 94 patients with advanced epithelial ovarian cancer (EOC) and 30 ovarian cystadenomas were obtained. We analyzed the association of the immunohistochemical-determined expression of these two factors and clinicopathologic variables, overall survival, and angiogenesis. We established SKOV3 cells stably overexpressing XAF1 and explored the possible functions of XAF1 in ovarian cancer cells in vitro and in vivo. The protein expression of XAF1 was significantly lower and that of XIAP higher in malignant than nonmalignant tissues. Low XAF1 expression was associated with high-grade tumors and poor overall survival for patients. XAF1 expression was associated with microvessel density. Overexpression of XAF1 suppressed cell proliferation and enhanced SKOV3 cells sensitivity to cisplatin, as well as inhibited tumor growth and decreased MVD in vivo. Overexpression of XAF1 induced XIAP inactivation, caspase-3 activation and cytosolic expression of cytochrome c. These results suggested that XAF1 may be involved in ovarian cancer development and up-regulation of XAF1 may confer sensitivity of ovarian cancer cells to cisplatin-mediated apoptosis.

Born to be alive: a role for the BCL-2 family in melanoma tumor cell survival, apoptosis, and treatment

The global incidence of melanoma has dramatically increased during the recent decades, yet the advancement of primary and adjuvant therapies has not kept a similar pace. The development of melanoma is often centered on cellular signaling that hyper-activates survival pathways, while inducing a concomitant blockade to cell death. Aberrations in cell death signaling not only promote tumor survival and enhanced metastatic potential, but also create resistance to anti-tumor strategies. Chemotherapeutic agents target melanoma tumor cells by inducing a form of cell death called apoptosis, which is governed by the BCL-2 family of proteins. The BCL-2 family is comprised of anti-apoptotic proteins (e.g., BCL-2, BCL-xL, and MCL-1) and pro-apoptotic proteins (e.g., BAK, BAX, and BIM), and their coordinated regulation and function are essential for optimal responses to chemotherapeutics. Here we will discuss what is currently known about the mechanisms of BCL-2 family function with a focus on the signaling pathways that maintain melanoma tumor cell survival. Importantly, we will critically evaluate the literature regarding how chemotherapeutic strategies directly impact on BCL-2 family function and offer several suggestions for future regimens to target melanoma and enhance patient survival.

The enigmatic roles of caspases in tumor development

One function ascribed to apoptosis is the suicidal destruction of potentially harmful cells, such as cancerous cells. Hence, their growth depends on evasion of apoptosis, which is considered as one of the hallmarks of cancer. Apoptosis is ultimately carried out by the sequential activation of initiator and executioner caspases, which constitute a family of intracellular proteases involved in dismantling the cell in an ordered fashion. In cancer, therefore, one would anticipate caspases to be frequently rendered inactive, either by gene silencing or by somatic mutations. From clinical data, however, there is little evidence that caspase genes are impaired in cancer. Executioner caspases have only rarely been found mutated or silenced, and also initiator caspases are only affected in particular types of cancer. There is experimental evidence from transgenic mice that certain initiator caspases, such as caspase-8 and -2, might act as tumor suppressors. Loss of the initiator caspase of the intrinsic apoptotic pathway, caspase-9, however, did not promote cellular transformation. These data seem to question a general tumor-suppressive role of caspases. We discuss several possible ways how tumor cells might evade the need for alterations of caspase genes. First, alternative splicing in tumor cells might generate caspase variants that counteract apoptosis. Second, in tumor cells caspases might be kept in check by cellular caspase inhibitors such as c-FLIP or XIAP. Third, pathways upstream of caspase activation might be disrupted in tumor cells. Finally, caspase-independent cell death mechanisms might abrogate the selection pressure for caspase inactivation during tumor development. These scenarios, however, are hardly compatible with the considerable frequency of spontaneous apoptosis occurring in several cancer types. Therefore, alternative concepts might come into play, such as compensatory proliferation. Herein, apoptosis and/or non-apoptotic functions of caspases may even promote tumor development. Moreover, experimental evidence suggests that caspases might play non-apoptotic roles in processes that are crucial for tumorigenesis, such as cell proliferation, migration, or invasion. We thus propose a model wherein caspases are preserved in tumor cells due to their functional contributions to development and progression of tumors.

X-linked inhibitor of apoptosis protein as a therapeutic target in metastatic melanoma

Melanoma is the most dangerous type of skin cancer, with notorious resistance to current chemotherapy and immunotherapy. In this issue, Hiscutt et al. report on the prognostic significance of X-linked inhibitor of apoptosis protein (XIAP) in melanoma. The finding that XIAP inhibition significantly increases endoplasmic reticulum stress-mediated apoptosis in melanoma cells suggests that XIAP is a potential therapeutic target for melanoma therapy.

Biomarkers: the useful and the not so useful--an assessment of molecular prognostic markers for cutaneous melanoma

Among individuals with localized (Stage I-II) melanoma, stratifying patients by a number of phenotypic variables (e.g., depth of invasion, ulceration) yields a wide range of 10-year melanoma-specific survival rates. With the possible exception of Ki-67, no molecular assessment is routinely used. However, there have been a tremendous number of studies assessing protein expression by immunohistochemistry toward the goal of better prediction of recurrence. In a previous systematic review, which required publication of multivariable prognostic models as a strict inclusion criterion, we identified 37 manuscripts that collectively reported on 62 proteins. Data for 324 proteins extracted from 418 manuscripts did not meet our inclusion criteria for that study, but are revisited here, emphasizing trends of protein expression across either melanocytic lesion progression or gradations of tumor thickness. These identified 101 additional proteins that stratify melanoma, organized according to the Hanahan and Weinberg functional capabilities of cancer.

Targeting X-linked inhibitor of apoptosis protein to increase the efficacy of endoplasmic reticulum stress-induced apoptosis for melanoma therapy

Melanoma remains notoriously resistant to current chemotherapeutics, leaving an acute need for novel therapeutic approaches. The aim of this study was to determine the prognostic and therapeutic significance of X-linked inhibitor of apoptosis protein (XIAP) in melanoma through correlation of XIAP expression with disease stage, RAS/RAF mutational status, clinical outcome, and susceptibility to endoplasmic reticulum (ER) stress-induced cell death. XIAP expression and N-RAS/B-RAF mutational status were retrospectively determined in a cohort of 55 primary cutaneous melanocytic lesions selected and grouped according to the American Joint Committee on Cancer staging system. Short hairpin RNA interference of XIAP was used to analyze the effect of XIAP expression on ER stress-induced apoptosis in response to fenretinide or bortezomib in vitro. The results showed that XIAP positivity increased with progressive disease stage, although there was no significant correlation between XIAP positivity and combined N-RAS/B-RAF mutational status or clinical outcome. However, XIAP knockdown significantly increased ER stress-induced apoptosis of melanoma cells in a caspase-dependant manner. The correlation of XIAP expression with disease stage, as well as data showing that XIAP knockdown significantly increases fenretinide and bortezomib-induced apoptosis of metastatic melanoma cells, suggests that XIAP may prove to be an effective therapeutic target for melanoma therapy.

Expression of X-linked inhibitor of apoptosis protein in human colorectal cancer and its correlation with prognosis

BACKGROUND

X-linked inhibitor of apoptosis protein (XIAP) is a member of the inhibitor of apoptosis family of proteins and deregulation of XIAP can result in tumorigenicity. The objective of this study was to evaluate the prognostic significance of XIAP expression in colorectal cancer (CRC).

METHODS

RT-PCR was performed to detect the expression of XIAP mRNA in CRC cells and tissues. The expression of XIAP protein in tissues was measured by immunohistochemistry. The correlation of XIAP expression with clinicopathologic factors and prognosis of CRC patients was evaluated.

RESULTS

CRC cells showed significantly higher levels of XIAP mRNA expression than normal human intestinal epithelial cell. The expression level of XIAP mRNA in CRC samples was significantly higher than that in corresponding non-tumor samples. XIAP staining was positive in the cytoplasm of CRC cells. Higher XIAP protein expression was significantly correlated with tumor differentiation (P = 0.016), venous invasion (P = 0.039), and Duke's staging (P = 0.002). Moreover, XIAP-high group showed lower disease-free (P = 0.0136) and overall survival (P = 0.0084) rates than XIAP-low group. Multivariate analysis indicated that the status of XIAP expression was an independent prognostic factor for CRC (P = 0.0206; HR: 2.730; 95% CI: 1.226-5.445).

CONCLUSION

The status of XIAP expression might become an independent prognostic marker for CRC.

Immunohistochemical detection of p63 and XIAP in thymic hyperplasia and thymomas

We subjected 23 formalin-fixed, paraffin-embedded tissue blocks (11 cases of thymic hyperplasia and 12 thymomas [3 encapsulated, 8 with capsular invasion, and 1 atypical]) to incubation with monoclonal anti-X-linked inhibitor of apoptosis protein (XIAP) (BD Biosciences, San Jose, CA) and monoclonal anti-p63 (4A4, Santa Cruz, Santa Cruz, CA). Granular or heterogeneous cytoplasmic XIAP staining and nuclear p63 staining were considered positive. We compared thymic hyperplasia with thymoma and capsulated thymoma with thymoma with capsular invasion or atypia. p63 was positive in virtually all thymic epithelial cells in hyperplasia and thymoma. XIAP was negative in all hyperplasia cases except one. Of 12 thymomas, 9 were XIAP+ with focal/weak to diffuse/strong positivity: 2 of 3 encapsulated and 7 of 8 thymomas with capsular invasion were XIAP+. One atypical thymoma was XIAP-. XIAP expression differed significantly between hyperplasia and thymoma (P = .0007) but not between capsulated and invasive thymomas (P = .3797). p63 is consistently positive in nonneoplastic and neoplastic thymic epithelium. XIAP expression in thymoma suggests a possible role in the pathogenesis of thymoma and may be helpful in differentiating thymic hyperplasia from thymoma, especially in small biopsy specimens. However, the level of expression does not correlate with capsular invasion or atypia.

Caspases and inhibitor of apoptosis proteins in cutaneous and mucosal melanoma: expression profile and clinicopathologic significance

Malignant melanoma is characterized by apoptotic dysfunction, presumably due to abnormal expression of caspases and inhibitor of apoptosis proteins. However, the expression status and clinicopathologic significance of caspases and inhibitor of apoptosis proteins in cutaneous and particularly in mucosal melanomas have not been established. We investigated the expression of the major caspases (CASP3, 6, 7, 8, 9, and 10) and inhibitor of apoptosis proteins (survivin, CIAP1, CIAP2, XIAP, and Livin) by immunohistochemistry in 52 primary cutaneous and 25 mucosal melanomas, and 24 common nevi. Clinicopathologic and prognostic significance was investigated by statistical analysis. Our data showed that the significantly up-regulated inhibitor of apoptosis proteins in primary cutaneous and mucosal melanomas as compared with nevus (P < .01) were survivin, CIAP1, CIAP2, and Livin. Percentage of cases with positive caspase or IAP immunostaining was not significantly different between primary cutaneous and mucosal melanomas or between lower- and higher-stage melanomas (P > .05). Tumor location (cutaneous versus mucosal), stage, and positive cytoplasmic and nuclear survivin staining correlated significantly with prognosis by univariate analysis (P < .01). Multivariate analysis by Cox regression model showed that the most useful prognostic indicators were tumor location (cutaneous versus mucosal, relative risk = 6.021, 95% confidence interval = 2.623 approximately 13.821, P = .000), stage (relative risk = 4.129, 95% confidence interval = 1.817 approximately 9.383, P = .001), and nuclear survivin staining (relative risk = 3.383, 95% confidence interval = 1.137 approximately 10.008, P = .028).

Inhibiting XIAP expression by RNAi to inhibit proliferation and enhance radiosensitivity in laryngeal cancer cell line

OBJECTIVES

X-linked inhibitor of apoptosis protein (XIAP) is a novel member of the inhibitors of apoptosis (IAPs) family. The overexpression of XIAP is asscociated with radioresistance of human malignancies. The purpose of the present study was to investigate the effect of shRNA-targeted XIAP on the proliferation, apoptosis and radiosensitivity of human laryngeal carcinoma cells (Hep-2).

METHODS

A siRNA expression vector (pSilencer4.1-XIAPshRNA) was constructed and stably transfected into human laryngeal carcinoma cells (Hep-2). The downregulation of XIAP expression was evaluated by RT-PCR and Western blot analyses. Then, we investigated the effect of XIAP-shRNA on the proliferation, cell cycle changes and apoptosis in vitro of Hep-2 cells. Finally, the radiosensitivity of Hep-2 cells was investigated by clonogenic cell survival assay.

RESULTS

We established stably transfected cell line (Hep-2/XIAPshRNA) in which the expression of XIAP gene was downregulated. The cell viability of Hep-2/XIAP-RNA cells was obviously decreased compared with that of untransfected Hep-2 cells. Morever, XIAP-shRNA induced cell arrest in the G(0)/G(1) phase of cell cycle by flow cytometry analysis. Results of TUNEL assay indicated that Hep-2 cells stably transfected pSilencer4.1-XIAP-shRNA showed obvious apoptosis characters. Furthermore, the downregulation of XIAP expression could lead to significant radiosensitivity enhancement in laryngeal carcinoma cells.

CONCLUSIONS

RNAi-mediated downregulation of XIAP expression can inhibit proliferation, induce apoptosis and diminish the radioresistance of laryngeal carcinoma cells, so combined therapy with XIAP inhibition and radiation may be a potential strategy for the treatment of laryngeal carcinoma.