Abstract A042: Keratin 17 is a negative prognostic and predictive biomarker in pancreatic ductal adenocarcinoma

Background: There is an urgent need to understand why patients with clinically and histologically identical pancreatic ductal adenocarcinomas (PDACs) differ in response to treatment, disease progression, and survival. Although two standard chemotherapies are available, predictive biomarkers to guide regimen selection have not been defined. We previously reported that keratin 17 (K17) expression is a hallmark of PDAC cases with shortest patient survival. Furthermore, K17, explored using in vitro and in vivo murine models, drives resistance to gemcitabine and 5-fluorouracil, the most common chemotherapeutic agents in the two chemotherapies. Here, we aimed to validate the prognostic value of K17 and to further explore its role as a predictive biomarker. Methods: We used a cohort of 305 cases, with localized disease and who had gone tumor resection. An indirect immunoperoxidase method was used to detect K17 expression on surgical specimens, as previously described. Survival was plotted using the Kaplan–Meier method and hazard ratios (HRs) were calculated using Cox proportional hazard regressions for both, overall survival (OS) and progression-free survival (PFS). Results: Patients in the high-K17 expression group had shorter overall survival [median=25 mo., HR=1.511, p=0.0338] than those in the low-K17 expression group (median=42 mo.). In addition, high K17 expression was associated with shorter median OS (p=0.0280) and PFS (p=0.0434) in patients who were treated with gemcitabine (GEM) or with Gemcitabine/nab-Paclitaxel (GEMTAX) therapy (OS p=0.1959, PFS p=0.0724), compared to low-K17 counterparts. When we further separated our cohort by K17 level of expression and compared GEM and GEMTAX responses, we found that patients with tumors with high K17 expression do not benefit from GEMTAX adjuvant therapy (p=0.0356). Of note, K17 expression within the responder group was significantly lower than in tumors within the non-responder group for both Gem and GEMTAX (p<0.001). Conclusions: K17 expression is confirmed as a robust prognostic biomarker, significantly correlated with poor OS and PFS in two independent cohorts of PDAC patients. More importantly, our results indicate that K17 expression predicts PDAC resistance to gemcitabine and poor response to GEMTAX. Collectively, our findings have implications could guide the development of K17 as a predictive biomarker for gemcitabine-based interventions in both adjuvant and palliative settings, to optimize therapeutic efficacy for PDAC.

Citation Format: Lyanne Oblein, Lucia Roa-Peña, Sruthi Babu, Felicia D. Allard, Natalia D. Marchenko, Luisa F. Escobar-Hoyos, Kenneth R. Shroyer. Keratin 17 is a negative prognostic and predictive biomarker in pancreatic ductal adenocarcinoma [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer; 2022 Sep 13-16; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2022;82(22 Suppl):Abstract nr A042.

Abstract C071: Keratin 17 excludes CD8-positive T cells and recruits CD163-positive macrophages in pancreatic ductal adenocarcinoma

Background: Keratin 17 (K17) is a negative prognostic biomarker, overexpressed in the biologically most aggressive forms of pancreatic ductal adenocarcinoma (PDAC). In other anatomic sites and disease processes, K17 expression also correlates with immune cell infiltrates and could block T-cell infiltration. Thus, we hypothesized that K17 expression correlates with the inflammatory microenvironment in PDAC. In this study, we aimed to determine the relationship between the stromal immune cell infiltrates and K17 expression, using multiplexed immunohistochemistry (mIHC) and our suite of deep learning tools to quantitatively evaluate the expression of four biomarkers of T-cells and macrophages in PDAC. Methods: mIHC was performed on representative sections of 201 primary PDACs from Stony Brook University Hospital, Thomas Jefferson University Hospital, Cedars Sinai Medical Center, and from a national cohort (KYT, Pancreatic Cancer Action Network, and Perthera). Antibodies for CD4 (helper T-cells), CD8 (cytotoxic T-cells), CD16 (pan-macrophage), CD163 (M2 macrophages), pancytokeratin, and K17 were provided by Roche Diagnostics Corporation through a sponsored research agreement. mIHC was performed on a Discovery Ultra Autostainer (Roche), using horseradish peroxidase (HRP) and alkaline phosphatase (AP)-based protocols with multiple chromogens (Red: CD4, Purple: CD8, Yellow: CD16, Green: CD163, Teal: pancytokeratin, and Brown: K17) to enable multispectral imaging of diverse immune cell populations within the cancer microenvironment. A deep learning analysis workflow was used to detect and classify stromal inflammatory cells, in whole slide images (WSIs), generated using an Olympus VS120 digital microscope (Olympus, Tokyo, Japan). Pixel-wise predictions from a color auto-encoder (ColorAE) union UNET anchor UNET model were combined to create multi-class masks that were further analyzed to perform detection and classification. Results: The analysis of the inflammatory microenvironment focused on defining immune cell infiltrates located within 25 microns of the closest K17-positive versus K17-negative tumor cell in each representative section. Across the sum of K17-positive and negative zones/section, CD4 cell counts ranged from 0-10,617 (mean 2,709), CD8 cell counts ranged from 63-28,596 (mean 6,745), CD16 cell counts ranged from 4-7,797 (mean 3,024), and CD163 cell counts ranged from 35-34,696 (mean 14,968). CD4 T-helper cells, CD8 cytotoxic T cells, and CD16 macrophages were more numerous (respectively, p=0.0012; p=<0.0001; p=<0.0001) in K17-negative tumor zones compared to K17-positive zones. By contrast, the number of CD163 (M2) tumor-promoting macrophages was greater in K17 positive zones (p=0.0019). Conclusion: K17 expression by tumor cells impacts the chronic inflammatory microenvironment, shielding tumor cells from immune cell mediated cytotoxic responses, while recruiting tumor-promoting M2 macrophages, indicating that K17 impacts the immune response as a fundamental hallmark of aggression in PDAC.

Citation Format: Lyanne Oblein, Michael Horowitz, Mahmudul Hasan, Sruthi Babu, Mariana Torrente-Goncalves, Lucia Roa, Jaymie Oentoro, Jason Harper, Xin Yao Zheng, Wei Jiang, Andrew Hendifar, Natalie Moshayedi, Brent Larson, Veronica Placencio-Hickok, Edik Blais, Emmanuel Petricoin, Joel Saltz, Natalia D. Marchenko, Luisa F. Escobar-Hoyos, Kenneth Shroyer. Keratin 17 excludes CD8-positive T cells and recruits CD163-positive macrophages in pancreatic ductal adenocarcinoma [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer; 2022 Sep 13-16; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2022;82(22 Suppl):Abstract nr C071.

Abstract B049: Comparison of keratin 17 and GATA6 as prognostic markers in pancreatic cancer

Background: Clinically and histologically identical pancreatic ductal adenocarcinomas (PDACs) differ in response to treatment, disease progression, and survival. Clinical trials are testing the use of either keratin 17 (K17) or GATA6 as surrogate prognostic and predictive biomarkers of PDAC. Still, there is a need to contrast their expression and prognostic value in the same cohort of cases. K17, a biomarker of the most aggressive subtype of PDAC, promotes tumor growth and chemoresistance. GATA6 is a transcription factor necessary for normal pancreatic development, and a surrogate marker of the classical subtype of PDAC, associated with longer-term survival. Our goal was to test the hypothesis that there is a reciprocal relationship between K17 and GATA6 expression and that their detection informs prognostic stratification. Methods: Stony Brook University Hospital PDAC patients were selected, and representative sections were processed for dual-staining by indirect immunoperoxidase methods using antibodies to K17 (KDx Diagnostics) and GATA6 (R&D Systems AF1700). Manual scoring of the proportion of cells with strong (2+) nuclear and/or cytoplasmic staining was determined for K17 versus nuclear staining for GATA6 (PathSQ). Automated image analysis of GATA6 expression was further scored using QuPath v0.3.2 on annotated PDAC sections. Cut-off values for the best COX model for each marker were established to stratify for survival. Results: K17 expression was observed in cancer epithelial cells and in few benign proliferative ducts of chronic pancreatitis but no staining was detected in stromal or acinar cells. By contrast, GATA6 was detected in the nuclei of most cancer epithelial cells and was also consistently detected in benign ductal cells, acinar cells, and stromal cells. Defining high K17 cases as those with staining in ≥10% of tumor cells, high K17 correlated with shorter overall survival (p=0.03), supporting its use as a negative prognostic biomarker. Due to the high level of expression of GATA6 in PDAC cells (range 60-100% of tumor cells across all cases), however, no threshold was found to provide prognostic value. By dual-color IHC, K17 positive cells almost always had strong GATA6 nuclear staining but GATA6 was also detected in all K17-negative tumor cells. Computational analysis demonstrated 98% concordance with manual scoring for high GATA6 and 90% concordance with manual scoring for low GATA6. Conclusions: GATA6 was expressed in most PDAC tumor cells, as well as in benign stromal and epithelial cells. However, K17 expression was focal, relatively tumor cell-specific, and was not detected in most benign cellular components. Although high K17 status identified cases with relatively short survival, a survival threshold for GATA6 could not be detected. Contradiction in our observations of GATA6 compared to previous reports could result from differences in case selection or immunohistochemical protocol. Thus, further work is needed to explore the relationship between K17 and GATA6 as complementary biomarkers of PDAC patient survival.

Citation Format: Michael Horowitz, Lyanne Oblein, Jaymie Oentoro, Saumya Kasliwal, Natalia D. Marchenko, Luisa F. Escobar-Hoyos, Kenneth R. Shroyer. Comparison of keratin 17 and GATA6 as prognostic markers in pancreatic cancer [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer; 2022 Sep 13-16; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2022;82(22 Suppl):Abstract nr B049.

ErbB2 inhibition by lapatinib promotes degradation of mutant p53 protein in cancer cells

Mutations in the p53 tumor suppressor gene are the most prevalent genetic events in human Her2-positive breast cancer and are associated with poor prognosis and survival. Human clinical data and our in vitro and in vivo studies strongly suggest potent oncogenic cooperation between mutant p53 and Her2 (ErbB2). Yet, the translational significance of mutant p53 in Her2 positive breast cancer, especially with respect to Her2-targeted therapies, has not been evaluated. Our previous work identified novel oncogenic activity of mutant p53 whereby mutp53 amplifies ErbB2 signaling via the mutp53-HSF1-ErbB2 feed-forward loop. Here we report that pharmacological interception of this circuit by ErbB2 inhibitor lapatinib downregulates mutant p53 in vitro and in vivo. We found that ErbB2 inhibition by lapatinib inhibits transcription factor HSF1, and its target Hsp90, followed by mutant p53 degradation in MDM2 dependent manner. Thus, our data suggest that mutant p53 sensitizes cancer cells to lapatinib via two complementary mechanisms: mutant p53 mediated amplification of ErbB2 signaling, and simultaneous annihilation of both potent oncogenic drivers, ErbB2 and mutant p53. Hence, our study could provide valuable information for the optimization of therapeutic protocols to achieve superior clinical effects in the treatment of Her2 positive breast cancer.

p53 loss-of-heterozygosity is a necessary prerequisite for mutant p53 stabilization and gain-of-function in vivo

Missense mutations in TP53 comprise >75% of all p53 alterations in cancer, resulting in highly stabilized mutant p53 proteins that not only lose their tumor-suppressor activity, but often acquire oncogenic gain-of-functions (GOFs). GOF manifests itself in accelerated tumor onset, increased metastasis, increased drug resistance and shortened survival in patients and mice. A known prerequisite for GOF is mutant p53 protein stabilization, which itself is linked to aberrant protein conformation. However, additional determinants for mutant p53 stabilization likely exist. Here we show that in initially heterozygous mouse tumors carrying the hotspot GOF allele R248Q (p53Q/+), another necessary prerequisite for mutant p53 stabilization and GOF in vivo is loss of the remaining wild-type p53 allele, termed loss-of-heterozygosity (LOH). Thus, in mouse tumors with high frequency of p53 LOH (osteosarcomas and fibrosarcomas), we find that mutant p53 protein is stabilized (16/17 cases, 94%) and tumor onset is significantly accelerated compared with p53+/− tumors (GOF). In contrast, in mouse tumors with low frequency of p53 LOH (MMTV-Neu breast carcinomas), mutant p53 protein is not stabilized (16/20 cases, 80%) and GOF is not observed. Of note, human genomic databases (TCGA, METABRIC etc.) show a high degree of p53 LOH in all examined tumor types that carry missense p53 mutations, including sarcomas and breast carcinomas (with and without HER2 amplification). These data – while cautioning that not all genetic mouse models faithfully represent the human situation – demonstrate for the first time that p53 LOH is a critical prerequisite for missense mutant p53 stabilization and GOF in vivo.

Mutant p53 - Heat Shock Response Oncogenic Cooperation: A New Mechanism of Cancer Cell Survival

The main tumor suppressor function of p53 as a "guardian of the genome" is to respond to cellular stress by transcriptional activation of apoptosis, growth arrest, or senescence in damaged cells. Not surprisingly, mutations in the p53 gene are the most frequent genetic alteration in human cancers. Importantly, mutant p53 (mutp53) proteins not only lose their wild-type tumor suppressor activity but also can actively promote tumor development. Two main mechanisms accounting for mutp53 proto-oncogenic activity are inhibition of the wild-type p53 in a dominant-negative fashion and gain of additional oncogenic activities known as gain-of-function (GOF). Here, we discuss a novel mechanism of mutp53 GOF, which relies on its oncogenic cooperation with the heat shock machinery. This coordinated adaptive mechanism renders cancer cells more resistant to proteotoxic stress and provides both, a strong survival advantage to cancer cells and a promising means for therapeutic intervention.

Mitochondrial death functions of p53

The p53 tumor suppressor network plays a fundamental surveillance role in both homeostatic and adaptive cell biology. p53 is one of the most important barriers against malignant derailment of normal cells, orchestrating growth arrest, senescence, or cell death by linking many different pathways in response to genotoxic and non-genotoxic insults. p53 is the key broadband sensor for numerous cellular stresses such as DNA damage, hypoxia, oxidative stress, oncogenic signaling, and nucleolar stress. The crucial tumor suppressive and tissue homeostasis activity of p53 is its ability to activate cell death via multiple different pathways. A well-characterized biochemical function of p53 in the regulation of apoptosis is its role as a potent transcriptional regulator. p53 activates a panel of proapoptotic genes from the mitochondrial apoptotic and death receptor programs while repressing antiapoptotic Bcl2 family genes. In addition, over the last 10 y a growing body of evidence has also defined direct extranuclear non-transcriptional p53 activities within mitochondria-mediated cell death pathways that are based on p53 protein accumulation in cytosolic and mitochondrial compartments and protein-protein interactions. To date, transcription-independent p53-mediated cell death regulation has been described for apoptosis, necrosis, and autophagy. Because mitochondrial dysregulation is central to the development of a number of pathologic processes such as cancer and neurodegenerative and age-related diseases, understanding the direct roles of p53 protein in mitochondria has high translational impact and could facilitate the development of novel drug targets to combat these diseases. In this review we will mainly focus on mechanisms of p53-mediated transcription-independent cell death pathways at mitochondria.

p63 is a prosurvival factor in the adult mammary gland during post-lactational involution, affecting PI-MECs and ErbB2 tumorigenesis

In embryogenesis, p63 is essential to develop mammary glands. In the adult mammary gland, p63 is highly expressed in the basal cell layer that comprises myoepithelial and interspersed stem/progenitor cells, and has limited expression in luminal epithelial cells. In adult skin, p63 has a crucial role in the maintenance of epithelial stem cells. However, it is unclear whether p63 also has an equivalent role as a stem/progenitor cell factor in adult mammary epithelium. We show that p63 is essential in vivo for the survival and maintenance of parity-identified mammary epithelial cells (PI-MECs), a pregnancy-induced heterogeneous population that survives post-lactational involution and contain multipotent progenitors that give rise to alveoli and ducts in subsequent pregnancies. p63+/- glands are normal in virgin, pregnant and lactating states. Importantly, however, during the apoptotic phase of post-lactational involution p63+/- glands show a threefold increase in epithelial cell death, concomitant with increased activation of the oncostatin M/Stat3 and p53 pro-apoptotic pathways, which are responsible for this phase. Thus, p63 is a physiologic antagonist of these pathways specifically in this regressive stage. After the restructuring phase when involution is complete, mammary glands of p63+/- mice again exhibit normal epithelial architecture by conventional histology. However, using Rosa(LSL-LacZ);WAP-Cre transgenics (LSL-LacZ, lox-stop-lox β-galactosidase), a genetic in vivo labeling system for PI-MECs, we find that p63+/- glands have a 30% reduction in the number of PI-MEC progenitors and their derivatives. Importantly, PI-MECs are also cellular targets of pregnancy-promoted ErbB2 tumorigenesis. Consistent with their PI-MEC pool reduction, one-time pregnant p63+/- ErbB2 mice are partially protected from breast tumorigenesis, exhibiting extended tumor-free and overall survival, and reduced tumor multiplicity compared with their p63+/+ ErbB2 littermates. Conversely, in virgin ErbB2 mice p63 heterozygosity provides no survival advantage. In sum, our data establish that p63 is an important survival factor for pregnancy-identified PI-MEC progenitors in breast tissue in vivo.

p53 opens the mitochondrial permeability transition pore to trigger necrosis

Ischemia-associated oxidative damage leading to necrosis is a major cause of catastrophic tissue loss, and elucidating its signaling mechanism is therefore of paramount importance. p53 is a central stress sensor responding to multiple insults, including oxidative stress to orchestrate apoptotic and autophagic cell death. Whether p53 can also activate oxidative stress-induced necrosis is, however, unknown. Here, we uncover a role for p53 in activating necrosis. In response to oxidative stress, p53 accumulates in the mitochondrial matrix and triggers mitochondrial permeability transition pore (PTP) opening and necrosis by physical interaction with the PTP regulator cyclophilin D (CypD). Intriguingly, a robust p53-CypD complex forms during brain ischemia/reperfusion injury. In contrast, reduction of p53 levels or cyclosporine A pretreatment of mice prevents this complex and is associated with effective stroke protection. Our study identifies the mitochondrial p53-CypD axis as an important contributor to oxidative stress-induced necrosis and implicates this axis in stroke pathology.

Inhibiting the HSP90 chaperone destabilizes macrophage migration inhibitory factor and thereby inhibits breast tumor progression

In several human cancer cell lines, HSP90 inhibitors destabilize macrophage inhibitory factor protein; systemic treatment with an HSP90 inhibitor slows tumor growth and extends overall survival in a mouse model of HER2-positive human breast cancer.

Intracellular macrophage migration inhibitory factor (MIF) often becomes stabilized in human cancer cells. MIF can promote tumor cell survival, and elevated MIF protein correlates with tumor aggressiveness and poor prognosis. However, the molecular mechanism facilitating MIF stabilization in tumors is not understood. We show that the tumor-activated HSP90 chaperone complex protects MIF from degradation. Pharmacological inhibition of HSP90 activity, or siRNA-mediated knockdown of HSP90 or HDAC6, destabilizes MIF in a variety of human cancer cells. The HSP90-associated E3 ubiquitin ligase CHIP mediates the ensuing proteasome-dependent MIF degradation. Cancer cells contain constitutive endogenous MIF–HSP90 complexes. siRNA-mediated MIF knockdown inhibits proliferation and triggers apoptosis of cultured human cancer cells, whereas HSP90 inhibitor-induced apoptosis is overridden by ectopic MIF expression. In the ErbB2 transgenic model of human HER2-positive breast cancer, genetic ablation of MIF delays tumor progression and prolongs overall survival of mice. Systemic treatment with the HSP90 inhibitor 17AAG reduces MIF expression and blocks growth of MIF-expressing, but not MIF-deficient, tumors. Together, these findings identify MIF as a novel HSP90 client and suggest that HSP90 inhibitors inhibit ErbB2-driven breast tumor growth at least in part by destabilizing MIF.

SAHA shows preferential cytotoxicity in mutant p53 cancer cells by destabilizing mutant p53 through inhibition of the HDAC6-Hsp90 chaperone axis

Mutant p53 (mutp53) cancers are surprisingly dependent on their hyperstable mutp53 protein for survival, identifying mutp53 as a potentially significant clinical target. However, exploration of effective small molecule therapies targeting mutp53 has barely begun. Mutp53 hyperstabilization, a hallmark of p53 mutation, is cancer cell-specific and due to massive upregulation of the HSP90 chaperone machinery during malignant transformation. We recently showed that stable complex formation between HSP90 and its mutp53 client inhibits E3 ligases MDM2 and CHIP, causing mutp53 stabilization. Histone deacetylase (HDAC) inhibitors (HDACi) are a new class of promising anti-cancer drugs, hyperacetylating histone and non-histone targets. Currently, suberoylanilide hydroxamic acid (SAHA) is the only FDA-approved HDACi. We show that SAHA exhibits preferential cytotoxicity for mutant, rather than wild-type and null p53 human cancer cells. Loss/gain-of-function experiments revealed that although able to exert multiple cellular effects, SAHA's cytotoxicity is caused to a significant degree by its ability to strongly destabilize mutp53 at the level of protein degradation. The underlying mechanism is SAHA's inhibition of HDAC6, an essential positive regulator of HSP90. This releases mutp53 and enables its MDM2- and CHIP-mediated degradation. SAHA also strongly chemosensitizes mutp53 cancer cells for chemotherapy due to its ability to degrade mutp53. This identifies a novel action of SAHA with the prospect of SAHA becoming a centerpiece in mutp53-specific anticancer strategies.

Functional inactivation of endogenous MDM2 and CHIP by HSP90 causes aberrant stabilization of mutant p53 in human cancer cells

The tight control of wild-type p53 by mainly MDM2 in normal cells is permanently lost in tumors harboring mutant p53, which exhibit dramatic constitutive p53 hyperstabilization that far exceeds that of wild-type p53 tumors. Importantly, mutant p53 hyperstabilization is critical for oncogenic gain of function of mutant p53 in vivo. Current insight into the mechanism of this dysregulation is fragmentary and largely derived from ectopically constructed cell systems. Importantly, mutant p53 knock-in mice established that normal mutant p53 tissues have sufficient enzymatic reserves in MDM2 and other E3 ligases to maintain full control of mutant p53. We find that in human cancer cells, endogenous mutant p53, despite its ability to interact with MDM2, suffers from a profound lack of ubiquitination as the root of its degradation defect. In contrast to wild-type p53, the many mutant p53 proteins which are conformationally aberrant are engaged in complexes with the HSP90 chaperone machinery to prevent its aggregation. In contrast to wild-type p53 cancer cells, we show that in mutant p53 cancer cells, this HSP90 interaction blocks the endogenous MDM2 and CHIP (carboxy-terminus of Hsp70-interacting protein) E3 ligase activity. Interference with HSP90 either by RNA interference against HSF1, the transcriptional regulator of the HSP90 pathway, or by direct knockdown of Hsp90 protein or by pharmacologic inhibition of Hsp90 activity with 17AAG (17-allylamino-17-demethoxygeldanamycin) destroys the complex, liberates mutant p53, and reactivates endogenous MDM2 and CHIP to degrade mutant p53. Of note, 17AAG induces a stronger viability loss in mutant p53 than in wild-type p53 cancer cells. Our data support the rationale that suppression of mutant p53 levels in vivo in established cancers might achieve clinically significant effects.

The transcription-independent mitochondrial p53 program is a major contributor to nutlin-induced apoptosis in tumor cells

Strategies to induce p53 activation in tumors that retain wild-type p53 are promising for cancer therapy. Nutlin is a potent and selective pharmacological MDM2 inhibitor that competitively binds to its p53-binding pocket, thereby leading to non-genotoxic p53 stabilization and activation of growth arrest and apoptosis pathways. Nutlin-induced apoptosis is thought to occur via p53's transcriptional program. Here we report that the transcription-independent mitochondrial p53 program plays an important role in Nutlin-induced p53-mediated tumor cell death. Aside from nuclear stabilization, Nutlin causes cytoplasmic p53 accumulation and translocation to mitochondria. Monoubiquitinated p53, originating from a distinct cytoplasmic pool, is the preferred p53 species that translocates to mitochondria in response to stress. Nutlin does not interfere with MDM2's ability to monoubiquitinate p53, due to the fact that MDM2-p53 complexes are only partially disrupted and that Nutlin-stabilized MDM2 retains its E3 ubiquitin ligase activity. Nutlin-induced mitochondrial p53 translocation is rapid and associated with cytochrome C release that precedes induction of p53 target genes. Specific inhibition of mitochondrial p53 translocation by Pifithrin mu reduces the apoptotic Nutlin response by 2.5-fold, underlining the significance of p53's mitochondrial program in Nutlin-induced apoptosis. Surprisingly, blocking the transcriptional arm of p53, either via alpha-Amanitin or the p53-specific transcriptional inhibitor Pifithrin alpha, not only fails to inhibit, but greatly potentiates Nutlin-induced apoptosis. In sum, the direct mitochondrial program is a major mechanism in Nutlin-induced p53-mediated apoptosis. Moreover, at least in some tumors the transcriptional p53 activities in net balance not only are dispensable for the apoptotic Nutlin response, but appear to actively block its therapeutic effect.

A role of HAUSP in tumor suppression in a human colon carcinoma xenograft model

The protease HAUSP is a critical component of the p53-Mdm2 pathway and acts as a specific deubiquitinase for both p53 and Mdm2 and thus is important for p53 regulation. In knock-down and knock-out cellular systems it was observed that ablation of HAUSP induces profound stabilization of p53 due to enhanced degradation of Mdm2. Thus, inhibiting HAUSP by small compound interference has been proposed as a rational therapeutic strategy to activate p53 in p53 wild type tumors. However, HAUSP-mediated effects in the p53-Mdm2 axis are highly complex and non-linear and to date the role of HAUSP in tumor suppression in vivo remains unexplored. Here we investigate the effect of HAUSP up and downregulation on cell proliferation, apoptosis and tumor growth in vitro and in a xenograft model in vivo, using an inducible isogenic human colon carcinoma cell system. Importantly, in the absence of stress, both HAUSP up and downregulation inhibit cell proliferation in vitro and tumor growth in vivo due to constitutively elevated p53 levels. Moreover, tumors with HAUSP up and downregulation respond to radiotherapy with further growth inhibition. However, HAUSP downregulation causes resistance to Camptothecin- and irradiation-induced apoptosis, which correlates with suppressed mitochondrial translocation of p53. Our data suggest that changes in HAUSP modulate tumor growth and apoptotic sensitivity in vivo.

Hyperubiquitylation of wild-type p53 contributes to cytoplasmic sequestration in neuroblastoma

Neuroblastoma (NB) is the most common solid malignancy in childhood and its prognosis is still generally poor. In contrast to many other cancers, mutations of the p53 tumor suppressor are rare. Instead, significant cytosolic sequestration of wtp53 is one of several mechanisms that attenuate p53 function in this cancer. Here, we report that aberrant p53 hyperubiquitylation contributes to p53 cytoplasmic sequestration in NB. NB lines constitutively harbor an elevated portion of wtp53 as stable ubiquitylated species confined to the cytoplasm. p53 hyperubiquitylation is not due to dysregulation by Hdm2 or proteasomal dysfunction. Instead, the defect lies in p53 regulation by HAUSP, a major p53-deubiquitylating enzyme. In contrast to non-NB cancer cells with nuclear p53 and normal ubiquitylation, p53 from NB cells shows impaired HAUSP interaction. Conversely, interference with p53 hyperubiquitylation in NB cells by Nutlin 3a or by a C-terminal p53 peptide (aa 305-393) results in p53 relocalization from the cytoplasm to the nucleus, and in case of Nutlin, in reactivation of p53's transcriptional and apoptotic functions. Moreover, nutlin and camptothecin act synergistically in inducing NB cell apoptosis. Hence, this study strengthens the rationale for targeting p53 deubiquitylation by drugs like Nutlin as a promising new strategy in NB therapy.

β-Catenin regulates the gene of MMP-26, a novel matrix metalloproteinase expressed both in carcinomas and normal epithelial cells

There are several unorthodox features, which distinguish the non-redundant and unique novel matrix metalloproteinase-26 (MMP-26) (an enzyme that has recently evolved and does not exist in rodents but is present in humans) from other members of the MMP superfamily. This report describes our recent efforts to gain a better understanding of the mechanisms which restrict expression of MMP-26 to certain cell/tissue types. We examined transcriptional regulation of the human MMP-26 gene in normal and malignant cells. The AP-1 and Tcf-4 sites of the MMP-26 promoter appear most potent in regulating the expression of the MMP-26-luciferase chimera in HEK293 embryonic kidney and MCF7 breast carcinoma cells. Key regulators of the Wnt pathway (beta-catenin and lymphoid enhancer-binding factor/T-cell factor with which beta-catenin associates) enhanced the transcriptional activity of MMP-26 suggesting that the MMP-26 gene is a likely target of the Wnt pathway. Immunostaining, gene arrays and reverse-transcriptase polymerase chain reaction (RT-PCR) confirm the presence of MMP-26 in normal cells, including the apical epithelial conjunctiva cells of the human eye, as well as in malignant cells of epithelial origin. MMP-26 predominantly accumulates in its proenzyme form in the intracellular milieu of the transfected breast carcinoma MCF7 cells. This study brings us a step forward towards a better understanding of the unconventional role, regulation and functions of epithelial cell MMP-26 in physiological conditions and in neoplasms.

Aberrant, persistent inclusion into lipid rafts limits the tumorigenic function of membrane type-1 matrix metalloproteinase in malignant cells

Membrane type-1 matrix metalloproteinase (MT1-MMP) is a key enzyme in cell locomotion and tissue remodeling. Trafficking to the plasma membrane and internalization into the transient storage compartment both regulate the cell surface presentation of MT1-MMP. Our data indicate that mutant MT1-MMP lacking the cytoplasmic tail is recruited to the caveolae-enriched lipid raft membrane microdomains in breast carcinoma MCF7 cells. In contrast, the wild-type protease is not permanently associated with lipid rafts. Trafficking to lipid rafts correlated with poor internalization and the persistent presentation of MT1-MMP at the cell surface. The tail mutant efficiently functioned in inducing the activation of the latent proMMP-2 zymogen, matrix remodeling, and contraction of three-dimensional collagen lattices. Recruitment of the tail mutant to lipid raft antagonized, however, the cleavage of the plasma membrane-associated E-cadherin. These events limited the contribution of the tail mutant to cell locomotion and malignant growth. It is conceivable that the tail peptide sequence plays a crucial role in the translocations of MT1-MMP across the cell and contributes to coordinated cellular functions. It is tempting to hypothesize that the mechanisms involved in trafficking of MT1-MMP to caveolin-enriched lipid rafts may be targeted in a clinically advantageous manner.

The structure and regulation of the human and mouse matrix metalloproteinase-21 gene and protein

Matrix metalloproteinases (MMPs) play key roles in tissue remodelling under normal development and, especially, in diseases ranging from malignancies to stroke. We cloned and thoroughly characterized the novel human and mouse MMP gene encoding MMP-21. MMP-21 is the last uncharacterized MMP coded by the human genome. Human and mouse MMP-21 is the orthologue of Xenopus laevis X-MMP. The latent proenzyme of MMP-21 (569 amino acid residues) consists of the prodomain, the catalytic domain and the haemopexin-like domain, and is potentially capable of being activated in its secretory pathway to the extracellular milieu by furin-like proprotein convertases. Human MMP-21 is the probable target gene of the Wnt pathway. In addition, the expression of MMP-21 is controlled uniquely by Pax and Notch transcription factors known to be critical for organogenesis. MMP-21 is expressed transiently in mouse embryogenesis and increased in embryonic neuronal tissues. Our observations clearly indicate that there is an important specific function for MMP-21 in embryogenesis, especially in neuronal cells.

A quantitative analysis of rate-limiting steps in the metastatic cascade using human-specific real-time polymerase chain reaction

A quantitative assessment of rate-limiting steps in metastasis has always been challenging because of the difficulty of detecting small tumor cell populations. We have developed a highly sensitive assay for monitoring the metastatic dissemination of human tumor cells in the chick embryo and used this assay to investigate the relative efficacy of sequential stages in the metastatic cascade for two malignant human tumor cells lines, HEp3 and HT1080. This assay is based on the real-time PCR amplification of human alu sequences and exhibits a high sensitivity (25 cells/lung) with a large linear range (50-100,000 cell/lung). The assay is optimized for a high number of replicate in vivo assays (50-100 animals/assay) and can be applied in both experimental and spontaneous metastasis models. Using quantitative alu PCR, we determined that HEp3 spontaneously metastasizes very efficiently and rapidly, generating secondary growth in the lung exceeding 1-2 x 10(4) cells/lung in 7 days. In contrast, spontaneous HT1080 metastasis is 50-100-fold less efficient, resulting in only 200-400 cells/lung in 7 days. By taking advantage of the sensitivity and specificity of the real-time alu PCR assay we were also able to quantitatively assess multiple steps in metastasis including intravasation, arrest of tumor cells in secondary organs of the embryo, and the initial growth and expansion of the arrested tumor cells. A comparative analysis of HEp3 and HT1080 metastasis demonstrates that the relatively low-to-moderate metastatic rate of HT1080 is caused by two distinct deficiencies, an 8-10-fold lower rate of intravasation and a delayed onset of HT1080 growth expansion in the secondary organ. Thus, a very facile metastasis model system coupled with the sensitive, real-time PCR-based assay allows for the identification and quantification of rate-limiting steps in the metastatic cascade for select human tumor cell lines.

Unconventional activation mechanisms of MMP-26, a human matrix metalloproteinase with a unique PHCGXXD cysteine-switch motif

ProMMP-26 has the unique Pro-His(81)-Cys-Gly-Xaa-Xaa-Asp cysteine-switch motif that discriminates this protease from all other matrix metalloproteinases (MMPs) known so far. The conserved, free cysteine residue of the conventional PRCXXPD sequence interacts with the zinc ion of the catalytic domain and provides the fourth coordination site for the catalytic zinc, thereby preventing latent proMMPs from becoming active. MMPs become functionally active when proteolytic cleavage releases the prodomain and the PRCXXPD sequence and exposes the zinc atom. Here, we report that the Pro-His(81)-Cys-Gly-Xaa-Xaa-Asp motif is not functional in proMMP-26 and consequently is not involved in the activation mechanisms. Organomercurial treatment failed to activate proMMP-26. The autolytic Lys-Lys-Gln(59) downward arrow Gln(60)-Phe-His cleavage upstream of the Pro-His(81)-Cys-Gly-Xaa-Xaa-Asp motif induced the proteolytic activity of recombinant proMMP-26 whereas any further cleavage inactivated the enzyme. The His(81) --> Arg(81) mutation restored the conventional cysteine-switch sequence in the prodomain but failed to induce the cysteine-switch activation mechanism. These data and computer modeling studies allowed us to hypothesize that the presence of His(81) significantly modified the fold of proMMP-26, abolished the functionality of the cysteine-switch motif, and stimulated an alternative intramolecular activation pathway of the proenzyme.

Promoter characterization of the novel human matrix metalloproteinase-26 gene: regulation by the T-cell factor-4 implies specific expression of the gene in cancer cells of epithelial origin

A novel matrix metalloproteinase-26 (MMP-26) is known to be specifically expressed in epithelial carcinomas. To facilitate studies of MMP-26 transcriptional regulation, we have cloned and characterized a 1 kb 5'-flanking region of the human MMP-26 gene. Altogether, our findings indicate that the MMP-26 promoter has distinctive structural and functional features among MMP genes. An unusual polyadenylation site proximal to the transcription-factor-binding sites protects transcription of the MMP-26 gene from the upstream promoters and represents a part of the stringent transcriptional regulation of the gene. The MMP-26 gene has a consensus TATA-box and one transcriptional start site located 60 and 35 nucleotides upstream of the translational start site, respectively. The MMP-26 promoter was able to drive luciferase expression in human A549 lung carcinoma, HT1080 fibrosarcoma and HEK293 embryonic kidney cells. The basal transcription efficiency of the MMP-26 promoter is relatively low, thereby explaining the minute expression of the gene in most cells and tissues. When compared with other MMP genes, the MMP-26 promoter contains binding sites for a few transcription factors. Sequential deletion and mutation analysis, and electrophoretic mobility-shift assay have identified the T-cell factor-4 (Tcf-4) motif and the activator protein-1 site as the major regulatory elements of the MMP-26 promoter. Since previous studies have established that the Tcf-4 transcription factor is subjected exclusively to regulation through the beta-catenin/E(epithelial)-cadherin pathway, this implies the specific expression of MMP-26 in cancer cells of epithelial origin.

Hypoxia death stimulus induces translocation of p53 protein to mitochondria. Detection by immunofluorescence on whole cells

Evidence suggests that p53 induces cell death by a dual mode of action involving activation of target genes and transcriptionally independent direct signaling. Mitochondria are major signal transducers in apoptosis. We recently discovered that a fraction of induced p53 protein rapidly translocates to mitochondria during p53-dependent apoptosis, but not during p53-independent apoptosis or p53-mediated cell cycle arrest. Importantly, specific targeting of p53 to mitochondria was sufficient to induce apoptosis in p53-deficient tumor cells. This led us to propose a model where p53 exerts a direct apoptogenic role at the mitochondria, thereby enhancing the transcription-dependent apoptosis of p53. Here we show for the first time that mitochondrial localization of endogenous p53 can be visualized by immunofluorescence of whole cells when stressed by hypoxic conditions. Suborganellar localization by limited trypsin digestion of isolated mitochondria from stressed cells suggests that a significant amount of mitochondrial p53 is located at the surface of the organelle. This mitochondrial association can be reproduced in vitro with purified p53. Together, our data provide further evidence for an apoptogenic signaling role of p53 protein in vivo at the level of the mitochondria.

Death signal-induced localization of p53 protein to mitochondria. A potential role in apoptotic signaling

The mechanism of p53-mediated apoptosis after cellular stress remains poorly understood. Evidence suggests that p53 induces cell death by a multitude of molecular pathways involving activation of target genes and transcriptionally independent direct signaling. Mitochondria play a key role in apoptosis. We show here that a fraction of p53 protein localizes to mitochondria at the onset of p53-dependent apoptosis but not during p53-independent apoptosis or p53-mediated cell cycle arrest. The accumulation of p53 to mitochondria is rapid (within 1 h after p53 activation) and precedes changes in mitochondrial membrane potential, cytochrome c release, and procaspase-3 activation. Immunoelectron microscopy and immuno-fluorescence-activated cell sorter analysis of isolated mitochondria show that the majority of mitochondrial p53 localizes to the membranous compartment, whereas a fraction is found in a complex with the mitochondrial import motor mt hsp70. After induction of ectopic p53 without additional DNA damage in p53-deficient cells, p53 again partially localizes to mitochondria, preceding the onset of apoptosis. Overexpression of anti-apoptotic Bcl-2 or Bcl-xL abrogates stress signal-mediated mitochondrial p53 accumulation and apoptosis but not cell cycle arrest, suggesting a feedback signaling loop between p53 and mitochondrial apoptotic regulators. Importantly, bypassing the nucleus by targeting p53 to mitochondria using import leader fusions is sufficient to induce apoptosis in p53-deficient cells. We propose a model where p53 can contribute to apoptosis by direct signaling at the mitochondria, thereby amplifying the transcription-dependent apoptosis of p53.

Organization of threonine biosynthesis genes from the obligate methylotroph Methylobacillus flagellatus

The genes encoding aspartate kinase (ask), homoserine dehydrogenase (hom), homoserine kinase (thrB) and threonine synthase (thrC) from the obligate methylotroph Methylobacillus flagellatus were cloned. In maxicells hom and thrC directed synthesis of 51 and 48 kDa polypeptides, respectively. The hom, thrB and thrC genes and adjacent DNA areas were sequenced. Of the threonine biosynthesis genes, only hom and thrC were tightly linked in the order hom-thrC. The gene for thymidylate synthase (thyA) followed thrC and the gene for aspartate aminotransferase (aspC) preceded hom. All four genes (aspC-hom-thrC-thyA) were transcribed in the same direction. mRNA analysis indicated that hom-thrC are apparently transcribed in one 7.5 kb transcript in M. flagellatus. Promoter analysis showed the presence of a functional promoter between aspC and hom. No functional promoter was found to be associated with the DNA stretch between hom and thrC. The thrB gene encoded an unusual type of homoserine kinase and was not linked to other threonine biosynthesis genes.

Overexpression of the wild type p73 gene in breast cancer tissues and cell lines

The p73 gene is a structural and, in overexpression systems, functional p53 homologue. Ectopic p73 expression can activate a broad subset of p53-responsive genes, induce apoptosis, and act as a growth suppressor. Yet, viral oncoproteins that antagonize p53 (adenovirus E1B 55K, SV40 large T, and human papillomavirus E6) do not antagonize p73. This could suggest that inactivation of p73, in contrast to p53, is not required for tumorigenesis. Also, p73 is not activated by DNA damage. Because intragenic p73 mutations in tumors have not been reported and imprinting is idiosyncratic, tumor-specific changes in wild-type p73 expression levels become the most reliable guide toward identifying the normal function of p73 and its role in tumorigenesis. We analyzed 77 invasive breast cancers and 7 breast cancer cell lines for p73 mRNA expression levels, allelic origin, intragenic mutations, and COOH-terminal splice variants. A range of normal tissues, including breast, showed very low p73 expression, with little variation from tissue to tissue. In contrast, 38% (29 cases) of breast cancers had elevated p73 mRNA ranging from 5-25-fold above normal, with the remaining tumors (64%) falling within the normal range. Moreover, five of seven cell lines (71%) also exhibited p73 overexpression (13-73-fold). Yet, no correlation with p21 mRNA and protein levels was present, although four of the five lines were mutant for p53. Mutation analysis of the eight highest expressers showed wild type status. Eight of 14 informative samples were biallelic, whereas the remaining 6 samples showed monoallelic expression. Tumors and cell lines with p73 overexpression tended to exhibit a complex profile of up to six different COOH-terminal splice variants, whereas normal and transformed tissues with low p73 mRNA predominantly expressed p73 alpha. We confirm the previously described variants p73 gamma and delta in breast tissue and describe two novel isoforms, p73 epsilon and phi, thereby further enlarging combinatorial possibilities. Together, our in vivo data show that p73 does not have a role as a classic Knudson-type tumor suppressor in breast cancer.

A leucine-rich nuclear export signal in the p53 tetramerization domain: regulation of subcellular localization and p53 activity by NES masking

Appropriate subcellular localization is crucial for regulating p53 function. We show that p53 export is mediated by a highly conserved leucine-rich nuclear export signal (NES) located in its tetramerization domain. Mutation of NES residues prevented p53 export and hampered tetramer formation. Although the p53-binding protein MDM2 has an NES and has been proposed to mediate p53 export, we show that the intrinsic p53 NES is both necessary and sufficient for export. This report also demonstrates that the cytoplasmic localization of p53 in neuroblastoma cells is due to its hyperactive nuclear export: p53 in these cells can be trapped in the nucleus by the export-inhibiting drug leptomycin B or by binding a p53-tetramerization domain peptide that masks the NES. We propose a model in which regulated p53 tetramerization occludes its NES, thereby ensuring nuclear retention of the DNA-binding form. We suggest that attenuation of p53 function involves the conversion of tetramers into monomers or dimers, in which the NES is exposed to the proteins which mediate their export to the cytoplasm.

Loss of p53 function in uterine papillary serous carcinoma

In contrast to endometrioid carcinoma, uterine papillary serous carcinoma (UPSC) is an aggressive type of endometrial cancer. Loss of p53 function is critical for the molecular pathogenesis of UPSC. Both UPSC and its putative precursor, endometrial intraepithelial carcinoma (EIC), show abnormal p53 overexpression in most tumors. To further assess the nature of p53 alterations in UPSC, we systematically reevaluated a subset of our previous cohort of UPSC patients. In the current study, we correlate mutations of the p53 gene as detected by direct sequencing of exons 5 through 8 with p53 accumulation and expression of Waf-1 in 32 UPSC tumors. Waf-1 is a downstream effector of p53-mediated G1 arrest after DNA damage and, thus, an indicator of p53 functionality. Although 78% of tumors exhibited strong nuclear p53 immunoreactivity in 100% of tumor cells, we were able to detect p53 mutations in 53%. As expected, all p53 mutant tumors (17 cases) exhibited p53 overexpression. Seventy percent of those (12 tumors) showed concomitant lack of Waf-1 expression consistent with transcriptionally inactive p53, whereas the other five tumors showed Waf-1 staining in only a minor fraction of tumor cells consistent with p53-independent Waf-1 expression. In contrast, 47% (15 cases) of tumors failed to exhibit p53 mutations; interestingly, more than half of those (eight cases) showed strong nuclear p53 accumulation in all tumor cells but lacked concomitant Waf-1 expression. These findings are consistent with a mutation-dependent and -independent type of p53 inactivation in UPSC that are both associated with nuclear overexpression. Our findings suggest that the combined immunocytochemical analysis of p53 and Waf-1 is a valuable means of assessing the functional status of p53. In summary, p53 alterations are common in UPSC and probably responsible for its aggressive biological behavior.

Nuclear overexpression of p53 protein does not correlate with gene mutation in primary peritoneal carcinoma

Primary peritoneal carcinoma (PPC) is an aggressive malignancy of the female coelomic epithelium. Previously we had analyzed 29 cases of PPC for p53 protein accumulation by immunocytochemistry. P53 was overexpressed in 83% (24 of 29) of PPCs, including 21 tumors with diffuse intense staining of 100% of tumor nuclei and three additional tumors with significant focal staining. Here we report results of a mutational analysis on the entire p53 coding sequence of 22 of these cases (comprising 18 p53-positive and four negative tumors), using single-strand conformation polymorphism (SSCP) and direct sequence analysis. Only 2 of 22 (9%) patients harbored a p53 mutation (which, interestingly, were identical and consisted of a codon 259 Asp --> His exchange), despite diffuse overexpression of high levels of nuclear p53 protein in most cases. This result indicates that (1) the abnormal p53 expression is usually not caused by mutations of the p53 gene in PPC and (2) PPC is part of a growing number of tumors that share evidence of p53 dysfunction in the absence of mutation.