Targeting the Proteasome in Advanced Renal Cell Carcinoma: Complexity and Limitations of Patient-Individualized Preclinical Drug Discovery.

BACKGROUND: Systemic treatment options for metastatic renal cell carcinoma (RCC) have significantly expanded in recent years. However, patients refractory to tyrosine kinase and immune checkpoint inhibitors still have limited treatment options and patient-individualized approaches are largely missing. PATIENTS AND METHODS: In vitro drug screening of tumor-derived short-term cultures obtained from seven patients with clear cell RCC was performed. For one patient, a patient-derived xenograft (PDX) mouse model was established for in vivo validation experiments. Drug effects were further investigated in established RCC cell lines. RESULTS: The proteasome inhibitor carfilzomib was among the top hits identified in three of four patients in which an in vitro drug screening could be performed successfully. Carfilzomib also showed significant acute and long-term cytotoxicity in established RCC cell lines. The in vivo antitumoral activity of carfilzomib was confirmed in a same-patient PDX model. The cytotoxicity of carfilzomib was found to correlate with the level of accumulation of ubiquitinated proteins. CONCLUSIONS: In this proof-of-concept study, we show that patient-individualized in vitro drug screening and preclinical validation is feasible. However, the fact that carfilzomib failed to deliver a clinical benefit in RCC patients in a recent phase II trial unrelated to the present study underscores the complexities and limitations of this strategy.

Cullin 5 is a novel candidate tumor suppressor in renal cell carcinoma involved in the maintenance of genome stability.

Clear cell renal cell carcinoma (ccRCC) is intimately associated with defects in ubiquitin-mediated protein degradation. Herein, we report that deficiency in the E3 ligase subunit cullin 5 (CUL5) promotes chromosomal instability and is an independent negative prognostic factor in ccRCC. CUL5 was initially identified in an RNA interference screen as a novel regulator of centrosome duplication control. We found that depletion of CUL5 rapidly promotes centriole overduplication and mitotic errors. Downregulation of CUL5 also caused an increase of DNA damage that was found to involve impaired DNA double-strand break repair. Using immunohistochemistry, CUL5 protein expression was found to be below detection level in the majority of RCCs. A re-analysis of the TCGA ccRCC cohort showed that a reduced CUL5 gene expression or CUL5 deletion were associated with a significantly worse overall patient survival. In conclusion, our results indicate that CUL5 functions as a novel tumor suppressor with prognostic relevance in ccRCC and is critically involved in the maintenance of genome stability.

Ureterocystoneostomy in complex oncological cases with an "Uebelhoer" modified Boari bladder flap.

PURPOSE: The study aims to describe the technique and analyze the outcome of an arcuated bladder incision with building of a triangular flap, first described by Uebelhoer (UBBF), as a modification of the classical rectangular Boari bladder flap (BBF), that is often viable, but can present difficulties, such as reduced flap vascularization and mobility in pretreated patients. METHODS: Twelve consecutive patients with distal or mid ureteral leakage or stenosis, that underwent UBBF, were retrospectively analyzed. We assessed postoperative morbidity using Clavien-Dindo classification. Short- and long-term functional outcomes were assessed using glomerular filtration rate (GFR), ultrasound, and renal scintigraphy. RESULTS: Patients underwent UBBF during initial oncological surgery in five cases and due to ureteral defects following oncological surgery or radiotherapy in seven cases. Median patient age was 57 (interquartile range (IQR) 46-72), defect length was 7.5 cm (IQR 5-8 cm), and median follow-up period was 41 (IQR 36-48) months. In short-term follow-up, 11/13 postoperative morbidities were Clavien-Dindo level I-II complications, mostly infections. Two level IIIa complications occurred. One anastomotic leakage was treated sufficiently with temporarily ureteral stenting and one voiding disorder needed intervention. In the long-term follow-up, 84% of patients had improved or constant GFR. In the one-year renal scintigraphy, no urodynamically relevant voiding disorder occurred. CONCLUSIONS: The UBBF is a reliable procedure to reconstruct ureteral trauma even in complex oncological, pretreated patients suffering from distal or mid ureteral defects. It can be performed easily by a modified arcuate incision and provides good long-term functional outcomes.

Spatial niche formation but not malignant progression is a driving force for intratumoural heterogeneity.

Intratumoural heterogeneity (ITH) is a major cause of cancer-associated lethality. Extensive genomic ITH has previously been reported in clear cell renal cell carcinoma (ccRCC). Here we address the question whether ITH increases with malignant progression and can hence be exploited as a prognostic marker. Unexpectedly, precision quantitative image analysis reveals that the degree of functional ITH is virtually identical between primary ccRCCs of the lowest stage and advanced, metastatic tumours. Functional ITH was found to show a stage-independent topological pattern with peak proliferative and signalling activities almost exclusively in the tumour periphery. Exome sequencing of matching peripheral and central primary tumour specimens reveals various region-specific mutations. However, these mutations cannot directly explain the zonal pattern suggesting a role of microenvironmental factors in shaping functional ITH. In conclusion, our results indicate that ITH is an early and general characteristic of malignant growth rather than a consequence of malignant progression.

Prognostic Significance and Functional Role of CEP57 in Prostate Cancer.

We have recently shown that centrosomal protein 57 (CEP57) is overexpressed in a subset of human prostate cancers. CEP57 is involved in intracellular transport processes, and its overexpression causes mitotic defects as well as abnormal microtubule nucleation and bundling. In the present study, we further characterized the prognostic and functional role of CEP57 in prostate cancer. Unexpectedly, we found that high CEP57 expression is an independent prognostic factor for a more favorable biochemical recurrence-free survival in two large patient cohorts. To reconcile this finding with the ability of CEP57 to cause cell division errors and thus potentially promote malignant progression, we hypothesized that alterations of microtubule-associated transport processes, in particular nuclear translocation of the androgen receptor (AR), may play a role in our finding. However, CEP57 overexpression and microtubule bundling had, surprisingly, no effect on the nuclear translocation of the AR. Instead, we found a significant increase of cells with disarranged microtubules and a cellular morphology suggestive of a cytokinesis defect. Because mitotic dysfunction leads to a reduced daughter cell formation, it can explain the survival benefit of patients with increased CEP57 expression. In contrast, we show that a reduced expression of CEP57 is associated with malignant growth and metastasis. Taken together, our findings underscore that high CEP57 expression is associated with mitotic impairment and less aggressive tumor behavior. Because the CEP57-induced microtubule stabilization had no detectable effect on AR nuclear translocation, our results furthermore suggest that microtubule-targeting therapeutics used in advanced prostate cancer such as docetaxel may have modes of action that are at least in part independent of AR transport inhibition.

Identification of cell-free microRNAs in the urine of patients with prostate cancer.

OBJECTIVES: Current methods for the early detection of prostate cancer (PCa), in particular prostate-specific antigen screening, are likely to benefit from complementary molecular analyses to enhance specificity. MicroRNAs (miRNA) are small endogenously expressed noncoding RNAs that negatively regulate the expression of protein-coding genes at the transcriptional or translational level. Accumulating evidence suggests that miRNAs play an important role in tumorigenesis, are differentially expressed in different cancer types, and can be found in all bodily fluids so-far tested, including urine. METHODS AND MATERIALS: This study was undertaken to determine if miRNA could be isolated from the cell-free fraction of freely voided urine of PCa patients and if a miRNA signature could be found that would identify patients with cancer. RESULTS: In a first set of proof-of-concept experiments, we isolated RNA from the supernatant of cultured PCa cells, as well as cellular RNA, and compared the expression of cell-free miRNAs vs. cellular miRNAs. We identified miRNA-483-5p, miRNA-1275, and miRNA-1290 among the most abundant cell-free miRNAs. We then tested the expression of these miRNAs in patient urine samples. A total of 18 patients without detectable PCa by transperineal template-saturation biopsy and 71 patients with diagnosed biopsy-proven PCa were retrospectively analyzed. We could confirm that cell-free miRNAs found in cultures of PCa cells can in fact be isolated from freely voided patients' urine. Furthermore, we found that patients with PCa express miR-483-5p in the cell-free urine fraction at a higher level than control patients do. CONCLUSIONS: The present study is among the first to show that miRNAs can be detected in the cell-free, non-exosome-enriched fraction of urine collected from patients with PCa. As the method used here does not require isolation of exosomes, it could potentially simplify the future use of miRNAs as urine-based biomarkers.

The tyrosine kinase inhibitor nilotinib has antineoplastic activity in prostate cancer cells but up-regulates the ERK survival signal-Implications for targeted therapies.

BACKGROUND: Novel therapeutic options beyond hormone ablation and chemotherapy are urgently needed for patients with advanced prostate cancer. Tyrosine kinase inhibitors (TKIs) are an attractive option as advanced prostate cancers show a highly altered phosphotyrosine proteome. However, despite favorable initial clinical results, the combination of the TKI dasatinib with docetaxel did not result in improved patient survival for reasons that are not known in detail. METHODS: The National Cancer Institute-Approved Oncology Drug Set II was used in a phenotypic drug screen to identify novel compounds with antineoplastic activity in prostate cancer cells. Validation experiments were carried out in vitro and in vivo. RESULTS: We identified the TKI nilotinib as a novel compound with antineoplastic activity in hormone-refractory prostate cancer cells. However, further analyses revealed that treatment with nilotinib was associated with a significant up-regulation of the phospho-extracellular-signal-regulated kinases (ERK) survival signal. ERK blockade alone led to a significant antitumoral effect and enhanced the cytotoxicity of nilotinib when used in combination. CONCLUSIONS: Our findings underscore that TKIs, such as nilotinib, have antitumoral activity in prostate cancer cells but that survival signals, such as ERK up-regulation, may mitigate their effectiveness. ERK blockade alone or in combination with TKIs may represent a promising therapeutic strategy in advanced prostate cancer.

Evolving therapeutic concepts in prostate cancer based on genome-wide analyses (review).

Treatment of castration resistant prostate cancer (CRPC) continues to represent a major urooncological challenge due to tumor heterogeneity and the inevitable development of therapy resistance. Although androgen deprivation therapy retains an important role in the management of CRPC, recent evidence suggests that a broader spectrum of therapeutic targets may improve patient response and delay development of advanced disease. Genome-wide analyses have identified four major signaling nodes that are most frequently altered in prostate cancer: i) the androgen receptor (AR); ii) the PI3K pathway; iii) the Ras/Raf/MEK/ERK pathway; and iv) the retinoblastoma protein (pRB) signaling pathway. Extensive crosstalk and redundancy exists between these signaling pathways, which underscores the need for combination therapies. There are several novel AR pathway inhibitors currently in clinical use. Clinical trials are being performed on single-agent PI3K inhibitors with some success in tumors with genetically altered PI3K components. MEK/ERK inhibitors are also in clinical trials and the importance of pRB inactivation in prostate cancer is becoming more widely recognized. A greater understanding of the effects of single agent therapy on compensatory signaling pathway activation that can potentially thwart antitumoral responses is urgently needed and will provide additional insight into the mechanism of therapy resistance and how to further delay the progression to lethal disease.

Phenotypic drug screening and target validation for improved personalized therapy reveal the complexity of phenotype-genotype correlations in clear cell renal cell carcinoma.

OBJECTIVES: Novel personalized therapeutic approaches are urgently needed for patients with metastatic clear cell renal cell carcinoma (ccRCC). METHODS AND MATERIALS: We combined the development of a primary patient-derived ccRCC cell line with a phenotypic drug screen consisting of 101 approved anticancer compounds. RESULTS: We identified the MNNG HOS transforming gene (MET)-anaplastic lymphoma receptor tyrosine kinase (ALK) inhibitor crizotinib as the top hit of our drug screen, whereas compounds targeting the vascular endothelial growth factor (VEGF) or mammalian target of rapamycin (mTOR) pathway showed no or only minor in vitro activity. Among the known major crizotinib targets MET, ALK, and ROS-1, only MET was expressed in our ccRCC cell line. Subsequent sequence analysis revealed a heterozygous R988C mutation of the MET gene and a VHL deletion in both the primary tumor and the tumor-derived ccRCC cell line. However, we were unable to show an activation of MET and, further, MET knockdown did not result in increased apoptosis or cytotoxicity. Therefore, our results suggest that MET R988C does not function as a major oncogenic driver mutation but rather represents a sequence variant. However, we provide evidence that the cytotoxic effect of crizotinib in our cell line model correlates with its ability to inhibit P-glycoprotein (ABCB1)-associated transport functions. CONCLUSIONS: Our study shows that a phenotypic screen of a patient-derived tumor cell line can identify compounds with antitumor activity but with an unexpected mode of action. Our results underscore that target validation and phenotype-genotype correlations remain a major experimental challenge. The implications of our findings for a personalized management of patients with cancer are discussed.

High-risk prostate cancer: a disease of genomic instability.

OBJECTIVES: In this review, we will discuss the latest advances in our understanding of the relationship between the cellular DNA damage response and genomic instability in prostate cancer and the emerging possibilities to exploit these aberrations as prognostic biomarkers and guides for personalized patient management. METHODS: Important findings related to genomic instability in prostate cancer were retrieved from the literature and combined with our own results and a translational perspective. RESULTS: Prostate cancer is characterized by a highly altered genomic landscape with a wide spectrum of genomic alterations, including somatic mutations, copy number alterations (CNAs), gene fusions, complex chromosomal rearrangements, and aneuploidy. In addition, massive DNA damaging events, including chromothripsis and chromoplexy, which can lead to extensive genomic insults in a single step, have been identified. A number of these genomic aberrations have been found to provide prognostic information and can therefore help to identify high-risk patients. In addition, defects in the DNA damage checkpoint and repair machinery can potentially be harnessed for therapeutic purposes. CONCLUSIONS: Genomic instability plays a crucial role in the malignant progression of prostate cancer and can be exploited for the development of novel prognostic biomarkers and innovative therapies.

Unbiased compound screening identifies unexpected drug sensitivities and novel treatment options for gastrointestinal stromal tumors.

Most gastrointestinal stromal tumors (GIST) are caused by oncogenic KIT or platelet-derived growth factor receptor activation, and the small molecule kinase inhibitor imatinib mesylate is an effective first-line therapy for metastatic or unresectable GIST. However, complete remissions are rare and most patients ultimately develop resistance, mostly because of secondary mutations in the driver oncogenic kinase. Hence, there is a need for novel treatment options to delay failure of primary treatment and restore tumor control in patients who progress under therapy with targeted agents. Historic data suggest that GISTs do not respond to classical chemotherapy, but systematic unbiased screening has not been performed. In screening a compound library enriched for U.S. Food and Drug Administration (FDA)-approved chemotherapeutic agents (NCI Approved Oncology Drugs Set II), we discovered that GIST cells display high sensitivity to transcriptional inhibitors and topoisomerase II inhibitors. Mechanistically, these compounds exploited the cells' dependency on continuous KIT expression and/or intrinsic DNA damage response defects, explaining their activity in GIST. Mithramycin A, an indirect inhibitor of the SP1 transcription factor, and mitoxantrone, a topoisomerase II inhibitor, exerted significant antitumor effects in mouse xenograft models of human GIST. Moreover, these compounds were active in patient-derived imatinib-resistant primary GIST cells, achieving efficacy at clinically relevant concentrations. Taken together, our findings reveal that GIST cells have an unexpectedly high and specific sensitivity to certain types of FDA-approved chemotherapeutic agents, with immediate implications for encouraging their clinical exploration.

The DREAM complex mediates GIST cell quiescence and is a novel therapeutic target to enhance imatinib-induced apoptosis.

Gastrointestinal stromal tumors (GIST) can be successfully treated with imatinib mesylate (Gleevec); however, complete remissions are rare and patients frequently achieve disease stabilization in the presence of residual tumor masses. The clinical observation that discontinuation of treatment can lead to tumor progression suggests that residual tumor cells are, in fact, quiescent and, therefore, able to re-enter the cell-division cycle. In line with this notion, we have previously shown that imatinib induces GIST cell quiescence in vitro through the APC(CDH1)-SKP2-p27(Kip1) signaling axis. Here, we provide evidence that imatinib induces GIST cell quiescence in vivo and that this process also involves the DREAM complex, a multisubunit complex that has recently been identified as an additional key regulator of quiescence. Importantly, inhibition of DREAM complex formation by depletion of the DREAM regulatory kinase DYRK1A or its target LIN52 was found to enhance imatinib-induced cell death. Our results show that imatinib induces apoptosis in a fraction of GIST cells while, at the same time, a subset of cells undergoes quiescence involving the DREAM complex. Inhibition of this process enhances imatinib-induced apoptosis, which opens the opportunity for future therapeutic interventions to target the DREAM complex for more efficient imatinib responses.

The centrosome as potential target for cancer therapy and prevention.

INTRODUCTION: Cancer initiation and propagation is not possible without cell division. Besides microtubules, which are targeted by taxanes as part of a number of standard chemotherapy regimens, mitosis depends on small cellular organelles known as centrosomes. Centrosome abnormalities are a common finding in tumors including major human malignancies such as prostate or breast cancer. Centrosome aberrations can drive chromosome missegregation and aneuploidy, thereby promoting malignant progression. Nonetheless, these important cellular structures have not yet been directly exploited for targeted interventions. AREAS COVERED: This review will summarize the current knowledge of normal and aberrant centrosome duplication. We will highlight the principal pathways leading to aberrant centrosome numbers and the evidence for a role of centrosome amplification in malignant progression. Strategies to target centrosome-mediated cell division errors will be discussed. Lastly, we will review the evidence for centrosome clustering as a druggable cellular process. EXPERT OPINION: Recent advances in the understanding of centrosome biogenesis have revealed a number of potential centrosomal drug targets including Polo-like kinases, Cyclin-dependent kinases, Aurora kinases, and molecular motor proteins. For some of these proteins, targeted inhibitory compounds are available and in vitro experiments have provided the proof-of-concept that blocking centrosome overduplication can result in a reduction of aneuploid cells. In addition, inhibition of centrosomal clustering has antitumor activity in vitro and in vivo. Nonetheless, further in vitro and preclinical studies are required to determine the most effective way to exploit the centrosome for therapeutic or preventive anticancer strategies.

FGF-2 disrupts mitotic stability in prostate cancer through the intracellular trafficking protein CEP57.

Malignant tumors with deregulated FGF-2 expression such as prostate cancer are also frequently aneuploid. Aneuploidy can be caused by cell division errors due to extra centrosomes and mitotic spindle poles. However, a link between FGF-2 overexpression and chromosome missegregation has so far been elusive. Here, we show that FGF-2 rapidly uncouples centrosome duplication from the cell division cycle in prostate cancer cells through CEP57, an intracellular FGF-2-binding and trafficking factor. CEP57 was initially identified as a regulator of centriole overduplication in an RNA interference screen. We subsequently found that CEP57 rapidly stimulates centriole overduplication and mitotic defects when overexpressed and is required not only for FGF-2-induced centriole overduplication but also for normal centriole duplication. We provide evidence that CEP57 functions by modulating tubulin acetylation, thereby promoting daughter centriole stability. CEP57 was found to be overexpressed on the mRNA and protein level in a subset of prostate cancers, of which the vast majority also showed FGF-2 upregulation. Taken together, our results show an unexpected link between altered microenvironmental signaling cues such as FGF-2 overexpression and mitotic instability and provide a rationale for the therapeutic targeting of the FGF-2/FGFR1/CEP57 axis in prostate cancer. Cancer Res; 73(4); 1400-10. ©2012 AACR.

CAND1 promotes PLK4-mediated centriole overduplication and is frequently disrupted in prostate cancer.

Centrosomes play a crucial role in the maintenance of genome stability by orchestrating bipolar mitotic spindle formation. The centrosome normally duplicates precisely once before mitosis in a process that is extensively regulated by protein degradation including SKP1-Cullin 1 (CUL1)-F-box (SCF) E3 ubiquitin ligase activity. The core SCF component CUL1 has recently been found to be required to suppress the formation of supernumerary centrosomes and centrioles, the core-forming units of centrosomes. Here, we identify the CUL1-interacting protein cullin-associated and neddylation-dissociated 1 (CAND1) as a novel centrosomal protein with a role in centriole duplication control. CAND1 was found to synergize with Polo-like kinase 4 (PLK4), a master regulator of centriole biogenesis, in the induction of centriole overduplication. We provide evidence that CAND1 functions in this process by increasing PLK4 protein stability. Furthermore, mutants of CUL1 that lack the ability to interact with CAND1 and are unable to assemble functional E3 ubiquitin ligase complexes were impaired in their ability to restrain aberrant daughter centriole synthesis. To corroborate a role of CAND1 in human carcinogenesis, we analyzed a series of prostate adenocarcinomas and found altered expression of CAND1 on the mRNA or protein level in 52.9% and 40.8%, respectively, of the tumor samples analyzed. These results highlight the role of altered SCF components in cancer in general and encourage further studies to explore the SCF-CAND1 axis for the development of novel predictive biomarkers and therapeutic approaches in prostate cancer.

Neurl4, a novel daughter centriole protein, prevents formation of ectopic microtubule organizing centres.

Here we identify Neuralized homologue 4 (Neurl4) as a protein that interacts with CP110, a centrosomal protein that regulates centrosome duplication. Neurl4 uses a Neuralized homology repeat to preferentially localize to procentrioles and daughter centrioles. Neurl4 depletion results in ectopic microtubular organizing centres (MTOCs), leading to accumulation of CP110 and recruitment of a cohort of centrosomal proteins. We show that these ectopic MTOCs persist through mitosis and assemble aberrant mitotic spindles. Interestingly, Neurl4 promotes ubiquitylation of CP110, thereby destabilizing this protein. Our results indicate that Neurl4 counteracts accumulation of CP110, thereby maintaining normal centriolar homeostasis and preventing formation of ectopic MTOCs.

The HPV-16 E7 oncoprotein induces centriole multiplication through deregulation of Polo-like kinase 4 expression.

BACKGROUND: Infection with high-risk human papillomaviruses (HPVs) such as HPV-16 is intimately associated with squamous cell carcinomas (SCCs) of the anogenital tract and a subset of oropharyngeal carcinomas. Such lesions, including pre-invasive precursors, frequently show multipolar mitoses and aneuploidy. The high-risk HPV-16-encoded E7 oncoprotein has been shown to rapidly induce centrosome abnormalities thereby causing the formation of supernumerary mitotic spindle poles and increasing the risk for chromosome missegregation. HPV-16 E7 has been found to rapidly induce centriole overduplication, in part, through the simultaneous formation of more than one daughter centriole at single maternal centrioles (centriole multiplication). The precise molecular mechanism that underlies HPV-16 E7-induced centriole multiplication, however, remains poorly understood. FINDINGS: Here, we show that human keratinocytes engineered to stably express the HPV-16 E7 oncoprotein exhibit aberrant Polo-like kinase 4 (PLK4) protein expression at maternal centrioles. Real-time quantitative reverse transcriptase (qRT-PCR) analysis of these cells revealed an increase of PLK4 mRNA levels compared to control cells. Importantly, the ability of the HPV-16 E7 oncoprotein to induce centriole multiplication was found to correlate with its ability to activate the PLK4 promoter and to up-regulate PLK4 mRNA. CONCLUSIONS: These results highlight the critical role of PLK4 transcriptional deregulation in centriole multiplication in HPV-16 E7-expressing cells. Our findings encourage further experiments to test transcriptional inhibitors or small molecules targeting PLK4 to prevent centriole abnormalities, mitotic infidelity and malignant progression in HPV-associated neoplasms and other tumors in which PLK4 regulation is disrupted.

Genomic instability and cancer: lessons learned from human papillomaviruses.

High-risk HPV E6 and E7 oncoproteins cooperate to subvert critical host cell cycle checkpoint control mechanisms in order to promote viral genome replication. This results not only in aberrant proliferation but also in host cellular changes that can promote genomic instability. The HPV-16 E7 oncoprotein was found to induce centrosome abnormalities thereby disrupting mitotic fidelity and increasing the risk for chromosome missegregation and aneuploidy. In addition, expression of the high-risk HPV E7 oncoprotein stimulates DNA replication stress as a potential source of DNA breakage and structural chromosomal instability. Proliferation of genomically unstable cells is sustained by several mechanisms including the accelerated degradation of claspin by HPV-16 E7 and the degradation of p53 by the high-risk HPV E6 oncoprotein. These results highlight the oncogenic potential of aberrant proliferation and opens new avenues for prevention of malignant progression, not only in HPV-associated cervical cancer but also in non-virally associated malignancies with disrupted cell cycle checkpoint control mechanisms.

Daughter centriole elongation is controlled by proteolysis.

The centrosome is the major microtubule-organizing center of most mammalian cells and consists of a pair of centrioles embedded in pericentriolar material. Before mitosis, the two centrioles duplicate and two new daughter centrioles form adjacent to each preexisting maternal centriole. After initiation of daughter centriole synthesis, the procentrioles elongate in a process that is poorly understood. Here, we show that inhibition of cellular proteolysis by Z-L3VS or MG132 induces abnormal elongation of daughter centrioles to approximately 4 times their normal length. This activity of Z-L3VS or MG132 was found to correlate with inhibition of intracellular protease-mediated substrate cleavage. Using a small interfering RNA screen, we identified a total of nine gene products that either attenuated (seven) or promoted (two) abnormal Z-L3VS-induced daughter centriole elongation. Our hits included known regulators of centriole length, including CPAP and CP110, but, interestingly, several proteins involved in microtubule stability and anchoring as well as centrosome cohesion. This suggests that nonproteasomal functions, specifically inhibition of cellular proteases, may play an important and underappreciated role in the regulation of centriole elongation. They also highlight the complexity of daughter centriole length control and provide a framework for future studies to dissect the molecular details of this process.

A novel role of the aryl hydrocarbon receptor (AhR) in centrosome amplification - implications for chemoprevention.

BACKGROUND: Centrosome aberrations can cause genomic instability and correlate with malignant progression in common human malignancies such as breast and prostate cancer. Deregulation of cyclin/cyclin-dependent kinase 2 (CDK2) activity has previously been shown to be critically involved in centrosome overduplication. We therefore test here whether small molecule CDK inhibitors derived from the bis-indole indirubin can be used to suppress centrosome aberrations as a novel approach to chemoprevention of malignant progression. RESULTS: As expected, we found that the CDK inhibitor indirubin-3'-oxime (IO) suppresses centrosome amplification in breast cancer cells. However, we made the unexpected discovery that indirubin-derived compounds that have been chemically modified to be inactive as kinase inhibitors such as 1-methyl-indirubin-3'-oxime (MeIO) still significantly reduced centrosome amplification. All indirubins used in the present study are potent agonists of the aryl hydrocarbon receptor (AhR), which is known for its important role in the cellular metabolism of xenobiotics. To corroborate our results, we first show that the coincidence of nuclear AhR overexpression, reflecting a constitutive activation, and numerical centrosome aberrations correlates significantly with malignancy in mammary tissue specimens. Remarkably, a considerable proportion (72.7%) of benign mammary tissue samples scored also positive for nuclear AhR overexpression. We furthermore provide evidence that continued expression of endogenous AhR is critical to promote centriole overduplication induced by cyclin E and that AhR and cyclin E may function in the same pathway. Overexpression of the AhR in the absence of exogenous ligands was found to rapidly disrupt centriole duplication control. Nonetheless, the AhR agonists IO and MeIO were still found to significantly reduce centriole overduplication stimulated by ectopic AhR expression. CONCLUSIONS: Our results indicate that continued expression of endogenous AhR promotes centrosome amplification in breast cancer cells in a pathway that involves cyclin E. AhR agonists such as indirubins inhibit centrosome amplification even when stimulated by ectopic expression of the AhR suggesting that these compounds are potentially useful for the chemoprevention of centrosome-mediated cell division errors and malignant progression in neoplasms in which the AhR is overexpressed. Future studies are warranted to determine whether individuals in which nuclear AhR overexpression is detected in benign mammary tissue are at a higher risk for developing pre-cancerous or cancerous breast lesions.