Age alters the oncogenic trajectory toward luminal mammary tumors that activate unfolded proteins responses

A major limitation in the use of mouse models in breast cancer research is that most mice develop estrogen receptor-alpha (ERα)-negative mammary tumors, while in humans, the majority of breast cancers are ERα-positive. Therefore, developing mouse models that best mimic the disease in humans is of fundamental need. Here, using an inducible MMTV-rtTA/TetO-NeuNT mouse model, we show that despite being driven by the same oncogene, mammary tumors in young mice are ERα-negative, while they are ERα-positive in aged mice. To further elucidate the mechanisms for this observation, we performed RNAseq analysis and identified genes that are uniquely expressed in aged female-derived mammary tumors. We found these genes to be involved in the activation of the ERα axis of the mitochondrial UPR and the ERα-mediated regulation of XBP-1s, a gene involved in the endoplasmic reticulum UPR. Collectively, our results indicate that aging alters the oncogenic trajectory towards the ERα-positive subtype of breast cancers, and that mammary tumors in aged mice are characterized by the upregulation of multiple UPR stress responses regulated by the ERα.

Can THEM6 targeting stop resistance to prostate cancer treatment?

Prostate cancer (PCa) clinical management relies heavily on androgen‐deprivation therapy (ADT). However, despite experiencing initial clinical benefit, patients getting ADT for non‐resectable PCa eventually relapse and develop fatal castration‐resistant PCa (CRPC). Multiple mechanisms of acquired resistance to treatment have been reported, including metabolic adaptation (Marine et al, 2020). Notably, activation of the endoplasmic reticulum (ER) unfolded protein response (UPR) has been associated with oncogenic transformation (Hart et al, 2012), tumor progression, metastasis dissemination, and resistance to therapy (Chen & Cubillos‐Ruiz, 2021). Targeting different branches of ER UPR has been found to be an effective tool against aggressive PCa (Nguyen et al, 2018; Sheng et al, 2019). Therefore, a better understanding of these pathways may lead to the identification of novel drug targets.

Folding Mitochondrial-Mediated Cytosolic Proteostasis Into the Mitochondrial Unfolded Protein Response

Several studies reported that mitochondrial stress induces cytosolic proteostasis. How mitochondrial stress activates proteostasis in the cytosol remains unclear. However, the cross-talk between the mitochondria and cytosolic proteostasis has far reaching implications for treatment of proteopathies including neurodegenerative diseases. This possibility appears within reach since selected drugs have begun to emerge as being able to stimulate mitochondrial-mediated cytosolic proteostasis. In this review, we focus on studies describing how mitochondrial stress activates proteostasis in the cytosol across multiple model organisms. A model is proposed linking mitochondrial-mediated regulation of cytosolic translation, folding capacity, ubiquitination, and proteasome degradation and autophagy as a multi layered control of cytosolic proteostasis that overlaps with the integrated stress response (ISR) and the mitochondrial unfolded protein response (UPR^mt). By analogy to the conductor in an orchestra managing multiple instrumental sections into a dynamically integrated musical piece, the cross-talk between these signaling cascades places the mitochondria as a major conductor of cellular integrity.

Are the estrogen receptor and SIRT3 axes of the mitochondrial UPR key regulators of breast cancer sub-type determination according to age?

Aging is a major risk factor of developing breast cancer. Despite the fact that post-menopausal women have lower levels of estrogen, older women have a higher rate of estrogen receptor alpha (ERα) positive breast cancer. Conversely, young women who have elevated levels of estrogen tend to develop ERα negative disease that is associated with higher rate of metastasis. This perspective proposes a unifying model centered around the importance of mitochondrial biology in cancer and aging to explain these observations. Mitochondria are essential for the survival of cancer cells and therefore pathways that maintain the functionality of the mitochondrial network in cancer cells fulfill a critical role in the survival of cancer cells. The ERα and the mitochondrial sirtuin-3 (SIRT3) have been reported to be key players of the mitochondrial unfolded protein response (UPR^mt) ^1-5. The UPR^mt is a complex retrograde signaling cascade that regulates the communication between the mitochondria and the nucleus to restore mitochondrial fitness in response to oxidative stress ^5-7. SIRT3 is a major regulator of aging ⁸. Its level decreases with age and single nucleotide polymorphisms (SNPs) that preserve its expression at higher levels are observed in centenarians ^9,10. We propose a model whereby the ERα axis of the UPR^mt acts to compensate for the loss of SIRT3 observed with age, and becomes the dominant axis of the UPR^mt to maintain the integrity of the mitochondria during transformation, thus explaining the selective advantage of ERα positive luminal cells in breast cancer arising from older women.

Raloxifene is a Female-specific Proteostasis Therapeutic in the Spinal Cord

Several neurodegenerative disorders are characterized by proteasome dysfunctions leading to protein aggregations and pathogenesis. Since we showed that estrogen receptor alpha (ERα) activates the proteasome, drugs able to stimulate ERα in the central nervous system (CNS) could hold potential for therapeutic intervention. However, the transcriptional effects of selective estrogen receptor modulators (SERMs), such as tamoxifen and raloxifene, can be tissue specific. A direct comparison of the effects of different SERMs on gene transcription in the CNS has never been performed. Here, we report an RNA-seq analysis of the spinal cord treated with estrogen, tamoxifen, or raloxifene. We find stark SERM and sex-specific differences in gene expression profiles in the spinal cord. Notably, raloxifene, but not estrogen or tamoxifen, modulates numerous deubiquitinating enzymes, proteasome subunits and assembly factors, and these effects translate into decreased protein aggregates. In the SOD1-G93A mouse model of amyotrophic lateral sclerosis, we found that even a low dose of raloxifene causes a significant decrease in mutant SOD1 aggregates in the spinal cord, accompanied by a delay in the decline of muscle strength in females, but not in males. These results strongly indicate SERM-selective as well as sex-specific effects, and emphasize the importance of sex as a biological variable to be considered for the careful selection of specific SERM for use in clinical trials for neurodegenerative diseases.

The Multi-Faced Role of PAPP-A in Post-Partum Breast Cancer: IGF-Signaling is Only the Beginning

Insulin-like growth factor (IGF) signaling and control of local bioavailability of free IGF by the IGF binding proteins (IGFBP) are important regulators of both mammary development and breast cancer. A recent genome-wide association study (GWAS) identified small nucleotide polymorphisms that reduce the expression of IGFBP-5 as a risk factor of developing breast cancer. This observation suggests that genetic alterations leading to a decreased level of IGFBP-5 may also contribute to breast cancer. In the current review, we focus on Pregnancy-Associated Plasma Protein A (PAPP-A), a protease involved in the degradation of IGFBP-5. PAPP-A is overexpressed in the majority of breast cancers but its role in cancer has only begun to be explored. More specifically, this review aims at highlighting the role of post-partum involution in the oncogenic function of PAPP-A. Notably, we summarize recent studies indicating that PAPP-A plays a role not only in the degradation of IGFBP-5 but also in the deposition of collagen and activation of the collagen receptor discoidin 2 (DDR2) during post-partum involution. Finally, considering the immunosuppressive microenvironment of post-partum involution, we also discuss the unexpected finding made in Ewing Sarcoma that PAPP-A plays a role in immune evasion. While the immunosuppressive role of PAPP-A in breast cancer remains to be determined, collectively these studies highlight the multifaced role of PAPP-A in cancer that extends well beyond its effect on IGF-signaling.

Mitohormesis Primes Tumor Invasion and Metastasis

Moderate mitochondrial stress can lead to persistent activation of cytoprotective mechanisms - a phenomenon termed mitohormesis. Here, we show that mitohormesis primes a subpopulation of cancer cells to basally upregulate mitochondrial stress responses, such as the mitochondrial unfolded protein response (UPR^mt) providing an adaptive metastatic advantage. In this subpopulation, UPR^mt activation persists in the absence of stress, resulting in reduced oxidative stress indicative of mitohormesis. Mechanistically, we showed that the SIRT3 axis of UPR^mt is necessary for invasion and metastasis. In breast cancer patients, a 7-gene UPR^mt signature demonstrated that UPR^mt-HIGH patients have significantly worse clinical outcomes, including metastasis. Transcriptomic analyses revealed that UPR^mt-HIGH patients have expression profiles characterized by metastatic programs and the cytoprotective outcomes of mitohormesis. While mitohormesis is associated with health and longevity in non-pathological settings, these results indicate that it is perniciously used by cancer cells to promote tumor progression.

Postpartum Involution and Cancer: An Opportunity for Targeted Breast Cancer Prevention and Treatments?

Childbirth at any age confers a transient increased risk for breast cancer in the first decade postpartum and this window of adverse effect extends over two decades in women with late-age first childbirth (>35 years of age). Crossover to the protective effect of pregnancy is dependent on age at first pregnancy, with young mothers receiving the most benefit. Furthermore, breast cancer diagnosis during the 5- to 10-year postpartum window associates with high risk for subsequent metastatic disease. Notably, lactation has been shown to be protective against breast cancer incidence overall, with varying degrees of protection by race, multiparity, and lifetime duration of lactation. An effect for lactation on breast cancer outcome after diagnosis has not been described. We discuss the most recent data and mechanistic insights underlying these epidemiologic findings. Postpartum involution of the breast has been identified as a key mediator of the increased risk for metastasis in women diagnosed within 5-10 years of a completed pregnancy. During breast involution, immune avoidance, increased lymphatic network, extracellular matrix remodeling, and increased seeding to the liver and lymph node work as interconnected pathways, leading to the adverse effect of a postpartum diagnosis. We al discuss a novel mechanism underlying the protective effect of breastfeeding. Collectively, these mechanistic insights offer potential therapeutic avenues for the prevention and/or improved treatment of postpartum breast cancer.

Proteasome mapping reveals sexual dimorphism in tissue-specific sensitivity to protein aggregations

Defects in the proteasome can result in pathological proteinopathies. However, the pathogenic role of sex‐ and tissue‐specific sensitivity to proteotoxic stress remains elusive. Here, we map the proteasome activity across nine tissues, in male and female mice, and demonstrate strong sexual dimorphism in proteasome activity, where females have significantly higher activity in several tissues. Further, we report drastic differences in proteasome activity among tissues, independently of proteasome concentration, which are exacerbated under stress conditions. Sexual dimorphism in proteasome activity is confirmed in a SOD1 ALS mouse model, in which the spinal cord, a tissue with comparatively low proteasome activity, is severely affected. Our results offer mechanistic insight into tissue‐specific sensitivities to proteostasis stress and into sex differences in the progression of neurodegenerative proteinopathies.

Collagen and PAPP-A in the Etiology of Postpartum Breast Cancer

Pregnancy has a dual effect on the risk of breast cancer. On one hand, pregnancy at a young age is known to be protective. However, pregnancy is also associated with a transient increased risk of breast cancer. For women that have children after the age of 30, the risk remains higher than women who never had children for decades. Involution of the breast has been identified as a window of mammary development associated with the adverse effect of pregnancy. In this review, we summarize the current understanding of the role of involution and describe the role of collagen in this setting. We also discuss the role of a collagen-dependent protease, pappalysin-1, in postpartum breast cancer and its role in activating both insulin-like growth factor signaling and discoidin domain collagen receptor 2, DDR2. Together, these novel advances in our understanding of postpartum breast cancer open the way to targeted therapies against this aggressive breast cancer sub-type.

Mitohormesis, UPRmt, and the Complexity of Mitochondrial DNA Landscapes in Cancer

The discovery of the Warburg effect, the preference of cancer cells to generate ATP via glycolysis rather than oxidative phosphorylation, has fostered the misconception that cancer cells become independent of the electron transport chain (ETC) for survival. This is inconsistent with the need of ETC function for the generation of pyrimidines. Along with this misconception, a large body of literature has reported numerous mutations in mitochondrial DNA (mtDNA), further fueling the notion of nonfunctional ETC in cancer cells. More recent findings, however, suggest that cancers maintain oxidative phosphorylation capacity and that the role of mtDNA mutations in cancer is likely far more nuanced in light of the remarkable complexity of mitochondrial genetics. This review aims at describing the various model systems that were developed to dissect the role of mtDNA in cancer, including cybrids, and more recently mitochondrial-nuclear exchange and conplastic mice. Furthermore, we put forward the notion of mtDNA landscapes, where the surrounding nonsynonymous mutations and variants can enhance or repress the biological effect of specific mtDNA mutations. Notably, we review recent studies describing the ability of some mtDNA landscapes to activate the mitochondrial unfolded protein response (UPR^mt) but not others. Furthermore, the role of the UPR^mt in maintaining cancer cells in the mitohormetic zone to provide selective adaptation to stress is discussed. Among the genes activated by the UPR^mt, we suggest that the dismutases SOD2 and SOD1 may play key roles in the establishment of the mitohormetic zone. Finally, we propose that using a UPR^mt nuclear gene expression signature may be a more reliable readout than mtDNA landscapes, given their diversity and complexity.

SOD1 is essential for oncogene-driven mammary tumor formation but dispensable for normal development and proliferation

We previously reported that the dismutase SOD1 is overexpressed in breast cancer. However, whether SOD1 plays an active role in tumor formation in vivo has never been demonstrated. Further, as luminal cells of normal breast epithelial cells are enriched in SOD1, whether SOD1 is essential for normal mammary gland development has never been determined. We initiated this study to investigate the role of SOD1 in mammary gland tumorigenesis as well as in normal mammary gland development. We crossed the inducible erbB2 (MMTV-iErbB2) and Wnt (MMTV-Wnt) transgenic mice to the SOD1 heterozygote or knockout mice. Our results show that SOD1 is essential for oncogene-driven proliferation, but not normal proliferation of the mammary gland associated with pregnancy or other normal proliferative tissues such as skin and intestines. We show that activation of the oncogene ErbB2 is associated with increased ROS and that high ROS sub-population of ErbB2 cancer cells show elevated SOD1. In the same cells, decrease in SOD1 is associated with an elevation in both apoptosis as well as oncogene-induced senescence. Based on these results, we suggest that SOD1 carries a housekeeping function that maintains ROS levels below a threshold that supports oncogene-dependent proliferation, while allowing escape from oncogene-induced senescence, independently of the oncogene driving tumor formation. These results identify SOD1 as an ideal target for cancer therapy as SOD1 inhibitors hold the potential to prevent the growth of cancers cells of diverse genotypes, activate multiple modes of cell death therefore making acquired resistance more difficult, while sparing normal tissues.

Parity predisposes breasts to the oncogenic action of PAPP-A and activation of the collagen receptor DDR2

Background

Women who had children at a young age (less than 25) show a reduced overall risk of breast cancer. However, epidemiological studies showed that for all other women, pregnancy increases the risk of breast cancer and the risk remains higher for decades. Further, even in women who had children at a young age, there is a transient increase risk that peaks 6 years after pregnancy. Women diagnosed with breast cancer following pregnancy show a higher rate of metastasis. Yet, the factors that increase the predisposition of post-partum breasts to more aggressive cancers remain unknown. Pregnancy-associated plasma protein A (PAPP-A) is a secreted protease that is overexpressed in more than 70% of breast cancers. However, PAPP-A is a collagen-dependent oncogene. We initiated this study to test the effect of PAPP-A on the predisposition of post-partum breasts.

Methods

We used PAPP-A mouse models for the analysis of its effect on virgin, involuting, or post-partum mammary glands. We performed second-harmonic generation microscopy for the analysis of collagen, defined tumor-associated collagen signature (TACS), the rate of mammary tumors, and the status of the collagen-DDR2-Snail axis of metastasis. We knockdown DDR2 by CRISPR and performed invasion assays. A transcriptomic approach was used to define a PAPP-A and parity-dependent genetic signature and assess its correlation with breast cancer recurrence in humans.

Results

We confirmed that post-partum mammary glands have a higher level of collagen than virgin glands and that this collagen is characterized by an anti-proliferative architecture. However, PAPP-A converts the anti-proliferative post-partum collagen into pro-tumorigenic collagen. We show that PAPP-A activates the collagen receptor DDR2 and metastasis. Further, deletion of DDR2 by CRISPR abolished the effect of PAPP-A on invasion. We defined a PAPP-A-driven genetic signature that identifies patients at higher risk of metastasis.

Conclusions

These results support the notion that information about pregnancy may be critical in the prognosis of breast cancer as passage through a single pregnancy predisposes to the oncogenic action of PAPP-A. Our data indicate that history of pregnancy combined with the expression of PAPP-A-driven genetic signature may be useful to identify patients at higher risk of metastatic disease.

Electronic supplementary material

The online version of this article (10.1186/s13058-019-1142-z) contains supplementary material, which is available to authorized users.

Cross talk between SOD1 and the mitochondrial UPR in cancer and neurodegeneration

The mitochondrial unfolded protein response (UPR^mt) is rapidly gaining attention. While the CHOP (ATF4/5) axis of the UPR^mt was the first to be described, other axes have subsequently been reported. Validation of this complex pathway in C. elegans has been extensively studied. However, validation of the UPR^mt in mouse models of disease known to implicate mitochondrial reprogramming or dysfunction, such as cancer and neurodegeneration, respectively, is only beginning to emerge. This review summarizes recent findings and highlights the major role of the superoxide dismutase SOD1 in the communication between the mitochondria and the nucleus in these settings. While SOD1 has mostly been studied in the context of familial amyotrophic lateral sclerosis (fALS), recent studies suggest that SOD1 may be a potentially important mediator of the UPR^mt and converge to emphasize an increasingly vital role of SOD1 as a therapeutic target in cancer.

Autophagy is a gatekeeper of hepatic differentiation and carcinogenesis by controlling the degradation of Yap

Activation of the Hippo pathway effector Yap underlies many liver cancers, however no germline or somatic mutations have been identified. Autophagy maintains essential metabolic functions of the liver, and autophagy-deficient murine models develop benign adenomas and hepatomegaly, which have been attributed to activation of the p62/Sqstm1-Nrf2 axis. Here, we show that Yap is an autophagy substrate and mediator of tissue remodeling and hepatocarcinogenesis independent of the p62/Sqstm1-Nrf2 axis. Hepatocyte-specific deletion of Atg7 promotes liver size, fibrosis, progenitor cell expansion, and hepatocarcinogenesis, which is rescued by concurrent deletion of Yap. Our results shed new light on mechanisms of Yap degradation and the sequence of events that follow disruption of autophagy, which is impaired in chronic liver disease.

Increased levels of the Yap oncoprotein stimulate liver growth and promote hepatocarcinogenesis. Here the authors show that hepatocyte-specific loss of Atg7 in mice leads to decreased autophagic degradation of Yap and liver overgrowth, and further establish this association in human liver cancer tissues.

Lactation opposes pappalysin-1-driven pregnancy-associated breast cancer

Pregnancy is associated with a transient increase in risk for breast cancer. However, the mechanism underlying pregnancy‐associated breast cancer (PABC) is poorly understood. Here, we identify the protease pappalysin‐1 (PAPP‐A) as a pregnancy‐dependent oncogene. Transgenic expression of PAPP‐A in the mouse mammary gland during pregnancy and involution promotes the deposition of collagen. We demonstrate that collagen facilitates the proteolysis of IGFBP‐4 and IGFBP‐5 by PAPP‐A, resulting in increased proliferative signaling during gestation and a delayed involution. However, while studying the effect of lactation, we found that although PAPP‐A transgenic mice lactating for an extended period of time do not develop mammary tumors, those that lactate for a short period develop mammary tumors characterized by a tumor‐associated collagen signature (TACS‐3). Mechanistically, we found that the protective effect of lactation is associated with the expression of inhibitors of PAPP‐A, STC1, and STC2. Collectively, these results identify PAPP‐A as a pregnancy‐dependent oncogene while also showing that extended lactation is protective against PAPP‐A‐mediated carcinogenesis. Our results offer the first mechanism that explains the link between breast cancer, pregnancy, and breastfeeding.

From discovery of the CHOP axis and targeting ClpP to the identification of additional axes of the UPRmt driven by the estrogen receptor and SIRT3

The mitochondrial UPR (UPR^mt) is rapidly gaining attention. While most studies on the UPR^mt have focused on its role in aging, emerging studies suggest an important role of the UPR^mt in cancer. Further, several of the players of the UPR^mt in mammalian cells have well reported roles in the maintenance of the organelle. The goal of this review is to emphasize aspects of the UPR^mt that have been overlooked in the current literature, describe the role of specific players of the UPR^mt in the biology of the mitochondria and highlight the intriguing possibility that targeting the UPR^mt in cancer may be already within reach.

The Mitochondrial Unfolded Protein Response as a Non-Oncogene Addiction to Support Adaptation to Stress during Transformation in Cancer and Beyond

Upon accumulation of misfolded proteins in the mitochondria, the mitochondrial unfolded protein response (UPR^mt) is activated. This review focuses on the role of this response in cancer. We discuss evidence that during transformation, the UPR^mt may play an essential role in the maintenance of the integrity of the mitochondria in the face of increased oxidative stress. However, the role of the UPR^mt in other diseases is also emerging and is therefore also briefly discussed.

Patient-derived Interstitial Fluids and Predisposition to Aggressive Sporadic Breast Cancer through Collagen Remodeling and Inactivation of p53

Purpose: Despite the fact that interstitial fluid (IF) represents a third of our body fluid, it is the most poorly understood body fluid in medicine. Increased IF pressure is thought to result from the increased deposition of extracellular matrix in the affected tissue preventing its reabsorption. In the cancer field, increased rigidity surrounding a cancerous mass remains the main reason that palpation and radiologic examination, such as mammography, are used for cancer detection. While the pressure produced by IF has been considered, the biochemical composition of IF has not been considered in its effect on tumors.Experimental Design: We classified 135 IF samples from bilateral mastectomy patients based on their ability to promote the invasion of breast cancer cells.Results: We observed a wide range of invasion scores. Patients with high-grade primary tumors at diagnosis had higher IF invasion scores. In mice, injections of high-score IF (IF^High) in a normal mammary gland promotes ductal hyperplasia, increased collagen deposition, and local invasion. In a mouse model of residual disease, IF^High increased disease progression and promoted aggressive visceral metastases. Mechanistically, we found that IF^High induces myofibroblast differentiation and collagen production through activation of CLIC4. IF^High also downregulates RYBP, leading to degradation of p53. Furthermore, in mammary glands of heterozygous p53-mutant knock-in mice, IF^High promotes spontaneous tumor formation.Conclusions: Our study indicates that IF can increase the deposition of extracellular matrix and raises the provocative possibility that they play an active role in the predisposition, development, and clinical course of sporadic breast cancers. Clin Cancer Res; 23(18); 5446-59. ©2017 AACR.

Sex specific activation of the ERα axis of the mitochondrial UPR (UPRmt) in the G93A-SOD1 mouse model of familial ALS

The mitochondrial unfolded protein response (UPRmt) is a transcriptional program aimed at restoring proteostasis in mitochondria. Upregulation of mitochondrial matrix proteases and heat shock proteins was initially described. Soon thereafter, a distinct UPRmt induced by misfolded proteins in the mitochondrial intermembrane space (IMS) and mediated by the estrogen receptor alpha (ERα), was found to upregulate the proteasome and the IMS protease OMI. However, the IMS-UPRmt was never studied in a neurodegenerative disease in vivo. Thus, we investigated the IMS-UPRmt in the G93A-SOD1 mouse model of familial ALS, since mutant SOD1 is known to accumulate in the IMS of neural tissue and cause mitochondrial dysfunction. As the ERα is most active in females, we postulated that a differential involvement of the IMS-UPRmt could be linked to the longer lifespan of females in the G93A-SOD1 mouse. We found a significant sex difference in the IMS-UPRmt, because the spinal cords of female, but not male, G93A-SOD1 mice showed elevation of OMI and proteasome activity. Then, using a mouse in which G93A-SOD1 was selectively targeted to the IMS, we demonstrated that the IMS-UPRmt could be specifically initiated by mutant SOD1 localized in the IMS. Furthermore, we showed that, in the absence of ERα, G93A-SOD1 failed to activate OMI and the proteasome, confirming the ERα dependence of the response. Taken together, these results demonstrate the IMS-UPRmt activation in SOD1 familial ALS, and suggest that sex differences in the disease phenotype could be linked to differential activation of the ERα axis of the IMS-UPRmt.

mtDNA, Metastasis, and the Mitochondrial Unfolded Protein Response (UPRmt)

While several studies have confirmed a link between mitochondrial DNA (mtDNA) mutations and cancer cell metastasis, much debate remains regarding the nature of the alternations in mtDNA leading to this effect. Meanwhile, the mitochondrial unfolded protein response (UPR^mt) has gained much attention in recent years, with most studies of this pathway focusing on its role in aging. However, the UPR^mt has also been studied in the context of cancer. More recent work suggests that rather than a single mutation or alternation, specific combinatorial mtDNA landscapes able to activate the UPR^mt may be those that are selected by metastatic cells, while mtDNA landscapes unable to activate the UPR^mt do not. This review aims at offering an overview of the confusing literature on mtDNA mutations and metastasis and the more recent work on the UPR^mt in this setting.

An acetyltransferase-independent function of Eso1 regulates centromere cohesion

Eukaryotes contain three essential Structural Maintenance of Chromosomes (SMC) complexes: cohesin, condensin, and Smc5/6. Cohesin forms a ring-shaped structure that embraces sister chromatids to promote their cohesion. The cohesiveness of cohesin is promoted by acetylation of N-terminal lysines of the Smc3 subunit by the acetyltransferases Eco1 in Saccharomyces cerevisiae and the homologue, Eso1, in Schizosaccharomyces pombe. In both yeasts, these acetyltransferases are essential for cell viability. However, whereas nonacetylatable Smc3 mutants are lethal in S. cerevisiae, they are not in S. pombe We show that the lethality of a temperature-sensitive allele of eso1 (eso1-H17) is due to activation of the spindle assembly checkpoint (SAC) and is associated with premature centromere separation. The lack of cohesion at the centromeres does not correlate with Psm3 acetylation or cohesin levels at the centromeres, but is associated ith significantly reduced recruitment of the cohesin regulator Pds5. The SAC activation in this context is dependent on Smc5/6 function, which is required to remove cohesin from chromosome arms but not centromeres. The mitotic defects caused by Smc5/6 and Eso1 dysfunction are cosuppressed in double mutants. This identifies a novel function (or functions) for Eso1 and Smc5/6 at centromeres and extends the functional relationships between these SMC complexes.

Randomized phase II trial of fulvestrant alone or in combination with bortezomib in hormone receptor-positive metastatic breast cancer resistant to aromatase inhibitors: a New York Cancer Consortium trial

The proteasome inhibitor bortezomib enhances the effect of the selective estrogen receptor (ER) downregulator (SERD) fulvestrant by causing accumulation of cytoplasmic ER aggregates in preclinical models. The purpose of this trial was to determine whether bortezomib enhanced the effectiveness of fulvestrant. One hundred eighteen postmenopausal women with ER-positive metastatic breast cancer resistant to aromatase inhibitors (AIs) were randomized to fulvestrant alone (Arm A-500 mg intramuscular (i.m.) day -14, 1, 15 in cycle 1, and day 1 of additional cycles) or in combination with bortezomib (Arm B-1.6 mg/m² intravenous (i.v.) on days 1, 8, 15 of each cycle). The study was powered to show an improvement in median progression-free survival (PFS) from 5.4 to 9.0 months and compare PFS rates at 6 and 12 months (α=0.10, β=0.10). Patients with progression on fulvestrant could cross over to the combination (arm C). Although there was no difference in median PFS (2.7 months in both arms), the hazard ratio for PFS in Arm B versus Arm A (referent) was 0.73 (95% confidence interval (CI)=0.49, 1.09, P=0.06, 1-sided log-rank test, significant at the prespecified 1-sided 0.10 α level). At 12 months, the PFS proportion in Arm A and Arm B was 13.6% and 28.1% (P=0.03, 1-sided χ²-test; 95% CI for difference (14.5%)=-0.06, 29.1%). Of 27 patients on arm A who crossed over to the combination (arm C), 5 (18%) were progression-free for at least 24 weeks. Bortezomib likely enhances the effectiveness of fulvestrant in AI-resistant, ER-positive metastatic breast cancer by reducing acquired resistance, supporting additional evaluation of proteasome inhibitors in combination with SERDs.

SOD2 and the Mitochondrial UPR: Partners Regulating Cellular Phenotypic Transitions

ATP and reactive oxygen species (ROS) are signaling molecules that control cellular function and phenotype. Mitochondria produce both ATP and ROS. Since the electrons needed to generate either ATP or ROS originate from NADH/FADH2, the mechanism through which electrons flow towards oxygen determines yields and whether ATP or ROS prevails. Alterations in the electron flow impact cells dramatically, such as by supporting specialization (which requires high ATP) or imposing dedifferentiation. High ROS, facilitated by enzymes such as superoxide dismutase 2 (SOD2) that enhance mitochondrial hydrogen peroxide (mtH2O2), are normally linked to dedifferentiation of somatic cells. Here we propose that combined high mtH2O2 and mitochondrial unfolded protein response (UPR(mt)) activation are essential for somatic dedifferentiation programs and the acquisition of stem-like properties in reparative processes and disease.

Sirtuins and the Estrogen Receptor as Regulators of the Mammalian Mitochondrial UPR in Cancer and Aging

By being both the source of ATP and the mediator of apoptosis, the mitochondria are key regulators of cellular life and death. Not surprisingly alterations in the biology of the mitochondria have implications in a wide array of diseases including cancer and age-related diseases such as neurodegeneration. To protect the mitochondria against damage the mitochondrial unfolded protein response (UPR(mt)) orchestrates several pathways, including the protein quality controls, the antioxidant machinery, oxidative phosphorylation, mitophagy, and mitochondrial biogenesis. While several reports have implicated an array of transcription factors in the UPR(mt), most of the focus has been on studies of Caenorhabditis elegans, which led to the identification of ATFS-1, for which the mammalian homolog remains unknown. Meanwhile, there are studies which link the UPR(mt) to sirtuins and transcription factors of the Foxo family in both C. elegans and mammalian cells but those have been largely overlooked. This review aims at emphasizing the potential importance of these studies by building on the large body of literature supporting the key role of the sirtuins in the maintenance of the integrity of the mitochondria in both cancer and aging. Further, the estrogen receptor alpha (ERα) and beta (ERβ) are known to confer protection against mitochondrial stress, and at least ERα has been linked to the UPR(mt). Considering the difference in gender longevity, this chapter also includes a discussion of the link between the ERα and ERβ and the mitochondria in cancer and aging.

Abstract P1-12-08: Bortezomib enhances the efficacy of fulvestrant by promoting the aggregation of the ER in the cytoplasm

Background: Aromatase inhibitors (AIs) are standard treatment for Estrogen Receptor (ER)+ breast cancers in post-menopausal women where the main source of estradiol comes from adipose tissue when the aromatase enzyme converts androgens to estrogens. However, in premenopausal women, functioning ovaries flood the body with estrogens, and aromatase inhibitors used alone offer no therapeutic benefit. In addition to tamoxifen and aromatase inhibitors, estrogen receptor down-regulators, are a third type of anti-estrogen. The first of this class to be FDA approved is Fulvestrant, which acts by promoting the proteosomal degradation of the ER. Like tamoxifen, fulvestrant binds directly to the ER but while tamoxifen has both antagonist and agonist effects on the ER, fulvestrant is a pure antagonist. Other important advantages of fulvestrant over tamoxifen are that 1) fulvestrant prevents the ER from binding DNA, 2) fulvestrant is not linked to increased risk of endometrial cancer, 3) fulvestrant promotes permanent degradation of the ER.

The molecular machinery leading to the degradation of the ER in the nucleus following fulvestrant treatment is well described and correlates with its ubiquitination in the nucleus. A less well-recognized mechanism, is fulvestrant’s ability to promote the aggregation of the newly synthesized ER in the cytoplasm. Understanding that protein aggregates are toxic when not eliminated by the proteasome, we took advantage of this effect of fulvestrant to ask whether combining fulvestrant with the proteasome inhibitor Bortezomib could enhance the efficacy of Fulvestrant.

Results: We found that bortezomib enhances the aggregation of the ER in the cytoplasm following treatment with fulvestrant. Further, these aggregates were found to be insoluble and to activate the unfolded protein response (UPR), a stress response that leads to cell death. Further, bortezomib is able to prevent the activation of cytoprotective responses linked to the acquisition of fulvestrant resistance. Furthermore, in a breast cancer mouse model of tamoxifen resistance, the combination induced tumor regression. We currently are testing new generation proteasome inhibitors and the results will be presented at the meeting.

Conclusion: We conclude that adding bortezomib to fulvestrant enhances its efficacy by taking advantage of a previously poorly recognized mechanism–fulvestrant’s induction of ER aggregation in the cytoplasm. Further, our data suggest that this strategy will block the ability of cells to acquired resistance to fulvestrant. Our group has developed a clinical trial that has tested this combination and the results of this trial presented at the meeting.

Citation Format: Doris Germain, Kerin Adelson, George Raptis, Samuel Waxman. Bortezomib enhances the efficacy of fulvestrant by promoting the aggregation of the ER in the cytoplasm [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P1-12-08.

Abstract S6-03: Randomized phase II trial of fulvestrant alone or in combination with bortezomib in hormone receptor-positive metastatic breast cancer resistant to aromatase inhibitors: A New York cancer consortium trial

Purpose: Fulvestrant (F) is a selective estrogen receptor downregulator (SERD) with activity in aromatase-inhibitor (AI) resistant estrogen receptor (ER)-positive metastatic breast cancer (MBC). In preclinical studies, the proteasome inhibitor bortezomib (B) enhances the antineoplastic effects of F, in part by promoting accumulation of large ER-aggregates that lead to cell death (Ishii et al. Clin Cancer Res 2011 17:2292). The objective of this study was to determine if the combination of F+B was more efficacious than F alone in MBC after AI progression.

Patients and Methods: Postmenopausal women with ER-positive MBC who had progressive disease after prior AI therapy were eligible. They were randomized to F alone (500 mg IM days -15, 1, 15 in cycle 1, and day 1 of each subsequent cycle) or in combination with B (1.6 mg/m2 IV on days 1, 8, 15). The primary endpoint was progression free survival (PFS), measured from cycle 1, day 1 of starting F. A sample size of 118 was pre-specified in order to provide sufficient power to detect an improvement in median PFS from 5.4 to 9.0 months, and compare PFS rates after 6 and 12 months (1-sided alpha=0.10, beta=0.10). Patients with progression on F could cross over to the F+B combination.

Results: Of 118 patients enrolled, 59 received F alone (arm A), 57 received F+B (arm B), and 2 assigned to arm B never initiated protocol therapy. There were no significant differences in patient characteristics between arms with regard to median age (57 vs. 59 years), ECOG performance status (0 and 1, 64% and 36%, respectively), prior chemotherapy for metastasis (25%), or liver metastases (37%), although patients in arm A had longer median interval between diagnosis and metastasis (49 vs. 28 months) and were more likely to present with metastasis (32% vs. 26%). Patients in arm B had more adverse events (all grades), including nausea (63% vs. 29%), diarrhea (47% vs. 8%), sensory neuropathy (46% vs. 29%), and limb edema (37% vs. 19%), although grade 3-4 events were uncommon, and only 11% discontinued B due to toxicity. At 12 months, the PFS proportion in Arm A and Arm B was 13.6% vs. 28.1%, respectively (P=0.03, 1-sided chi-square test) (95% CI for difference [14.5%] = -0.06%, 29.1%). Although median PFS was similar in the two arms (2.69 vs. 2.73 months, respectively), the hazard ratio for Arm B vs. Arm A (referent) was 0.73 (95% CI = 0.49, 1.09, P=0.06, 1-sided log rank test). Both results were significant at the pre-specified 1-sided 0.10 alpha level. Of 27 patients on arm A who crossed over to F+B at progression, 4 (15%) were progression-free for at least 24 weeks and had periods of disease control that were longer than when treated with F alone.

Conclusion: Adding bortezomib to fulvestrant in AI-resistant ER-positive MBC enhances its effectiveness by delaying acquired fulvestrant resistance. These results support additional evaluation of proteasome inhibitors in combination with SERDs.

Acknowledgement: Supported by contract N01-CM-62204 to the New York Cancer Consortium (P.I. J. Sparano) and grant P30 CA013330 (P.I. D. Goldman) from the National Institutes of Health, and by a grant from Millennium, Inc.

Citation Format: Kerin B Adelson, Bhuvaneswari Ramaswamy, Joseph A Sparano, Paul J Christos, John J Wright, George Raptis, Miguel C Villalona, Cynthia X Ma, Dawn Hershman, Joseph Baar, Paula Klein, Tessa Cigler, G Thomas Budd, Yelena Novik, Antoinette R Tan, Susan Tannenbaum, Anupama Goel, Ellis Levine, Charles L Shapiro, Eleni Andreopoulou, Michael Naughton, Kevin Kalinsky, Samuel Waxman, Doris Germain. Randomized phase II trial of fulvestrant alone or in combination with bortezomib in hormone receptor-positive metastatic breast cancer resistant to aromatase inhibitors: A New York cancer consortium trial [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr S6-03.

Toca-1 is suppressed by p53 to limit breast cancer cell invasion and tumor metastasis

Introduction

Transducer of Cdc42-dependent actin assembly-1 (Toca-1) recruits actin regulatory proteins to invadopodia, and promotes breast tumor metastasis. Since metastatic breast tumors frequently harbor mutations in the tumor suppressor p53, we tested whether p53 regulates Toca-1 expression.

Methods

Normal mammary epithelial cells (HBL-100, MCF10A) and breast cancer cell lines expressing wild-type (WT) p53 (DU4475, MTLn3) were treated with camptothecin or Nutlin-3 to stabilize p53 to test effects on Toca-1 mRNA and protein levels. Chromatin immunoprecipitation (ChIP) assays were performed to identify p53 binding site in Toca-1 gene. Stable silencing of p53 and Toca-1 were performed in MTLn3 cells to test effects on invadopodia and cell invasion in vitro, and tumor metastasis in vivo.

Results

We observed that breast cancer cell lines with mutant p53 have high levels of Toca-1 compared to those with WT p53. Stabilization of WT p53 led to further reduction in Toca-1 mRNA and protein levels in normal breast epithelial cells and breast cancer cells. ChIP assays revealed p53 binding within intron 2 of toca1, and reduced histone acetylation within its promoter region upon p53 upregulation or activation. Stable silencing of WT p53 in MTLn3 cells led to increased extracellular matrix degradation and cell invasion compared to control cells. Interestingly, the combined silencing of p53 and Toca-1 led to a partial rescue of these effects of p53 silencing in vitro and reduced lung metastases in mice. In human breast tumors, Toca-1 levels were high in subtypes with frequent p53 mutations, and high Toca-1 transcript levels correlated with increased risk of relapse.

Conclusions

Based on these findings, we conclude that loss of p53 tumor suppressor function in breast cancers leads to upregulation of Toca-1, and results in enhanced risk of developing metastatic disease.

Electronic supplementary material

The online version of this article (doi:10.1186/s13058-014-0503-x) contains supplementary material, which is available to authorized users.

SOD1, an unexpected novel target for cancer therapy

Cancer cells have elevated levels of reactive oxygen species (ROS), which are generated in majority by the mitochondria. In the mitochondrial matrix, the manganese dismutase SOD2 acts as a major anti-oxidant enzyme. The deacetylase SIRT3 regulates the activity of SOD2. Recently, SIRT3 was reported to be decreased in 87% of breast cancers, resulting therefore in a decrease in the activity of SOD2 and an elevation in ROS. In addition to SIRT3, we recently reported that SOD2 itself is down-regulated in breast cancer cell lines upon activation of oncogenes, such as Ras. Since in absence of SOD2, superoxide levels are elevated and may cause irreversible damage, mechanisms must exist to retain superoxide below a critical threshold and maintain viability of cancer cells. The copper/zinc dismutase SOD1 localizes in the cytoplasm, the inter-membrane space of the mitochondria and the nucleus. Emerging evidences from several groups now indicate that SOD1 is overexpressed in cancers and that the activity of SOD1 may be essential to maintain cellular ROS under this critical threshold. This review summarizes the studies reporting important roles of SOD1 in cancer and addresses the potential cross-talk between the overexpression of SOD1 and the regulation of the mitochondrial unfolded protein response (UPR(mt)). While mutations in SOD1 is the cause of 20% of cases of familial amyotrophic lateral sclerosis (fALS), a devastating neurodegenerative disease, these new studies expand the role of SOD1 to cancer.

Inactivation of Omi/HtrA2 protease leads to the deregulation of mitochondrial Mulan E3 ubiquitin ligase and increased mitophagy

Omi/HtrA2 is a nuclear encoded mitochondrial serine protease with dual and opposite functions that depend entirely on its subcellular localization. During apoptosis, Omi/HtrA2 is released into the cytoplasm where it participates in cell death. While confined in the inter-membrane space of the mitochondria, Omi/HtrA2 has a pro-survival function that may involve the regulation of protein quality control (PQC) and mitochondrial homeostasis. Loss of Omi/HtrA2's protease activity causes the neuromuscular disorder of the mnd2 (motor neuron degeneration 2) mutant mice. These mice develop multiple defects including neurodegeneration with parkinsonian features. Loss of Omi/HtrA2 in non-neuronal tissues has also been shown to cause premature aging. The normal function of Omi/HtrA2 in the mitochondria and how its deregulation causes neurodegeneration or premature aging are unknown. Here we report that the mitochondrial Mulan E3 ubiquitin ligase is a specific substrate of Omi/HtrA2. During exposure to H(2)O(2), Omi/HtrA2 degrades Mulan, and this regulation is lost in cells that carry the inactive protease. Furthermore, we show accumulation of Mulan protein in various tissues of mnd2 mice as well as in Omi/HtrA2(-/-) mouse embryonic fibroblasts (MEFs). This causes a significant decrease of mitofusin 2 (Mfn2) protein, and increased mitophagy. Our work describes a new stress-signaling pathway that is initiated in the mitochondria and involves the regulation of Mulan by Omi/HtrA2 protease. Deregulation of this pathway, as it occurs in mnd2 mutant mice, causes mitochondrial dysfunction and mitophagy, and could be responsible for the motor neuron disease and the premature aging phenotype observed in these animals.

SOD2 to SOD1 switch in breast cancer

Cancer cells are characterized by elevated levels of reactive oxygen species, which are produced mainly by the mitochondria. The dismutase SOD2 localizes in the matrix and is a major antioxidant. The activity of SOD2 is regulated by the deacetylase SIRT3. Recent studies indicated that SIRT3 is decreased in 87% of breast cancers, implying that the activity of SOD2 is compromised. The resulting elevation in reactive oxygen species was shown to be essential for the metabolic reprograming toward glycolysis. Here, we show that SOD2 itself is down-regulated in breast cancer cell lines. Further, activation of oncogenes, such as Ras, promotes the rapid down-regulation of SOD2. Because in the absence of SOD2, superoxide levels are elevated in the matrix, we reasoned that mechanisms must exist to retain low levels of superoxide in other cellular compartments especially in the intermembrane space of the mitochondrial to avoid irreversible damage. The dismutase SOD1 also acts as an antioxidant, but it localizes to the cytoplasm and the intermembrane space of the mitochondria. We report here that loss of SOD2 correlates with the overexpression of SOD1. Further, we show that mitochondrial SOD1 is the main dismutase activity in breast cancer cells but not in non-transformed cells. In addition, we show that the SOD1 inhibitor LCS-1 leads to a drastic fragmentation and swelling of the matrix, suggesting that in the absence of SOD2, SOD1 is required to maintain the integrity of the organelle. We propose that by analogy to the cadherin switch during epithelial-mesenchymal transition, cancer cells also undergo a SOD switch during transformation.

KLF6-SV1 drives breast cancer metastasis and is associated with poor survival

Metastasis is the major cause of cancer mortality. A more thorough understanding of the mechanisms driving this complex multistep process will aid in the identification and characterization of therapeutically targetable genetic drivers of disease progression. We demonstrate that KLF6-SV1, an oncogenic splice variant of the KLF6 tumor suppressor gene, is associated with increased metastatic potential and poor survival in a cohort of 671 lymph node-negative breast cancer patients. KLF6-SV1 overexpression in mammary epithelial cell lines resulted in an epithelial-to-mesenchymal-like transition and drove aggressive multiorgan metastatic disease in multiple in vivo models. Additionally, KLF6-SV1 loss-of-function studies demonstrated reversion to an epithelial and less invasive phenotype. Combined, these findings implicate KLF6-SV1 as a key driver of breast cancer metastasis that distinguishes between indolent and lethal early-stage disease and provides a potential therapeutic target for invasive breast cancer.

The inner ear of Megatherium and the evolution of the vestibular system in sloths

Extant tree sloths are uniquely slow mammals with a very specialized suspensory behavior. To improve our understanding of their peculiar evolution, we investigated the inner ear morphology of one of the largest and most popular fossil ground sloths, Megatherium americanum. We first address the predicted agility of this animal from the scaling of its semicircular canals (SC) relative to body mass, based on recent work that provided evidence that the size of the SC in mammals correlates with body mass and levels of agility. Our analyses predict intermediate levels of agility for Megatherium, contrasting with the extreme slowness of extant sloths. Secondly, we focus on the morphology of the SC at the inner ear scale and investigate the shape and proportions of these structures in Megatherium and in a large diversity of extant xenarthrans represented in our database. Our morphometric analyses demonstrate that the giant ground sloth clearly departs from the SC morphology of both extant sloth genera (Choloepus, Bradypus) and is in some aspects closer to that of armadillos and anteaters. Given the close phylogenetic relationships of Megatherium with the extant genus Choloepus, these results are evidence of substantial homoplasy of the SC anatomy in sloths. This homoplasy most likely corresponds to an outstanding convergent evolution between extant suspensory sloth genera.

Abstract 787: Tumor suppressor p53 inhibits expression of the pro-metastasis protein Toca-1

Transducer of Cdc42-dependent actin assembly-1 (Toca-1) is an adaptor protein that recruits key actin regulatory proteins to invadopodia in metastatic breast cancer cells. We recently showed that stable silencing of Toca-1 led to reduced invadopodia formation, cell invasion and tumor metastasis in an orthotopic xenograft model in mice. These results raise the possibility that Toca-1 upregulation may occur during tumor progression to metastatic disease. Here we show that Toca-1 expression is higher in triple negative breast cancers (HER2/ER/PR) compared to other subtypes. Since triple negative breast cancers frequently harbor mutations in the tumor suppressor p53, we tested whether p53 regulates Toca-1 expression. In wild-type (WT) p53-expressing normal breast epithelial and breast cancer cells, stabilization of WT p53 by inducing DNA damage, or treating cells with Mdm2 inhibitor Nutlin-3, led to dose-dependent reductions in Toca-1 mRNA and protein expression. Likewise, ectopic expression of WT p53 in p53-null HCC1806 cells also led to repression of Toca-1 levels. To test if this was a direct effect of p53 on the Toca-1 gene, chromatin immunoprecipitation (ChIP) studies were performed. Indeed, the proximal promoter region and intron 2 of the Toca-1 gene (which contained a predicted p53 binding site) were detected in p53 ChIP assays in WT p53-expressing cells, but not mutant p53 expressing breast cancer cells. Together, these results identify a novel pathway to inhibit Toca-1 expression in cells with WT p53, and imply that the loss of p53 in highly metastatic breast cancer subtypes may lead to elevated Toca-1 expression and progression to metastatic disease.

Citation Format: Harish Chander, Colin D. Brien, Peter Truesdell, Doris Germain, Andrew W. B. Craig. Tumor suppressor p53 inhibits expression of the pro-metastasis protein Toca-1. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 787. doi:10.1158/1538-7445.AM2013-787

Abstract 1121: ER-negative breast cancer survive mitochondrial IMS-stress via SIRT3 activation

Mitochondrial dysfunction is central in carcinogenesis and has been associated with elevated levels of Reactive Oxygen Species (ROS). While moderate levels of ROS activate signaling cascade promoting tumor progression, high levels of ROS can lead to cell death. We previously demonstrated that Estrogen Receptor (ER)-positive breast cancer cells activate ER to overcome the mitochondrial impairment and reduce high levels of ROS induced by stress in the inter-mitochondrial membrane space (IMS). The ultimate outcome of ER activation is induction of a novel Unfolded Protein Response (UPR) that involves up-regulation of NRF-1, a major regulator of mitochondrial biogenesis as well as induction of OMI and proteasome, two critical elements of protein quality control required to monitor the quality of IMS proteins. In the present work, we have evidence that ER-negative breast cancer cells trigger a distinct mechanism to cope up with IMS-stress. Due to the lack of ER, the stress from the IMS is extended into the matrix of mitochondria leading to activation of the matrix deacetylase, SIRT3. SIRT3 is required to increase the antioxidant capacity by up-regulating the antioxidant enzyme, MnSOD and therefore reducing ROS. In addition, our results reveal that SIRT3 is essential for removal of the irreversible impaired mitochondria through mitophagy. Overall, our data indicate that ER-positive and ER-negative breast cancer induce different mechanism to conquer mitochondrial malfunction coupled with ROS overproduction and maintain their cellular integrity.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1121. doi:1538-7445.AM2012-1121

Abstract 5314: An oncogenic splice variant of the KLF6 tumor suppressor gene is associated with poor survival and is a potent driver of breast cancer metastasis

Breast cancer is a leading causes of cancer death in women worldwide. It is a heterogeneous and genetically complex disease has left a significant proportion of patients with inadequate treatment options. Since metastasis and drug resistance pose significant challengea in breast cancer treatment, elucidating the mechanisms of these processes is critical for understanding the key drivers of disease progression and for the development of targeted therapies. Accumulating evidence from our laboratory and other groups suggest that the tumor suppressor gene, KLF6, and its oncogenic splice variant, KLF6-SV1, play a role in breast cancer progression, dissemination, and chemoresistance. Here we demonstrate that in multiple independent clinical cohorts of over 1200 breast cancer patients with defined clinical outcome, high KLF6-SV1 mRNA levels in the primary tumor associated with poor survival and disease recurrence. Specifically, upregulated KLF6-SV1 expression in the primary tumors correlated with poor survival independent of disease stage and grade. Thus, we hypothesized that KLF6-SV1 is an early driver/molecular determinant of invasive breast cancer. In order to investigate the functional/biological relevance of KLF6-SV1 in breast cancer development and progression, we performed a series of experiments using retroviral-based overexpression in multiple non-tumorigenic, tumorigenic, and metastatic breast cancer cell lines. We demonstrated that overexpression of KLF6-SV1 increased mesenchymal marker gene expression, cellular survival, invasion, as well as the migratory potential of KLF6-SV1 transduced cells. Interestingly, KLF6-SV1 did not increase growth rate of these mammary epithelial cell lines, similar to the clinical data which showed an absence of a correlation between KLF6-SV1 expression and primary tumor size. In a 3D model KLF6-SV1 upregulation disrupted mammary acinar morphogenesis promoting complex multiacinar structures. To assay whether the KLF6-SV1-induced EMT phenotype conferred increased metastatic potential in vivo, we injected tumorigenic cells expressing high levels of KLF6-SV1 subcutaneously into immunodeficient mice. Strikingly, KLF6-SV1 overexpression alone drove the entire metastatic cascade resulting in dissemination to many organs including the liver, kidney, heart, lung, and spleen. Furthermore, KLF6-SV1 overexpression in a metastatic breast cancer cell line increased metastasis to the lungs and liver in an orthotopic model of the disease. This is consistent with our clinical data in which high KLF6-SV1 expression was correlated with decreased overall survival and metastasis free survival. Together these findings suggest a role for the KLF6-SV1 splice variant as a driver of breast cancer metastasis and validate its potential utility as a novel biomarker and therapeutic target for breast cancer.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 5314. doi:1538-7445.AM2012-5314