Luminal Rank loss decreases cell fitness leading to basal cell bipotency in parous mammary glands

Rank signaling pathway regulates mammary gland homeostasis and epithelial cell differentiation. Although Rank receptor is expressed by basal cells and luminal progenitors, its role in each individual cell lineage remains unclear. By combining temporal/lineage specific Rank genetic deletion with lineage tracing techniques, we found that loss of luminal Rank reduces the luminal progenitor pool and leads to aberrant alveolar-like differentiation with high protein translation capacity in virgin mammary glands. These Rank-deleted luminal cells are unable to expand during the first pregnancy, leading to lactation failure and impairment of protein synthesis potential in the parous stage. The unfit parous Rank-deleted luminal cells in the alveoli are progressively replaced by Rank-proficient cells early during the second pregnancy, thereby restoring lactation. Transcriptomic analysis and functional assays point to the awakening of basal bipotency after pregnancy by the induction of Rank/NF-κB signaling in basal parous cell to restore lactation and tissue homeostasis.

Rank ectopic expression in the presence of Neu and MMTV oncogenes alters mammary epithelial cell populations and their tumorigenic potential

Determination of the mammary epithelial cell that serves as the cell of origin for breast cancer is key to understand tumor heterogeneity and clinical management. In this study, we aimed to decipher whether Rank expression in the presence of PyMT and Neu oncogenes might affect the cell of origin of mammary gland tumors. We observed that Rank expression in PyMT^+/- and Neu^+/- mammary glands alters the basal and luminal mammary cell populations already in preneoplasic tissue, which may interfere with the tumor cell of origin restricting their tumorigenesis ability upon transplantation assays. In spite of this, Rank expression eventually promotes tumor aggressiveness once tumorigenesis is established.

Inhibition of RANK signaling as a potential immunotherapy in breast cancer

RANK signaling in mouse mammary tumor cells exerts an immunosuppressive environment by promoting the infiltration of pro-tumorigenic neutrophils and preventing CD8 T cell recruitment. Single-agent denosumab led to an increased tumor immune infiltration by lymphocytes and CD8 + T cells in breast cancer patients, supporting the immunomodulatory role of RANK signaling.

Inhibition of RANK signaling in breast cancer induces an anti-tumor immune response orchestrated by CD8+ T cells

Most breast cancers exhibit low immune infiltration and are unresponsive to immunotherapy. We hypothesized that inhibition of the receptor activator of nuclear factor-κB (RANK) signaling pathway may enhance immune activation. Here we report that loss of RANK signaling in mouse tumor cells increases leukocytes, lymphocytes, and CD8⁺ T cells, and reduces macrophage and neutrophil infiltration. CD8⁺ T cells mediate the attenuated tumor phenotype observed upon RANK loss, whereas neutrophils, supported by RANK-expressing tumor cells, induce immunosuppression. RANKL inhibition increases the anti-tumor effect of immunotherapies in breast cancer through a tumor cell mediated effect. Comparably, pre-operative single-agent denosumab in premenopausal early-stage breast cancer patients from the Phase-II D-BEYOND clinical trial (NCT01864798) is well tolerated, inhibits RANK pathway and increases tumor infiltrating lymphocytes and CD8⁺ T cells. Higher RANK signaling activation in tumors and serum RANKL levels at baseline predict these immune-modulatory effects. No changes in tumor cell proliferation (primary endpoint) or other secondary endpoints are observed. Overall, our preclinical and clinical findings reveal that tumor cells exploit RANK pathway as a mechanism to evade immune surveillance and support the use of RANK pathway inhibitors to prime luminal breast cancer for immunotherapy.

Receptor activator of nuclear factor-κB (RANK)/RANK-ligand (RANKL) signaling regulates the tumor-immune crosstalk. Here the authors show that systemic RANKL inhibition promotes CD8 + T cell infiltration in patients with early breast cancer and that loss of RANK signaling in tumor cells drives a T cell-dependent anti-tumor response in preclinical models.

FGFR Inhibition Overcomes Resistance to EGFR-targeted Therapy in Epithelial-like Cutaneous Carcinoma

PURPOSE

Recurrent and/or metastatic unresectable cutaneous squamous cell carcinomas (cSCCs) are treated with chemotherapy or radiotherapy, but have poor clinical responses. A limited response (up to 45% of cases) to EGFR-targeted therapies was observed in clinical trials with patients with advanced and metastatic cSCC. Here, we analyze the molecular traits underlying the response to EGFR inhibitors, and the mechanisms responsible for cSCC resistance to EGFR-targeted therapy.

EXPERIMENTAL DESIGN

We generated primary cell cultures and patient cSCC-derived xenografts (cSCC-PDXs) that recapitulate the histopathologic and molecular features of patient tumors. Response to gefitinib treatment was tested and gefitinib-resistant (GefR) cSCC-PDXs were developed. RNA sequence analysis was performed in matched untreated and GefR cSCC-PDXs to determine the mechanisms driving gefitinib resistance.

RESULTS

cSCCs conserving epithelial traits exhibited strong activation of EGFR signaling, which promoted tumor cell proliferation, in contrast to mesenchymal-like cSCCs. Gefitinib treatment strongly blocked epithelial-like cSCC-PDX growth in the absence of EGFR and RAS mutations, whereas tumors carrying the E545K PIK3CA-activating mutation were resistant to treatment. A subset of initially responding tumors acquired resistance after long-term treatment, which was induced by the bypass from EGFR to FGFR signaling to allow tumor cell proliferation and survival upon gefitinib treatment. Pharmacologic inhibition of FGFR signaling overcame resistance to EGFR inhibitor, even in PIK3CA-mutated tumors.

CONCLUSIONS

EGFR-targeted therapy may be appropriate for treating many epithelial-like cSCCs without PIK3CA-activating mutations. Combined EGFR- and FGFR-targeted therapy may be used to treat cSCCs that show intrinsic or acquired resistance to EGFR inhibitors.

Chromosome 12p Amplification in Triple-Negative/BRCA1-Mutated Breast Cancer Associates with Emergence of Docetaxel Resistance and Carboplatin Sensitivity

Taxanes are the mainstay of treatment in triple-negative breast cancer (TNBC), with de novo and acquired resistance limiting patient's survival. To investigate the genetic basis of docetaxel resistance in TNBC, exome sequencing was performed on matched TNBC patient-derived xenografts (PDX) sensitive to docetaxel and their counterparts that developed resistance in vivo upon continuous drug exposure. Most mutations, small insertions/deletions, and copy number alterations detected in the initial TNBC human metastatic samples were maintained after serial passages in mice and emergence of resistance. We identified a chromosomal amplification of chr12p in a human BRCA1-mutated metastatic sample and the derived chemoresistant PDX, but not in the matched docetaxel-sensitive PDX tumor. Chr12p amplification was validated in a second pair of docetaxel-sensitive/resistant BRCA1-mutated PDXs and after short-term docetaxel treatment in several TNBC/BRCA1-mutated PDXs and cell lines, as well as during metastatic recurrence in a patient with BRCA1-mutated breast cancer who had progressed on docetaxel treatment. Analysis of clinical data indicates an association between chr12p amplification and patients with TNBC/basal-like breast cancer, a BRCA1 mutational signature, and poor survival after chemotherapy. Detection of chr12p amplification in a cohort of TNBC PDX models was associated with an improved response to carboplatin. Our findings reveal tumor clonal dynamics during chemotherapy treatments and suggest that a preexisting population harboring chr12p amplification is associated with the emergence of docetaxel resistance and carboplatin responsiveness in TNBC/BRCA1-mutated tumors. SIGNIFICANCE: Chr12p copy number gains indicate rapid emergence of resistance to docetaxel and increased sensitivity to carboplatin, therefore sequential docetaxel/carboplatin treatment could improve survival in TNBC/BRCA1 patients. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/79/16/4258/F1.large.jpg.

The Altered Transcriptome and DNA Methylation Profiles of Docetaxel Resistance in Breast Cancer PDX Models

Taxanes are standard therapy in clinical practice for metastatic breast cancer; however, primary or acquired chemoresistance are a common cause of mortality. Breast cancer patient-derived xenografts (PDX) are powerful tools for the study of cancer biology and drug treatment response. Specific DNA methylation patterns have been associated to different breast cancer subtypes but its association with chemoresistance remains unstudied. Aiming to elucidate docetaxel resistance mechanisms, we performed genome-wide DNA methylation in breast cancer PDX models, including luminal and triple-negative breast cancer (TNBC) models sensitive to docetaxel, their matched models after emergence of chemoresistance and residual disease after short-term docetaxel treatment. We found that DNA methylation profiles from breast cancer PDX models maintain the subtype-specific methylation patterns of clinical samples. Two main DNA methylation clusters were found in TNBC PDX and remain stable during the emergence of docetaxel resistance; however, some genes/pathways were differentially methylated according to docetaxel response. A DNA methylation signature of resistance able to segregate TNBC based on chemotherapy response was identified. Transcriptomic profiling of selected sensitive/resistant pairs and integrative analysis with methylation data demonstrated correlation between some differentially methylated and expressed genes in docetaxel-resistant TNBC PDX models. Multiple gene expression changes were found after the emergence of docetaxel resistance in TNBC. DNA methylation and transcriptional changes identified between docetaxel-sensitive and -resistant TNBC PDX models or residual disease may have predictive value for chemotherapy response in TNBC. IMPLICATIONS: Subtype-specific DNA methylation patterns are maintained in breast cancer PDX models. While no global methylation changes were found, we uncovered differentially DNA methylated and expressed genes/pathways associated with the emergence of docetaxel resistance in TNBC.

Targeting p38α Increases DNA Damage, Chromosome Instability, and the Anti-tumoral Response to Taxanes in Breast Cancer Cells

Breast cancer is the second leading cause of cancer-related death among women. Here we report a role for the protein kinase p38α in coordinating the DNA damage response and limiting chromosome instability during breast tumor progression, and identify the DNA repair regulator CtIP as a p38α substrate. Accordingly, decreased p38α signaling results in impaired ATR activation and homologous recombination repair, with concomitant increases in replication stress, DNA damage, and chromosome instability, leading to cancer cell death and tumor regression. Moreover, we show that pharmacological inhibition of p38α potentiates the effects of taxanes by boosting chromosome instability in murine models and patient-derived xenografts, suggesting the potential interest of combining p38α inhibitors with chemotherapeutic drugs that induce chromosome instability.

Tumor-initiating CD49f cells are a hallmark of chemoresistant triple negative breast cancer

Taxanes are mainstay treatment of triple negative breast cancer (TNBC) patients but resistance often develops. Using TNBC patient-derived orthoxenografts (PDX) we have recently discovered that a CD49f+ chemoresistant population with tumor-initiating ability is present in sensitive tumors and expands in tumors that have acquired resistance. Importantly, sensitivity to taxanes is recovered after long-term drug interruption. The characterization of this chemoresistant CD49f+ cells provides a unique opportunity to identify novel targets for the treatment of chemoresistant TNBC.

Stem cell-like transcriptional reprogramming mediates metastatic resistance to mTOR inhibition

Inhibitors of the mechanistic target of rapamycin (mTOR) are currently used to treat advanced metastatic breast cancer. However, whether an aggressive phenotype is sustained through adaptation or resistance to mTOR inhibition remains unknown. Here, complementary studies in human tumors, cancer models and cell lines reveal transcriptional reprogramming that supports metastasis in response to mTOR inhibition. This cancer feature is driven by EVI1 and SOX9. EVI1 functionally cooperates with and positively regulates SOX9, and promotes the transcriptional upregulation of key mTOR pathway components (REHB and RAPTOR) and of lung metastasis mediators (FSCN1 and SPARC). The expression of EVI1 and SOX9 is associated with stem cell-like and metastasis signatures, and their depletion impairs the metastatic potential of breast cancer cells. These results establish the mechanistic link between resistance to mTOR inhibition and cancer metastatic potential, thus enhancing our understanding of mTOR targeting failure.

RANK Signaling Blockade Reduces Breast Cancer Recurrence by Inducing Tumor Cell Differentiation

RANK expression is associated with poor prognosis in breast cancer even though its therapeutic potential remains unknown. RANKL and its receptor RANK are downstream effectors of the progesterone signaling pathway. However, RANK expression is enriched in hormone receptor negative adenocarcinomas, suggesting additional roles for RANK signaling beyond its hormone-dependent function. Here, to explore the role of RANK signaling once tumors have developed, we use the mouse mammary tumor virus-Polyoma Middle T (MMTV-PyMT), which mimics RANK and RANKL expression patterns seen in human breast adenocarcinomas. Complementary genetic and pharmacologic approaches demonstrate that therapeutic inhibition of RANK signaling drastically reduces the cancer stem cell pool, decreases tumor and metastasis initiation, and enhances sensitivity to chemotherapy. Mechanistically, genome-wide expression analyses show that anti-RANKL therapy promotes lactogenic differentiation of tumor cells. Moreover, RANK signaling in tumor cells negatively regulates the expression of Ap2 transcription factors, and enhances the Wnt agonist Rspo1 and the Sca1-population, enriched in tumor-initiating cells. In addition, we found that expression of TFAP2B and the RANK inhibitor, OPG, in human breast cancer correlate and are associated with relapse-free tumors. These results support the use of RANKL inhibitors to reduce recurrence and metastasis in breast cancer patients based on its ability to induce tumor cell differentiation. Cancer Res; 76(19); 5857-69. ©2016 AACR.

RANK as a therapeutic target in cancer

The RANK signaling pathway has emerged as a new target in breast cancer as receptor activator of nuclear factor κB ligand (RANKL) and its receptor RANK mediate the pro-tumorigenic role of progesterone in the mammary gland. Thousands of cancer patients worldwide are already taking RANKL inhibitors for the management of bone metastasis, given the relevance of this pathway in osteoclastogenesis and bone resorption. RANK signaling also has multiple divergent effects in immunity and inflammation, both in the generation of active immune responses and in the induction of tolerance: it is required for lymph node organogenesis, thymic medullary epithelial development and self-tolerance, and regulates activation of several immune cells and inflammatory processes. The RANK pathway interferes with mammary epithelial differentiation and mediates the major proliferative response of mammary epithelium to progesterone and progesterone-driven expansion of mammary stem cells; it also controls hair follicle and epidermal stem cell homeostasis, pointing to RANK as a key regulator of epithelial stemness. Here we revisit the main functions of RANK signaling in bone remodeling, immune cells and epithelial differentiation. We also discuss the mechanistic evidence that supports its pleiotropic effects on cancer: from bone metastasis to immune and cancer-cell-dependent effects.

Rankl Impairs Lactogenic Differentiation Through Inhibition of the Prolactin/Stat5 Pathway at Midgestation

Prolactin and progesterone both orchestrate the proliferation and differentiation of the mammary gland during gestation. Differentiation of milk secreting alveoli depends on the presence of prolactin receptor, the downstream Jak2-Stat5 pathway and the transcription factor Elf5. A strict regulation of Rank signaling is essential for the differentiation of the mammary gland and in particular for alveolar commitment. Impaired alveologenesis and lactation failure are observed in both, knockout and Rank overexpressing mice; however, the underlying molecular mechanism responsible for these phenotypes remains largely unknown. Using genome-wide expression analyses and functional studies, we show here that Rankl (RL) exposure leads to impaired secretory differentiation of alveolar cells not only in MMTV-RANK but also in wild-type (WT) mammary acini. Conversely, pharmacological blockage of Rank signaling at midgestation in WT mice leads to precocious and exacerbated lactogenesis. Mechanistically, RL negatively regulates Stat5 phosphorylation and Elf5 expression at the onset of lactogenesis. Continuous RL exposure leads to the expansion of basal and bipotent cells in WT and MMTV-RANK acini. Overall, we demonstrate that enhanced Rank signaling impairs secretory differentiation during pregnancy by inhibition of the prolactin/p-Stat5 pathway.

Lymphangioleiomyomatosis Biomarkers Linked to Lung Metastatic Potential and Cell Stemness

Lymphangioleiomyomatosis (LAM) is a rare lung-metastasizing neoplasm caused by the proliferation of smooth muscle-like cells that commonly carry loss-of-function mutations in either the tuberous sclerosis complex 1 or 2 (TSC1 or TSC2) genes. While allosteric inhibition of the mechanistic target of rapamycin (mTOR) has shown substantial clinical benefit, complementary therapies are required to improve response and/or to treat specific patients. However, there is a lack of LAM biomarkers that could potentially be used to monitor the disease and to develop other targeted therapies. We hypothesized that the mediators of cancer metastasis to lung, particularly in breast cancer, also play a relevant role in LAM. Analyses across independent breast cancer datasets revealed associations between low TSC1/2 expression, altered mTOR complex 1 (mTORC1) pathway signaling, and metastasis to lung. Subsequently, immunohistochemical analyses of 23 LAM lesions revealed positivity in all cases for the lung metastasis mediators fascin 1 (FSCN1) and inhibitor of DNA binding 1 (ID1). Moreover, assessment of breast cancer stem or luminal progenitor cell biomarkers showed positivity in most LAM tissue for the aldehyde dehydrogenase 1 (ALDH1), integrin-ß3 (ITGB3/CD61), and/or the sex-determining region Y-box 9 (SOX9) proteins. The immunohistochemical analyses also provided evidence of heterogeneity between and within LAM cases. The analysis of Tsc2-deficient cells revealed relative over-expression of FSCN1 and ID1; however, Tsc2-deficient cells did not show higher sensitivity to ID1-based cancer inhibitors. Collectively, the results of this study reveal novel LAM biomarkers linked to breast cancer metastasis to lung and to cell stemness, which in turn might guide the assessment of additional or complementary therapeutic opportunities for LAM.

Dual fatty acid synthase and HER2 signaling blockade shows marked antitumor activity against breast cancer models resistant to anti-HER2 drugs

Blocking the enzyme Fatty Acid Synthase (FASN) leads to apoptosis of HER2-positive breast carcinoma cells. The hypothesis is that blocking FASN, in combination with anti-HER2 signaling agents, would be an effective antitumor strategy in preclinical HER2+ breast cancer models of trastuzumab and lapatinib resistance. We developed and molecularly characterized in vitro HER2+ models of resistance to trastuzumab (SKTR), lapatinib (SKLR) and both (SKLTR). The cellular interactions of combining anti-FASN polyphenolic compounds (EGCG and the synthetic G28UCM) with anti-HER2 signaling drugs (trastuzumab plus pertuzumab and temsirolimus) were analyzed. Tumor growth inhibition after treatment with EGCG, pertuzumab, temsirolimus or the combination was evaluated in two in vivo orthoxenopatients: one derived from a HER2+ patient and another from a patient who relapsed on trastuzumab and lapatinib-based therapy. SKTR, SKLR and SKLTR showed hyperactivation of EGFR and p-ERK1/2 and PI3KCA mutations. Dual-resistant cells (SKLTR) also showed hyperactivation of HER4 and recovered levels of p-AKT compared with mono-resistant cells. mTOR, p-mTOR and FASN expression remained stable in SKTR, SKLR and SKLTR. In vitro, anti-FASN compounds plus pertuzumab showed synergistic interactions in lapatinib- and dual- resistant cells and improved the results of pertuzumab plus trastuzumab co-treatment. FASN inhibitors combined with temsirolimus displayed the strongest synergistic interactions in resistant cells. In vivo, both orthoxenopatients showed strong response to the antitumor activity of the combination of EGCG with pertuzumab or temsirolimus, without signs of toxicity. We showed that the simultaneous blockade of FASN and HER2 pathways is effective in cells and in breast cancer models refractory to anti-HER2 therapies.

Exploiting tumor vulnerabilities: epigenetics, cancer metabolism and the mTOR pathway in the era of personalized medicine

Patient stratification according to drug responses, together with the discovery of novel antitumor targets, is leading to a new era of personalized cancer treatments. With the aim of identifying emerging pathways and the challenges faced by clinicians during clinical trials, the IDIBELL Cancer Conference on Personalized Cancer Medicine took place in Barcelona on December 3-4, 2012. This conference brought together speakers working in different areas of cancer research (epigenetics, metabolism and the mTOR pathway, cell death and the immune system, clinical oncology) to discuss the latest developments in personalized cancer medicine.

Identification of NOG as a specific breast cancer bone metastasis-supporting gene

Metastasis requires numerous biological functions that jointly provide tumor cells from a primary site to seed and colonize a distant organ. Some of these activities are selected for in the primary site, whereas others are acquired at the metastatic niche. We provide molecular evidence showing that the BMP inhibitor, NOG, provides metastatic breast cancer cells with the ability to colonize the bone. NOG expression is acquired during the late events of metastasis, once cells have departed from the primary site, because it is not enriched in primary tumors with high risk of bone relapse. On the contrary, breast cancer bone metastatic lesions do select for high levels of NOG expression when compared with metastasis to the lung, liver, and brain. Pivotal to the bone colonization functions is the contribution of NOG to metastatic autonomous and nonautonomous cell functions. Using genetic approaches, we show that when NOG is expressed in human breast cancer cells, it facilitates bone colonization by fostering osteoclast differentiation and bone degradation and also contributes to metastatic lesions reinitiation. These findings reveal how aggressive cancer cell autonomous and nonautonomous functions can be mechanistically coupled to greater bone metastatic potential.

RANK induces epithelial-mesenchymal transition and stemness in human mammary epithelial cells and promotes tumorigenesis and metastasis

Paracrine signaling through receptor activator of NF-κB (RANK) pathway mediates the expansion of mammary epithelia that occurs during pregnancy, and activation of RANK pathway promotes mammary tumorigenesis in mice. In this study we extend these previous data to human cells and show that the RANK pathway promotes the development of mammary stem cells and breast cancer. Overexpression of RANK (FL-RANK) in a panel of tumoral and normal human mammary cells induces the expression of breast cancer stem and basal/stem cell markers. High levels of RANK in untransformed MCF10A cells induce changes associated with both stemness and transformation, including mammary gland reconstitution, epithelial-mesenchymal transition (EMT), increased migration, and anchorage-independent growth. In addition, spheroids of RANK overexpressing MCF10A cells display disrupted acinar formation, impair growth arrest and polarization, and luminal filling. RANK overexpression in tumor cells with nonfunctional BRCA1 enhances invasiveness in acinar cultures and increases tumorigenesis and metastasis in immunodeficient mice. High levels of RANK were found in human primary breast adenocarcinomas that lack expression of the hormone receptors, estrogen and progesterone, and in tumors with high pathologic grade and proliferation index; high RANK/RANKL expression was significantly associated with metastatic tumors. Together, our findings show that RANK promotes tumor initiation, progression, and metastasis in human mammary epithelial cells by increasing the population of CD44(+)CD24(-) cells, inducing stemness and EMT. These results suggest that RANK expression in primary breast cancer associates with poor prognosis.

Expression of mTert in primary murine cells links the growth-promoting effects of telomerase to transforming growth factor-β signaling

Here, we show that ectopic expression of the catalytic subunit of mouse telomerase (mTert) confers a growth advantage to primary murine embryonic fibroblasts (MEFs), which have very long telomeres, as well as facilitates their spontaneous immortalization and increases their colony-forming capacity upon activation of oncogenes. We demonstrate that these telomere length-independent growth-promoting effects of mTert overexpression require catalytically active mTert, as well as the formation of mTert/Terc complexes. The gene expression profile of mTert-overexpressing MEFs indicates that telomerase enhances growth in these cells through the repression of growth-inhibiting genes of the transforming growth factor-beta (TGF-beta) signaling network. We functionally validate this result by showing that mTert abrogates the growth-inhibitory effect of TGF-beta in MEFs, thus demonstrating that telomerase increments the proliferative potential of primary mouse embryonic fibroblasts by targeting the TGF-beta pathway.

Antagonistic effects of telomerase on cancer and aging in K5-mTert transgenic mice

Many degenerative diseases that occur with aging, as well as premature aging syndromes, are characterized by presenting cells with critically short telomeres. Telomerase reintroduction is envisioned as a putative therapy for diseases characterized by telomere exhaustion. K5-mTert transgenic mice overexpress telomerase in a wide spectrum of tissues. These mice have a higher incidence of both induced and spontaneous tumors, resulting in increased mortality during the first year of life. Here, we show that in spite of this elevated tumor incidence and the initial lower survival, K5-mTert mice show an extension of the maximum lifespan from 1.5 to 3 months, depending on the transgenic line, which represents up to a 10% increase in the mean lifespan compared to wild-type littermates. This longer lifespan is coincidental with a lower incidence of certain age-related degenerative diseases, mainly those related to kidney function and germline integrity. Importantly, these effects of telomerase overexpression cannot be attributed to dramatic differences in telomere length in aged K5-Tert mice compared to wild-type mice, as shown by quantitative telomeric FISH. These findings indicate that telomerase overexpression extends the maximum lifespan of mice.

Telomere dysfunction results in enhanced organismal sensitivity to the alkylating agent N-methyl-N-nitrosourea

Here, we use telomerase-deficient mice, Terc(-/-), to study the impact of telomerase abrogation in response to treatment with the alkylating agent N-Methyl-N-Nitrosourea (MNU), a potent carcinogen in the mouse. Wild-type mice treated with MNU developed lymphomas and carcinomas. In contrast, similarly treated G5 Terc(-/-) mice with critically short telomeres did not develop tumors and died of acute toxicity to the small intestine. G2 Terc(-/-) mice, which have long telomeres, were less susceptible to MNU-induced tumors than wild-type mice, as well as less sensitive to MNU toxicity than G5 Terc(-/-) mice. The results indicate that short telomeres suppress tumor growth and that lack of telomerase retards tumor progression, even in the presence of long telomeres. Finally, G5 Terc(-/-) hypersensitivity to MNU supports the notion that short telomeres interfere with proper DNA damage repair.

Cooperation between p53 mutation and high telomerase transgenic expression in spontaneous cancer development

Telomerase reintroduction in adult somatic tissues is envisioned as a way to extend their proliferative capacity. It is still a question, however, whether constitutive telomerase expression in adult tissues impacts the normal aging and spontaneous cancer incidence of an organism. Here, we studied the aging and spontaneous cancer incidence of mice with transgenic telomerase expression in a wide range of adult tissues, K5-Tert mice. For this, we maintained large colonies of K5-Tert mice for more than 2 years. K5-Tert mice showed a decreased life span compared to wild-type cohorts associated with a higher incidence of preneoplastic and neoplastic lesions in various tissue types. Neoplasias in K5-Tert mice were coincident with transgene expression in the affected tissues. These observations suggest that high telomerase activity may cooperate with genetic alterations that occur with age to promote tumorigenesis. Indeed, we demonstrate here that increased cancer incidence and the reduced viability of K5-Tert mice are aggravated in a p53(+/-) genetic background, indicating that telomerase cooperates with loss of p53 function in inducing tumorigenesis. Altogether, these results demonstrate that constitutive high levels of telomerase activity result in a decreased life span associated with an increased incidence of neoplasias as the organism ages.

Telomerase inhibition in RenCa, a murine tumor cell line with short telomeres, by overexpression of a dominant negative mTERT mutant, reveals fundamental differences in telomerase regulation between human and murine cells

In contrast to human primary fibroblasts, mouse embryonic fibroblasts have telomerase activity, immortalize spontaneously in culture, and can be neoplastically transformed by oncogenic insult. Ectopic expression of the human telomerase catalytic subunit, human telomerase reverse transcriptase (hTERT), in human primary cells allows both spontaneous immortalization and neoplastic transformation by oncogenes. This suggests that telomerase activity, as well as the fact that mouse telomeres are longer than human telomeres, may explain some of the differences in cellular control between human and murine cells. Telomerase inhibition in immortal or transformed human cells using dominant negative hTERT mutants leads to telomere shortening and cell death. Here we study the effect of expression of a dominant negative mutant of the catalytic subunit of mouse telomerase, mTERT-DN, in a murine kidney tumor cell line, RenCa, whose telomeres are similar in length to human telomeres. After showing initial telomerase activity inhibition and telomere shortening, all clones expressing mTERT-DN reactivated telomerase and showed normal viability, in contrast with that described for human cells. This efficient telomerase reactivation coincided with a significant increase in the endogenous TERT mRNA levels in the presence of mTERT-DN expression. The results presented here reveal the existence of fundamental differences in telomerase regulation between mice and man.

Normal telomere length and chromosomal end capping in poly(ADP-ribose) polymerase-deficient mice and primary cells despite increased chromosomal instability

Poly(ADP-ribose) polymerase (PARP)-1, a detector of single-strand breaks, plays a key role in the cellular response to DNA damage. PARP-1-deficient mice are hypersensitive to genotoxic agents and display genomic instability due to a DNA repair defect in the base excision repair pathway. A previous report suggested that PARP-1-deficient mice also had a severe telomeric dysfunction consisting of telomere shortening and increased end-to-end fusions (d'Adda di Fagagna, F., M.P. Hande, W.-M. Tong, P.M. Lansdorp, Z.-Q. Wang, and S.P. Jackson. 1999. NAT: Genet. 23:76-80). In contrast to that, and using a panoply of techniques, including quantitative telomeric (Q)-FISH, we did not find significant differences in telomere length between wild-type and PARP-1(-/)- littermate mice or PARP-1(-/)- primary cells. Similarly, there were no differences in the length of the G-strand overhang. Q-FISH and spectral karyotyping analyses of primary PARP-1(-/)- cells showed a frequency of 2 end-to-end fusions per 100 metaphases, much lower than that described previously (d'Adda di Fagagna et al., 1999). This low frequency of end-to-end fusions in PARP-1(-/)- primary cells is accordant with the absence of severe proliferative defects in PARP-1(-/)- mice. The results presented here indicate that PARP-1 does not play a major role in regulating telomere length or in telomeric end capping, and the chromosomal instability of PARP-1(-/)- primary cells can be explained by the repair defect associated to PARP-1 deficiency. Finally, no interaction between PARP-1 and the telomerase reverse transcriptase subunit, Tert, was found using the two-hybrid assay.

Increased epidermal tumors and increased skin wound healing in transgenic mice overexpressing the catalytic subunit of telomerase, mTERT, in basal keratinocytes

Telomerase transgenics are an important tool to assess the role of telomerase in cancer, as well as to evaluate the potential use of telomerase for gene therapy of age-associated diseases. Here, we have targeted the expression of the catalytic component of mouse telomerase, mTERT, to basal keratinocytes using the bovine keratin 5 promoter. These telomerase-transgenic mice are viable and show histologically normal stratified epithelia with high levels of telomerase activity and normal telomere length. Interestingly, the epidermis of these mice is highly responsive to the mitogenic effects of phorbol esters, and it is more susceptible than that of wild-type littermates to the development skin tumors upon chemical carcinogenesis. The epidermis of telomerase-transgenic mice also shows an increased wound-healing rate compared with wild-type littermates. These results suggest that, contrary to the general assumption, telomerase actively promotes proliferation in cells that have sufficiently long telomeres and unravel potential risks of gene therapy for age-associated diseases based on telomerase upregulation.

Telomerase-deficient mice with short telomeres are resistant to skin tumorigenesis

Inhibition of telomerase is proposed to limit the growth of cancer cells by triggering telomere shortening and cell death. Telomere maintenance by telomerase is sufficient, in some cell types, to allow immortal growth. Telomerase has been shown to cooperate with oncogenes in transforming cultured primary human cells into neoplastic cells, suggesting that telomerase activation contributes to malignant transformation. Moreover, telomerase inhibition in human tumour cell lines using dominant-negative versions of TERT leads to telomere shortening and cell death. These findings have led to the proposition that telomerase inhibition may result in cessation of tumour growth. The absence of telomerase from most normal cells supports the potential efficacy of anti-telomerase drugs for tumour therapy, as its inhibition is unlikely to have toxic effects. Mice deficient for Terc RNA (encoding telomerase) lack telomerase activity, and constitute a model for evaluating the role of telomerase and telomeres in tumourigenesis. Late-generation Terc-/- mice show defects in proliferative tissues and a moderate increase in the incidence of spontaneous tumours in highly proliferative cell types (lymphomas, teratocarcinomas). The appearance of these tumours is thought to be a consequence of chromosomal instability in these mice. These observations have challenged the expected effectiveness of anti-telomerase-based cancer therapies. Different cell types may nonetheless vary in their sensitivity to the chromosomal instability produced by telomere loss or to the activation of telomere-rescue mechanisms. Here we show that late-generation Terc-/- mice, which have short telomeres and are telomerase-deficient, are resistant to tumour development in multi-stage skin carcinogenesis. Our results predict that an anti-telomerase-based tumour therapy may be effective in epithelial tumours.