Conditional loss of PTEN leads to precocious development and neoplasia in the mammary gland

Luminal expression of PIK3CA mutant H1047R in the mammary gland induces heterogeneous tumors

The phosphoinositide 3-kinase (PI3K) signaling cascade, a key mediator of cellular survival, growth, and metabolism, is frequently altered in human cancer. Activating mutations in PIK3CA, which encodes the α-catalytic subunit of PI3K, occur in approximately 30% of breast cancers. These mutations result in constitutive activity of the enzyme and are oncogenic, but it is not known whether they are sufficient to induce mammary carcinomas in mice. In the present study, we show that the expression of mutant PIK3CA H1047R in the luminal mammary epithelium evokes heterogeneous tumors that express luminal and basal markers and are positive for the estrogen receptor. Our results suggest that the PIK3CA H1047R oncogene targets a multipotent progenitor cell and, furthermore, show that this model recapitulates features of human breast tumors with PIK3CA H1047R.

Elevated PI3K signaling drives multiple breast cancer subtypes

Most human breast tumors have mutations that elevate signaling through a key metabolic pathway that is induced by insulin and a number of growth factors. This pathway serves to activate an enzyme known as phosphatidylinositol 3' kinase (PI3K) as well as to regulate proteins that signal in response to lipid products of PI3K. The specific mutations that activate this pathway in breast cancer can occur in genes coding for tyrosine kinase receptors, adaptor proteins linked to PI3K, catalytic and regulatory subunits of PI3K, serine/threonine kinases that function downstream of PI3K, and also phosphatidylinositol 3' phosphatase tumor suppressors that function to antagonize this pathway. While each genetic change results in net elevation of PI3K pathway signaling, and all major breast cancer subtypes show pathway activation, the specific mutation(s) involved in any one tumor may play an important role in defining tumor subtype, prognosis and even sensitivity to therapy. Here, we describe mouse models of breast cancer with elevated PI3K signaling, and how they may be used to guide development of novel therapeutics.

MMTV-Cre transgenes can adversely affect lactation: considerations for conditional gene deletion in mammary tissue

CRE-loxP-mediated inactivation and activation of genes in mouse mammary epithelium have been widely used to study genetic pathways in normal development and neoplastic transformation in vivo. In 1997, we generated three distinct mouse lines carrying an identical MMTV-Cre transgene (lines A, D, and F). Because the presence of CRE recombinase can adversely affect the physiology of nonmammary cells, we explored whether transgenic females display lactational defects. Whereas dams from line D nurse their pups and display overtly normal mammary development, line A shows some impairment during lactation and females from line F completely fail to nurse their litters. The ability to nurse a litter correlates with the extent of alveolar development and differentiation. This study demonstrates the importance of including appropriate "Cre-only" controls and provides guidelines to avoid problems in data interpretation.

PTEN loss in the continuum of common cancers, rare syndromes and mouse models

PTEN is among the most frequently inactivated tumour suppressor genes in sporadic cancer. PTEN has dual protein and lipid phosphatase activity, and its tumour suppressor activity is dependent on its lipid phosphatase activity, which negatively regulates the PI3K-AKT-mTOR pathway. Germline mutations in PTEN have been described in a variety of rare syndromes that are collectively known as the PTEN hamartoma tumour syndromes (PHTS). Cowden syndrome is the best-described syndrome within PHTS, with approximately 80% of patients having germline PTEN mutations. Patients with Cowden syndrome have an increased incidence of cancers of the breast, thyroid and endometrium, which correspond to sporadic tumour types that commonly exhibit somatic PTEN inactivation. Pten deletion in mice leads to Cowden syndrome-like phenotypes, and tissue-specific Pten deletion has provided clues to the role of PTEN mutation and loss in specific tumour types. Studying PTEN in the continuum of rare syndromes, common cancers and mouse models provides insight into the role of PTEN in tumorigenesis and will inform targeted drug development.

Altered differentiation and paracrine stimulation of mammary epithelial cell proliferation by conditionally activated Smoothened

The Hedgehog (Hh) signaling network is critical for patterning and organogenesis in mammals, and has been implicated in a variety of cancers. Smoothened (Smo), the gene encoding the principal signal transducer, is overexpressed frequently in breast cancer, and constitutive activation in MMTV-SmoM2 transgenic mice caused alterations in mammary gland morphology, increased proliferation, and changes in stem/progenitor cell number. Both in transgenic mice and in clinical specimens, proliferative cells did not usually express detectable Smo, suggesting the hypothesis that Smo functioned in a non-cell autonomous manner to stimulate proliferation. Here, we employed a genetically tagged mouse model carrying a Cre-recombinase-dependent conditional allele of constitutively active Smo (SmoM2) to test this hypothesis. MMTV-Cre- or adenoviral-Cre-mediated SmoM2 expression in the luminal epithelium, but not in the myoepithelium, was required for the hyper-proliferative phenotypes. High levels of proliferation were observed in cells adjacent or in close-proximity to Smo expressing cells demonstrating that SmoM2 expressing cells were stimulating proliferation via a paracrine or juxtacrine mechanism. In contrast, Smo expression altered luminal cell differentiation in a cell-autonomous manner. SmoM2 expressing cells, purified by fluorescence activated cell sorting (FACS) via the genetic fluorescent tag, expressed high levels of Ptch2, Gli1, Gli2, Jag2 and Dll-1, and lower levels of Notch4 and Hes6, in comparison to wildtype cells. These studies provide insight into the mechanism of Smo activation in the mammary gland and its possible roles in breast tumorigenesis. In addition, these results also have potential implications for the interpretation of proliferative phenotypes commonly observed in other organs as a consequence of hedgehog signaling activation.

Disruption of reelin signaling alters mammary gland morphogenesis

Reelin signaling is required for appropriate cell migration and ductal patterning during mammary gland morphogenesis. Dab1, an intracellular adaptor protein activated in response to reelin signaling, is expressed in the developing mammary bud and in luminal epithelial cells in the adult gland. Reelin protein is expressed in a complementary pattern, first in the epithelium overlying the mammary bud during embryogenesis and then in the myoepithelium and periductal stroma in the adult. Deletion in mouse of either reelin or Dab1 induced alterations in the development of the ductal network, including significant retardation in ductal elongation, decreased terminal branching, and thickening and disorganization of the luminal wall. At later stages, some mutant glands overcame these early delays, but went on to exhibit enlarged and chaotic ductal morphologies and decreased terminal branching: these phenotypes are suggestive of a role for reelin in spatial patterning or structural organization of the mammary epithelium. Isolated mammary epithelial cells exhibited decreased migration in response to exogenous reelin in vitro, a response that required Dab1. These observations highlight a role for reelin signaling in the directed migration of mammary epithelial cells driving ductal elongation into the mammary fat pad and provide the first evidence that reelin signaling may be crucial for regulating the migration and organization of non-neural tissues.

Cooperation between Pik3ca and p53 mutations in mouse mammary tumor formation

PIK3CA, which codes for the p110α catalytic subunit of phosphatidylinositol 3-kinase, is one of the most frequently mutated genes in human breast cancer. Here, we describe a mouse model for PIK3CA-induced breast cancer by using the ROSA26 (R26) knock-in system, in which targeted Pik3ca alleles can be activated through transgenic expression of Cre recombinase. We mated Pik3ca(H1047R) and Pik3ca(wt) knock-in lines with MMTV-Cre transgenics, which express Cre in mammary epithelium. Starting at approximately 5 months of age, female R26-Pik3ca(H1047R);MMTV-Cre mice, but not control R26-Pik3ca(wt);MMTV-Cre mice, developed mammary tumors, as well as lymphoid and skin malignancies. R26-Pik3ca(H1047R);MMTV-Cre mammary tumors were typically either adenosquamous carcinoma or adenomyoepithelioma. As p53 is the most commonly mutated gene in breast cancer, we tested for genetic interaction between Pik3ca(H1047R) and p53 loss-of-function mutations in R26-Pik3ca(H1047R);p53(loxP/+);MMTV-Cre mice. This led to decreased survival of double-mutant animals, which developed lymphoma and mammary tumors with rapid kinetics. Mammary tumors that formed in p53(loxP/+);MMTV-Cre conditional mutants were either poorly differentiated adenocarcinoma or spindle cell/EMT, whereas R26-Pik3ca(H1047R);p53(loxP/+);MMTV-Cre mammary tumors were mostly adenosquamous carcinoma or spindle cell/EMT indicating that double-mutant mice develop a distinct spectrum of mammary tumors. Thus, an oncogenic variant of PIK3CA implicated in multiple human breast cancer subtypes can induce a very diverse spectrum of mammary tumors in mice. Furthermore, Pik3ca(H1047R) shows cooperation with p53, which altered the specific tumors that formed. Thus, the two most frequently mutated genes in human breast cancer show cooperation in mammary tumor formation.

Forced involution of the functionally differentiated mammary gland by overexpression of the pro-apoptotic protein bax

The mammary gland is a developmentally dynamic, hormone-responsive organ that undergoes proliferation and differentiation within the secretory epithelial compartment during pregnancy. The epithelia are maintained by pro-survival signals (e.g., Stat5, Akt1) during lactation, but undergo apoptosis during involution through inactivation of cell survival pathways and upregulation of pro-apoptotic proteins. To assess if the survival signals in the functionally differentiated mammary epithelial cells can override a pro-apoptotic signal, we generated transgenic mice that express Bax under the whey acidic protein (WAP) promoter. WAP-Bax females exhibited a lactation defect and were unable to nourish their offspring. Mammary glands demonstrated: (1) a reduction in epithelial content, (2) hallmark signs of mitochondria-mediated cell death, (3) an increase in apoptotic cells by TUNEL assay, and (4) precocious Stat3 activation. This suggests that upregulation of a single pro-apoptotic factor of the Bcl-2 family is sufficient to initiate apoptosis of functionally differentiated mammary epithelial cells in vivo.

Mouse models of inherited cancer syndromes

Animal models of cancer have been instrumental in understanding the progression and therapy of hereditary cancer syndromes. The ability to alter the genome of an individual mouse cell in both constitutive and inducible approaches has led to many novel insights into their human counterparts. In this review, knockout mouse models of inherited human cancer syndromes are presented and insights from the study of these models are highlighted.

Mammalian target of rapamycin-dependent acinar cell neoplasia after inactivation of Apc and Pten in the mouse salivary gland: implications for human acinic cell carcinoma

Cross-talk between the canonical Wnt and mammalian target of rapamycin (mTOR) signaling pathways occurs at multiple levels in the cell and likely contributes to the oncogenic effects of these pathways in human cancer. To gain more insight into the interplay between Wnt and mTOR signaling in salivary gland tumorigenesis, we developed a mouse model in which both pathways are constitutively activated by the conditional inactivation of the Apc and Pten tumor suppressor genes. Loss of either Apc or Pten alone did not cause tumor development. However, deletion of both genes resulted in the formation of salivary gland tumors with 100% penetrance and short latency that showed a remarkable morphologic similarity to human acinic cell carcinoma. Treatment of tumor-bearing mice using the mTOR inhibitor rapamycin led to complete regression of tumors, indicating that tumor growth was dependent on continued mTOR signaling. Importantly, we found that human salivary gland acinic cell carcinomas also express markers of activated mTOR signaling. Together, these results suggest that aberrant activation of mTOR signaling plays a pivotal role in acinar cell neoplasia of the salivary gland. Because rapamycin analogues are approved for treating other types of human malignancies, our findings suggest that rapamycin therapy should be evaluated for treating patients with salivary gland acinic cell carcinoma.

Deciphering the role of PI3K/Akt/mTOR pathway in breast cancer biology and pathogenesis

The phosphatidylinositol 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR) pathway mediates multiple cellular functions critical to tumor initiation, progression, and outcomes, including growth and proliferation, metabolism, motility, migration, invasion, angiogenesis, survival, and autophagy. Tight regulation of this pathway is paramount to ensure that multiple cellular inputs are integrated for appropriate cellular outcomes. Frequent deregulation and aberrations of this pathway have been implicated in breast cancer development and progression. This review focuses on the biology of this pathway and its role in breast cancer pathogenesis. The role of therapies directed at targeting mTOR in the PI3K/Akt/mTOR pathway, which are currently being evaluated in clinical trials, will also be reviewed.

Basal cell carcinoma and the carcinogenic role of aberrant Hedgehog signaling

Basal cell carcinoma (BCC) is the most frequent cancer in the white population and its incidence appears to be increasing worldwide. While the majority of BCCs arise sporadically, many cases are attributable to basal cell nevus syndrome, or Gorlin syndrome, an autosomal dominantly inherited disorder characterized by the occurrence of multiple BCCs and by extracutaneous tumors. Genetic studies on patients with basal cell nevus syndrome indicate deregulation of the Hedgehog (Hh) pathway in epidermal keratinocytes as the primary event in the pathogenesis of BCC. This article summarizes the recent progress in understanding Hh-dependent BCC tumorigenesis, as well as evidence for deregulation of other molecular pathways, primarily the Wnt developmental pathway. Understanding the molecular genetics of BCC development has provided new opportunities for molecular therapy of this cancer by targeting Hh and other signaling pathways.

Akt is required for Stat5 activation and mammary differentiation

INTRODUCTION

The Akt pathway plays a central role in regulating cell survival, proliferation and metabolism, and is one of the most commonly activated pathways in human cancer. A role for Akt in epithelial differentiation, however, has not been established. We previously reported that mice lacking Akt1, but not Akt2, exhibit a pronounced metabolic defect during late pregnancy and lactation that results from a failure to upregulate Glut1 as well as several lipid synthetic enzymes. Despite this metabolic defect, however, both Akt1-deficient and Akt2-deficient mice exhibit normal mammary epithelial differentiation and Stat5 activation.

METHODS

In light of the overlapping functions of Akt family members, we considered the possibility that Akt may play an essential role in regulating mammary epithelial development that is not evident in Akt1-deficient mice due to compensation by other Akt isoforms. To address this possibility, we interbred mice bearing targeted deletions in Akt1 and Akt2 and determined the effect on mammary differentiation during pregnancy and lactation.

RESULTS

Deletion of one allele of Akt2 in Akt1-deficient mice resulted in a severe defect in Stat5 activation during late pregnancy that was accompanied by a global failure of terminal mammary epithelial cell differentiation, as manifested by the near-complete loss in production of the three principal components of milk: lactose, lipid, and milk proteins. This defect was due, in part, to a failure of pregnant Akt1(-/-);Akt2(+/-) mice to upregulate the positive regulator of Prlr-Jak-Stat5 signaling, Id2, or to downregulate the negative regulators of Prlr-Jak-Stat5 signaling, caveolin-1 and Socs2.

CONCLUSIONS

Our findings demonstrate an unexpected requirement for Akt in Prlr-Jak-Stat5 signaling and establish Akt as an essential central regulator of mammary epithelial differentiation and lactation.

Rb deletion in mouse mammary progenitors induces luminal-B or basal-like/EMT tumor subtypes depending on p53 status

Breast cancer is a highly heterogeneous disease, with several different subtypes being characterized by distinct histology, gene expression patterns, and genetic alterations. The tumor suppressor gene retinoblastoma 1 (RB1) is frequently lost in both luminal-B and triple-negative tumor (TNT; i.e., estrogen receptor-, progesterone receptor-, and human epidermal growth factor receptor 2-negative) breast cancer subtypes. However, a causal role for RB1 loss in different subtypes remains undefined. Here we report that deletion of Rb alone or together with its relative p107 in mouse mammary stem/bipotent progenitor cells induced focal acinar hyperplasia with squamous metaplasia. These lesions progressed into histologically diverse, transplantable mammary tumors with features of either luminal-B or TNT subtypes. The TNTs included basal-like tumors as well as tumors that exhibited epithelial-to-mesenchymal transition (EMT). The EMT-type tumors and a subset of the basal-like tumors, but not luminal-B-like tumors, expressed mutant forms of the tumor suppressor p53. Accordingly, targeted deletion of both Rb and p53 in stem/bipotent progenitors led to histologically uniform, aggressive, EMT-type tumors. Reintroduction of Rb into these tumor cells suppressed growth in vitro and tumor formation in vivo. These results establish a causal role for Rb loss in breast cancer in mice and demonstrate that cooperating oncogenic events, such as mutations in p53, dictate tumor subtype after Rb inactivation.

Targeting breast cancer stem cells

The cancer stem cell (CSC) hypothesis postulates that tumors are maintained by a self-renewing CSC population that is also capable of differentiating into non-self-renewing cell populations that constitute the bulk of the tumor. Although, the CSC hypothesis does not directly address the cell of origin of cancer, it is postulated that tissue-resident stem or progenitor cells are the most common targets of transformation. Clinically, CSCs are predicted to mediate tumor recurrence after chemo- and radiation-therapy due to the relative inability of these modalities to effectively target CSCs. If this is the case, then CSC must be efficiently targeted to achieve a true cure. Similarities between normal and malignant stem cells, at the levels of cell-surface proteins, molecular pathways, cell cycle quiescence, and microRNA signaling present challenges in developing CSC-specific therapeutics. Approaches to targeting CSCs include the development of agents targeting known stem cell regulatory pathways as well as unbiased high-throughput siRNA or small molecule screening. Based on studies of pathways present in normal stem cells, recent work has identified potential "Achilles heals" of CSC, whereas unbiased screening provides opportunities to identify new pathways utilized by CSC as well as develop potential therapeutic agents. Here, we review both approaches and their potential to effectively target breast CSC.

PTEN and p53 cross-regulation induced by soy isoflavone genistein promotes mammary epithelial cell cycle arrest and lobuloalveolar differentiation

The tumor suppressors phosphatase and tensin homologue deleted on chromosome ten (PTEN) and p53 are closely related to the pathogenesis of breast cancer, yet pathway-specific mechanisms underlying their participation in mediating the protective actions of dietary bioactive components on breast cancer risk are poorly understood. We recently showed that dietary exposure to the soy isoflavone genistein (GEN) induced PTEN expression in mammary epithelial cells in vivo and in vitro, consistent with the breast cancer preventive effects of soy food consumption. Here, we evaluated PTEN and p53 functional interactions in the nuclear compartment of mammary epithelial cells as a mechanism for mammary tumor protection by GEN. Using the non-tumorigenic human mammary epithelial cells MCF10-A, we demonstrate that GEN increased PTEN expression and nuclear localization. We show that increased nuclear PTEN levels initiated an autoregulatory loop involving PTEN-dependent increases in p53 nuclear localization, PTEN-p53 physical association, PTEN-p53 co-recruitment to the PTEN promoter region and p53 transactivation of PTEN promoter activity. The PTEN-p53 cross talk induced by GEN resulted in increased cell cycle arrest; decreased pro-proliferative cyclin D1 and pleiotrophin gene expression and the early formation of mammary acini, indicative of GEN promotion of lobuloalveolar differentiation. Our findings provide support to GEN-induced PTEN as both a target and regulator of p53 action and offer a mechanistic basis for PTEN pathway activation to underlie the antitumor properties of dietary factors, with important implications for reducing breast cancer risk.

Simvastatin induces derepression of PTEN expression via NFkappaB to inhibit breast cancer cell growth

Sustained activation of Akt kinase acts as a focal regulator to increase cell growth and survival, which causes tumorigenesis including breast cancer. Statins, potent inhibitors of 3-hydroxy-3-methylglutaryl coenzyme A reductase, display anticancer activity. The molecular mechanisms by which statins block cancer cell growth are poorly understood. We demonstrate that in the tumors derived from MDA-MB-231 human breast cancer cell xenografts, simvastatin significantly inhibited phosphorylation of Akt with concomitant attenuation of the expression of the anti-apoptotic protein Bcl(XL). In many cancer cells, Bcl(XL) is a target of NFkappaB. Simvastatin inhibited the DNA binding and transcriptional activities of NFkappaB resulting in marked reduction in transcription of Bcl(XL). Signals transmitted by anti-neoplastic mechanism implanted in the cancer cells serve to obstruct the initial outgrowth of tumors. One such mechanism represents the action of the tumor suppressor protein PTEN, which negatively regulates Akt kinase activity. We provide the first evidence for significantly increased levels of PTEN in the tumors of simvastatin-administered mice. Importantly, simvastatin markedly prevented binding of NFkappaB to the two canonical recognition elements, NFRE-1 and NFRE-2 present in the PTEN promoter. Contrary to the transcriptional suppression of Bcl(XL), simvastatin significantly increased the transcription of PTEN. Furthermore, expression of NFkappaB p65 subunit inhibited transcription of PTEN, resulting in reduced protein expression, which leads to enhanced phosphorylation of Akt. Taken together, our data present a novel bifaceted mechanism where simvastatin acts on a nodal transcription factor NFkappaB, which attenuates the expression of anti-apoptotic Bcl(XL) and simultaneously derepresses the expression of anti-proliferative/proapoptotic tumor suppressor PTEN to prevent breast cancer cell growth.

Key signalling nodes in mammary gland development and cancer. Signalling downstream of PI3 kinase in mammary epithelium: a play in 3 Akts

The protein serine/threonine kinase Akt, also known as protein kinase B (PKB), is arguably the most important signalling nexus in the cell. Akt integrates a plethora of extracellular signals to generate diverse outcomes, including proliferation, motility, growth, glucose homeostasis, survival, and cell death. The phosphatidylinositol 3-kinase (PI3K)/Akt pathway is the second most frequently mutated pathway in cancer, after p53, and mutations in components of this pathway are found in around 70% of breast cancers. Thus, understanding how Akt relays input signals to downstream effectors is critically important for the design of therapeutic strategies to combat breast cancer. In this review, we will discuss the various signals upstream of Akt that impact on its activity, how Akt integrates these signals and modulates the activity of downstream targets to control mammary gland development, and how mutations in components of the pathway result in breast cancer.

Exploring the gain of function contribution of AKT to mammary tumorigenesis in mouse models

Elevated expression of AKT has been noted in a significant percentage of primary human breast cancers, mainly as a consequence of the PTEN/PI3K pathway deregulation. To investigate the mechanistic basis of the AKT gain of function-dependent mechanisms of breast tumorigenesis, we explored the phenotype induced by activated AKT transgenes in a quantitative manner. We generated several transgenic mice lines expressing different levels of constitutively active AKT in the mammary gland. We thoroughly analyzed the preneoplastic and neoplastic mammary lesions of these mice and correlated the process of tumorigenesis to AKT levels. Finally, we analyzed the impact that a possible senescent checkpoint might have in the tumor promotion inhibition observed, crossing these lines to mammary specific p53(R172H) mutant expression, and to p27 knock-out mice. We analyzed the benign, premalignant and malignant lesions extensively by pathology and at molecular level analysing the expression of proteins involved in the PI3K/AKT pathway and in cellular senescence. Our findings revealed an increased preneoplastic phenotype depending upon AKT signaling which was not altered by p27 or p53 loss. However, p53 inactivation by R172H point mutation combined with myrAKT transgenic expression significantly increased the percentage and size of mammary carcinoma observed, but was not sufficient to promote full penetrance of the tumorigenic phenotype. Molecular analysis suggest that tumors from double myrAKT;p53(R172H) mice result from acceleration of initiated p53(R172H) tumors and not from bypass of AKT-induced oncogenic senescence. Our work suggests that tumors are not the consequence of the bypass of senescence in MIN. We also show that AKT-induced oncogenic senescence is dependent of pRb but not of p53. Finally, our work also suggests that the cooperation observed between mutant p53 and activated AKT is due to AKT-induced acceleration of mutant p53-induced tumors. Finally, our work shows that levels of activated AKT are not essential in the induction of benign or premalignant tumors, or in the cooperation of AKT with other tumorigenic signal such as mutant p53, once AKT pathway is activated, the relative level of activity seems not to determine the phenotype.

Mammary-digital-nail (MDN) syndrome: a novel phenotype maps to human chromosome 22q12.3-13.1

Mammary-digital-nail syndrome is a novel phenotypic association consisting of anonychia onychodystrophy with hypoplasia or absence of distal phalanges in males and females, accompanied by juvenile hypertrophy of the breast in affected females. This newly described genetic trait presents an autosomal dominant inheritance pattern, with either reduced penetrance or germ-line mosaicism. Analysis of the pedigree, linkage studies followed by a genome-wide screen and by haplotype analysis defined the locus for the phenotype within a 12 cM (4.3 Mb) interval on chromosome 22q12.3-13.1. This chromosomal region has not been implicated before in genetic disorders of the mammary tissue or limbs. These data suggest a possibly novel signaling pathway affecting the organogenesis of limbs and mammary glands in humans.

Faithfull modeling of PTEN loss driven diseases in the mouse

A decade of work has indisputably defined PTEN as a pivotal player in human health and disease. Above all, PTEN has been identified as one of the most commonly lost or mutated tumor suppressor genes in human cancers. For this reason, the generation of a multitude of mouse models has been an invaluable strategy to dissect the function and consequences-of-loss of this essential, evolutionary conserved lipid phosphatase in tumor initiation and progression.In this chapter, we will summarize the mouse models that have allowed us to faithfully recapitulate features of human cancers and to highlight the network of connections between the PTEN signaling cascade and other oncogenic or tumor suppressive pathways.Notably, PTEN represents one of the most extensively modeled genes involved in human cancer and exemplifies the strength of genetic mouse modeling as an approach to gain information aimed to improve our understanding of and ability to alleviate human disease.

UVB-induced ERK/AKT-dependent PTEN suppression promotes survival of epidermal keratinocytes

Ultraviolet (UV) radiation in sunlight is the major environmental cause of skin cancer. PTEN (phosphatase and tensin homolog deleted on chromosome 10) is a proven critical tumor suppressor. We report here that UVB down-regulates PTEN in primary human keratinocytes, human HaCaT keratinocytes, and mouse skin. As compared to normal skin, the PTEN levels are reduced in human actinic keratosis, a precancerous skin lesion caused by solar UV. PTEN down-regulation is mediated by two mechanisms: (1) PTEN is cleaved by active caspase in apoptotic cells in which AKT activation is reduced; and (2) PTEN transcription is suppressed in surviving cells, and this suppression is independent of caspase activation and occurs in parallel with increased ERK and AKT activation. We report here that the combination of ERK and AKT activation is crucial for PTEN suppression in surviving cells following UVB irradiation. PTEN remains suppressed in these cells. AKT activation is higher in UVB-irradiated surviving cells as compared to UVB protected control cells. ERK and AKT pathways are involved in sustaining PTEN suppression in UVB-exposed cells. Increasing PTEN expression enhances apoptosis of keratinocytes in response to UVB radiation. Our findings indicate that (1) UVB radiation suppresses PTEN expression in keratinocytes, and (2) the ERK/AKT/PTEN axis may form a positive feedback loop following UVB irradiation. Identification of PTEN as a critical molecular target of UVB will add to our understanding of the pathogenesis of skin cancer.

Fish oil targets PTEN to regulate NFkappaB for downregulation of anti-apoptotic genes in breast tumor growth

The molecular mechanism for the beneficial effect of fish oil on breast tumor growth is largely undefined. Using the xenograft model in nude mice, we for the first time report that the fish oil diet significantly increased the level of PTEN protein in the breast tumors. In addition, the fish oil diet attenuated the PI 3 kinase and Akt kinase activity in the tumors leading to significant inhibition of NFkappaB activation. Fish oil diet also prevented the expression of anti-apoptotic proteins Bcl-2 and Bcl-XL in the breast tumors with concomitant increase in caspase 3 activity. To extend these findings we tested the functional effects of DHA and EPA, the two active omega-3 fatty acids of fish oil, on cultured MDA MB-231 cells. In agreement with our in vivo data, DHA and EPA treatment increased PTEN mRNA and protein expression and inhibited the phosphorylation of p65 subunit of NFkappaB in MDA MB-231 cells. Furthermore, DHA and EPA reduced expression of Bcl-2 and Bcl-XL. NFkappaB DNA binding activity and NFkappaB-dependent transcription of Bcl-2 and Bcl-XL genes were also prevented by DHA and EPA treatment. Finally, we showed that PTEN expression significantly inhibited NFkappaB-dependent transcription of Bcl-2 and Bcl-XL genes. Taken together, our data reveals a novel signaling pathway linking the fish oil diet to increased PTEN expression that attenuates the growth promoting signals and augments the apoptotic signals, resulting in breast tumor regression.

Minireview: physiological and pathological actions of RAS in the ovary

The small G proteins of the RAS superfamily act as molecular switches in the transduction of cellular signals critical for a wide range of normal developmental events as well as pathological processes. However, the functions of Ras genes in ovarian cells have only started to be unveiled. RAS, most likely KRAS that is highly expressed in granulosa cells of growing follicles, appears crucial for mediating the gonadotropin-induced events associated with the unique physiological process of ovulation. By contrast, conditional expression of a constitutively active Kras(G12D) mutant in granulosa cells results in ovulation defects due to the complete disruption of normal follicular growth, cessation of granulosa cell proliferation, and blockage of granulosa cell apoptosis and differentiation. When the tumor suppressor Pten is disrupted conditionally in the Kras(G12D)-expressing granulosa cells, granulosa cell tumors fail to develop. However, ovarian surface epithelial cells expressing the same Pten;Kras(G12D) mutations rapidly become ovarian surface epithelial serous cystadenocarcinomas. In this minireview, we summarize some of the physiological as well as pathological functions of RAS in the rodent ovary, discuss the implications of the Kras(G12D) mutant mouse models for understanding human diseases such as premature ovarian failure and ovarian cancers, and highlight new questions raised by the results of recent studies.

PTEN: new insights into its regulation and function in skin cancer

Skin cancer is the most common cancer in the United States. UV radiation in sunlight is the major environmental factor causing skin cancer development. PTEN (phosphatase and tensin homolog deleted on chromosome 10), a recently discovered tumor suppressor gene, is frequently mutated, deleted, or epigenetically silenced in various human cancers. PTEN negatively regulates the oncogenic phosphatidylinositol 3-kinase (PI3K)/protein kinase B (AKT) signaling pathways. PTEN is clearly a critical tumor suppressor for skin cancer in humans and in mice. This review summarizes the recent progress in the function of PTEN in the development of skin cancer, including basal-cell carcinoma, squamous-cell carcinoma, and melanoma. The regulation of PTEN by UV radiation is also discussed in association with skin carcinogenesis. Understanding the fundamental mechanisms that lead to the reduction of PTEN function in skin carcinogenesis and the essential association with UV radiation opens up new opportunities for molecular chemoprevention and therapy of skin cancer by targeting PTEN pathways.

Molecular mechanisms underlying the activation of mammalian primordial follicles

In humans and other mammalian species, the pool of resting primordial follicles serves as the source of developing follicles and fertilizable ova for the entire length of female reproductive life. One question that has intrigued biologists is: what are the mechanisms controlling the activation of dormant primordial follicles. Studies from previous decades have laid a solid, but yet incomplete, foundation. In recent years, molecular mechanisms underlying follicular activation have become more evident, mainly through the use of genetically modified mouse models. As hypothesized in the 1990s, the pool of primordial follicles is now known to be maintained in a dormant state by various forms of inhibitory machinery, which are provided by several inhibitory signals and molecules. Several recently reported mutant mouse models have shown that a synergistic and coordinated suppression of follicular activation provided by multiple inhibitory molecules is necessary to preserve the dormant follicular pool. Loss of function of any of the inhibitory molecules for follicular activation, including PTEN (phosphatase and tensin homolog deleted on chromosome 10), Foxo3a, p27, and Foxl2, leads to premature and irreversible activation of the primordial follicle pool. Such global activation of the primordial follicle pool leads to the exhaustion of the resting follicle reserve, resulting in premature ovarian failure in mice. In this review, we summarize both historical and recent results on mammalian primordial follicular activation and focus on the up-to-date knowledge of molecular networks controlling this important physiological event. We believe that information obtained from mutant mouse models may also reflect the molecular machinery responsible for follicular activation in humans. These advances may provide a better understanding of human ovarian physiology and pathophysiology for future clinical applications.

Mammary involution and breast cancer risk: transgenic models and clinical studies

Postlactational involution is the process following weaning during which the mammary gland undergoes massive cell death and tissue remodeling as it returns to the pre-pregnant state. Lobular involution is the process by which the breast epithelial tissue is gradually lost with aging of the mammary gland. While postlactational involution and lobular involution are distinct processes, recent studies have indicated that both are related to breast cancer development. Experiments using a variety of rodent models, as well as observations in human populations, suggest that deregulation of postlactational involution may act to facilitate tumor formation. By contrast, new human studies show that completion of lobular involution protects against subsequent breast cancer incidence.

Simultaneous loss of the DLC1 and PTEN tumor suppressors enhances breast cancer cell migration

The phosphatase and tensin homolog (PTEN) gene is a tumor suppressor frequently deleted or mutated in sporadic tumors of the breast, prostate, endometrium and brain. The protein acts as a dual specificity phosphatase for lipids and proteins. PTEN loss confers a growth advantage to cells, protects from apoptosis and favors cell migration. The deleted in liver cancer 1 (DLC1) gene has emerged as a novel tumor suppressor downregulated in a variety of tumor types including those of the breast. DLC1 contains a Rho GTPase activating domain that is involved in the inhibition of cell proliferation, migration and invasion. To investigate how simultaneous loss of PTEN and DLC1 contributes to cell transformation, we downregulated both proteins by RNA interference in the non-invasive MCF7 breast carcinoma cell line. Joint depletion of PTEN and DLC1 resulted in enhanced cell migration in wounding and chemotactic transwell assays. Interestingly, both proteins were found to colocalize at the plasma membrane and interacted physically in biochemical pulldowns and coimmunoprecipitations. We therefore postulate that the concerted local inactivation of signaling pathways downstream of PTEN and DLC1, respectively, is required for the tight control of cell migration.

Interaction of E-cadherin and PTEN regulates morphogenesis and growth arrest in human mammary epithelial cells

Phosphatase and tensin homologue deleted on chromosome 10 (PTEN) is a dual-function phosphatase with tumor suppressor function compromised in a wide spectrum of cancers. Because tissue polarity and architecture are crucial modulators of normal and malignant behavior, we postulated that PTEN may play a role in maintenance of tissue integrity. We used two nonmalignant human mammary epithelial cell lines that form polarized, growth-arrested structures (acini) when cultured in three-dimensional laminin-rich extracellular matrix gels (lrECM). As acini begin to form, PTEN accumulates both in the cytoplasm and at cell-cell contacts where it colocalizes with the E-cadherin/beta-catenin complex. Reduction of PTEN levels by shRNA in lrECM prevents formation of organized breast acini and disrupts growth arrest. Importantly, disruption of acinar polarity and cell-cell contact by E-cadherin function-blocking antibodies reduces endogenous PTEN protein levels and inhibits its accumulation at cell-cell contacts. Conversely, in Skbr-3 breast cancer cells lacking endogenous E-cadherin expression, exogenous introduction of E-cadherin gene causes induction of PTEN expression and its accumulation at sites of cell interactions. These studies provide evidence that E-cadherin regulates both the PTEN protein levels and its recruitment to cell-cell junctions in three-dimensional lrECM, indicating a dynamic reciprocity between architectural integrity and the levels and localization of PTEN. This interaction thus seems to be a critical integrator of proliferative and morphogenetic signaling in breast epithelial cells.

PTEN deficiency in a luminal ErbB-2 mouse model results in dramatic acceleration of mammary tumorigenesis and metastasis

Overexpression and/or amplification of the ErbB-2 oncogene as well as inactivation of the PTEN tumor suppressor are two important genetic events in human breast carcinogenesis. To address the biological impact of conditional inactivation of PTEN on ErbB-2-induced mammary tumorigenesis, we generated a novel transgenic mouse model that utilizes the murine mammary tumor virus (MMTV) promoter to directly couple expression of activated ErbB-2 and Cre recombinase to the same mammary epithelial cell (MMTV-NIC). Disruption of PTEN in the mammary epithelium of the MMTV-NIC model system dramatically accelerated the formation of multifocal and highly metastatic mammary tumors, which exhibited homogenous pathology. PTEN-deficient/NIC-induced tumorigenesis was associated with an increase in angiogenesis. Moreover, inactivation of PTEN in the MMTV-NIC mouse model resulted in hyperactivation of the phosphatidylinositol 3'-kinase/Akt signaling pathway. However, like the parental strain, tumors obtained from PTEN-deficient/NIC mice displayed histopathological and molecular features of the luminal subtype of primary human breast cancer. Taken together, our findings provide important implications in understanding the molecular determinants of mammary tumorigenesis driven by PTEN deficiency and ErbB-2 activation and could provide a valuable tool for testing the efficacy of therapeutic strategies that target these critical signaling pathways.

PTEN, stem cells, and cancer stem cells

Like normal stem cells, "cancer stem cells" have the capacity for indefinite proliferation and generation of new cancerous tissues through self-renewal and differentiation. Among the major intracellular signaling pathways, WNT, SHH, and NOTCH are known to be important in regulating normal stem cell activities, and their alterations are associated with tumorigenesis. It has become clear recently that PTEN (phosphatase and tensin homologue) is also critical for stem cell maintenance and that PTEN loss can cause the development of cancer stem cells and ultimately tumorigenesis.

Genetic modelling of the PTEN/AKT pathway in cancer research

The focus on targeted therapies has been fuelled by extensive research on molecular pathways and their role in tumorigenesis. Novel models of human cancer have been created to evaluate the role of specific genes in the different stages of cancer. Currently, mouse modelling of human cancer is possible through the expression of oncogenes, specific genetic mutations or the inactivation of tumour suppressor genes, and these models have begun to provide us with an understanding of the molecular pathways involved in tumour initiation and progression at the physiological level. Additionally, these mouse models serve as an excellent system to evaluate the efficacy of currently developed molecular targeted therapies and identify new potential targets for future therapies. The PTEN/AKT pathway is implicated in signal transduction through tyrosine kinase receptors and heterotrimeric G protein-linked receptors. Deregulation of the PTEN/AKT pathway is a common event in human cancer. Despite the abundant literature, the physiological role of each element of the pathway has begun to be uncovered thanks to genetically engineered mice. This review will summarise some of the key animal models which have helped us to understand this signalling network and its contribution to tumorigenesis.

PI3K/PTEN signaling in angiogenesis and tumorigenesis

Phosphatidylinositol 3-kinase (PI3K) and phosphatase and tensin homolog deleted on chromosome 10 (PTEN) signaling pathway play an important role in multiple cellular functions such as cell metabolism, proliferation, cell-cycle progression, and survival. PI3K is activated by growth factors and angiogenesis inducers such as vascular endothelial growth factor (VEGF) and angiopoietins. The amplification and mutations of PI3K and the loss of the tumor suppressor PTEN are common in various kinds of human solid tumors. The genetic alterations of upstream and downstream of PI3K signaling molecules such as receptor tyrosine kinases and AKT, respectively, are also frequently altered in human cancer. PI3K signaling regulates tumor growth and angiogenesis by activating AKT and other targets, and by inducing HIF-1 and VEGF expression. Angiogenesis is required for tumor growth and metastasis. In this review, we highlight the recent studies on the roles and mechanisms of PI3K and PTEN in regulating tumorigenesis and angiogenesis, and the roles of the downstream targets of PI3K for transmitting the signals. We also discuss the crosstalk of these signaling molecules and cellular events during tumor growth, metastasis, and tumor angiogenesis. Finally, we summarize the potential applications of PI3K, AKT, and mTOR inhibitors and their outcome in clinical trials for cancer treatment.

Epithelial Pten is dispensable for intestinal homeostasis but suppresses adenoma development and progression after Apc mutation

PTEN acts as a tumor suppressor in a range of tissue types and has been implicated in the regulation of intestinal stem cells. To study Pten function in the intestine, we used various conditional transgenic strategies to specifically delete Pten from the mouse intestinal epithelium. We show that Pten loss specifically within the adult or embryonic epithelial cell population does not affect the normal architecture or homeostasis of the epithelium. However, loss of Pten in the context of Apc deficiency accelerates tumorigenesis through increased activation of Akt, leading to rapid development of adenocarcinoma. We conclude that Pten is redundant in otherwise normal intestinal epithelium and epithelial stem cells but, in the context of activated Wnt signaling, suppresses progression to adenocarcinoma through modulation of activated Akt levels.

Distinct roles of the three Akt isoforms in lactogenic differentiation and involution

The three Akt isoforms differ in their ability to transduce oncogenic signals initiated by the Neu and PyMT oncogenes in mammary epithelia. As a result, ablation of Akt1 inhibits and ablation of Akt2 accelerates mammary tumor development by both oncogenes, while ablation of Akt3 is phenotypically almost neutral. Since the risk of breast cancer development in humans correlates with multiple late pregnancies, we embarked on a study to determine whether individual Akt isoforms also differ in their ability to transduce hormonal and growth factor signals during pregnancy, lactation and post-lactation involution. The results showed that the ablation of Akt1 delays the differentiation of the mammary epithelia during pregnancy and lactation, and that the ablation of Akt2 has the opposite effect. Finally, ablation of Akt3 results in minor defects, but its phenotype is closer to that of the wild type mice. Whereas the phenotype of the Akt1 ablation is cell autonomous, that of Akt2 is not. The ablation of Akt1 promotes apoptosis and accelerates involution, whereas the ablation of Akt2 inhibits apoptosis and delays involution. Mammary gland differentiation during pregnancy depends on the phosphorylation of Stat5a, which is induced by prolactin, a hormone that generates signals transduced via Akt. Here we show that the ablation of Akt1, but not the ablation of Akt2 or Akt3 interferes with the phosphorylation of Stat5a during late pregnancy and lactation. We conclude that the three Akt isoforms have different roles in mammary gland differentiation during pregnancy and this may reflect differences in hormonal signaling.

The roles of PTEN in development, physiology and tumorigenesis in mouse models: a tissue-by-tissue survey

In 1997, PTEN (phosphatase and tensin homologue deleted on chromosome 10, 10q23.3) was identified as an important tumor suppressor gene that is inactivated in a wide variety of human cancers. Ever since, PTEN's function has been extensively studied, and huge progress has been made in understanding PTEN's role in normal physiology and disease. In this review, we will systematically summarize the important data that have been gained from gene inactivation studies in mice and will put these data into physiological context using a tissue-by-tissue approach. We will cover mice exhibiting complete and constitutive inactivation of Pten as well as a large number of strains in which Pten has been conditionally deleted in specific tissues. We hope to highlight not only the tumor suppressive function of Pten but also its roles in embryogenesis and in the maintenance of the normal physiological functions of many organ systems.

Foxo3 is a PI3K-dependent molecular switch controlling the initiation of oocyte growth

In mammals, oocytes are packaged into compact structures-primordial follicles-which remain inert for prolonged intervals until individual follicles resume growth via a process known as primordial follicle activation. Here we show that the phosphoinositide 3-kinase (PI3K) signalling pathway controls primordial follicle activation through the forkhead transcription factor Foxo3. Within oocytes, Foxo3 is regulated by nucleocytoplasmic shuttling. Foxo3 is imported into the nucleus during primordial follicle assembly, and is exported upon activation. Oocyte-specific ablation of Pten resulted in PI3K-induced Akt activation, Foxo3 hyperphosphorylation, and Foxo3 nuclear export, thereby triggering primordial follicle activation, defining the steps by which the PI3K pathway and Foxo3 control this process. Inducible ablation of Pten and Foxo3 in adult oocytes using a new tool for genetic analysis of the germline, Vasa-Cre(ERT2), showed that this pathway functions throughout life. Thus, a principal physiologic role of the PI3K pathway is to control primordial follicle activation via Foxo3.

Signal transduction in transgenic mouse models of human breast cancer--implications for human breast cancer

The advent of genetically engineered mouse models (GEMs) of human breast cancer, have provided important insight into molecular basis or human breast cancer. This review will focus on two of the most extensively studied mouse models for human breast cancer involving mammary gland specific expression of the polyoma middle T (PyV MT) antigen and of the ErbB2. In addition, this review will discuss past and recent advances in understanding relative contribution of the signaling pathways in tumor induction and metastasis by these potent mammary oncogenes.

Genetic mosaic analysis reveals FGF receptor 2 function in terminal end buds during mammary gland branching morphogenesis

FGF signaling is associated with breast cancer and is required for mammary placode formation in the mouse. In this study, we employed a genetic mosaic analysis based on Cre-mediated recombination to investigate FGF receptor 2 (Fgfr2) function in the postnatal mammary gland. Mosaic inactivation of Fgfr2 by the MMTV-Cre transgene enabled us to compare the behavior of Fgfr2 null and Fgfr2 heterozygous cells in the same gland. Fgfr2 null cells were at a competitive disadvantage to their Fgfr2 heterozygous neighbors in the highly proliferative terminal end buds (TEBs) at the invasion front, owing to a negative effect of loss of Fgfr2 function on cell proliferation. However, Fgfr2 null cells were tolerated in mature ducts. In these genetic mosaic mammary glands, the epithelial network is apparently built by TEBs that over time are composed of a progressively larger proportion of Fgfr2-positive cells. However, subsequently, most cells lose Fgfr2 function, presumably due to additional rounds of Cre-mediated recombination. Using an independent strategy to create mosaic mammary glands, which employed an adenovirus-Cre that acts only once, we confirmed that Fgfr2 null cells were out-competed by neighboring Fgfr2 heterozygous cells. Together, our data demonstrate that Fgfr2 functions in the proliferating and invading TEBs, but it is not required in the mature ducts of the pubertal mammary gland.

Targeted disruption of Pten in ovarian granulosa cells enhances ovulation and extends the life span of luteal cells

FSH activates the phosphatidylinositol-3 kinase (PI3K)/acute transforming retrovirus thymoma protein kinase pathway and thereby enhances granulosa cell differentiation in culture. To identify the physiological role of the PI3K pathway in vivo we disrupted the PI3K suppressor, Pten, in developing ovarian follicles. To selectively disrupt Pten expression in granulosa cells, Ptenfl/fl mice were mated with transgenic mice expressing cAMP response element recombinase driven by Cyp19 promoter (Cyp19-Cre). The resultant Pten mutant mice were fertile, ovulated more oocytes, and produced moderately more pups than control mice. These physiological differences in the Pten mutant mice were associated with hyperactivation of the PI3K/acute transforming retrovirus thymoma protein kinase pathway, decreased susceptibility to apoptosis, and increased proliferation of mutant granulosa cells. Strikingly, corpora lutea of the Pten mutant mice persisted longer than those of control mice. Although the follicular and luteal cell steroidogenesis in Ptenfl/fl;Cyp19-Cre mice was similar to controls, viable nonsteroidogenic luteal cells escaped structural luteolysis. These findings provide the novel evidence that Pten impacts the survival/life span of granulosa/luteal cells and that its loss not only results in the facilitated ovulation but also in the persistence of nonsteroidogenic luteal structures in the adult mouse ovary.

PTEN deletion leads to deregulation of antioxidants and increased oxidative damage in mouse embryonic fibroblasts

Despite the significance of oxidative damage in carcinogenesis, the molecular mechanisms that lead to increased susceptibility to oxidative stress are not well understood. We now report a link between loss of protection against oxidative damage and loss of function of PTEN, a highly mutated tumor suppressor gene in a variety of human tumors. Using two-dimensional gel electrophoresis, combined with Western and Northern blot analyses, we found that PTEN deficiency in mouse embryonic fibroblasts (MEFs) displays deregulated expression of several antioxidant enzymes, including peroxiredoxins 1, 2, 5, and 6 and Cu, Zn superoxide dismutase. In these Pten-deleted MEFs, the basal levels of reactive oxygen species (ROS) were increased, and both the basal level and the ROS-induced oxidative damage of DNA were increased, as evidenced by increased levels of hydrogen peroxide (H2O2), superoxide anion, 8-hydroxy-2'-deoxyguanosine, and DNA double-strand breaks. We further show that Pten deletion is correlated with resistance to H2O2-induced expression of several antioxidants. These findings suggest an essential role for PTEN in maintaining the normal redox state of mouse embryonic fibroblasts against oxidative damage. They also provide a molecular link between PTEN, whose inactivation is known to be involved in a variety of human tumors, and antioxidants, whose perturbation leads to oxidative damage of cells.

Phosphatase and tensin homologue deleted on chromosome 10 deficiency accelerates tumor induction in a mouse model of ErbB-2 mammary tumorigenesis

Loss of the tumor suppressor phosphatase and tensin homologue deleted on chromosome 10 (PTEN) and amplification or elevated expression of ErbB-2 are both involved in human breast cancer. To directly test the importance of these genetic events in mammary tumorigenesis, we have assessed whether mammary-specific disruption of PTEN could cooperate with activation of ErbB-2. Transgenic mice expressing ErbB-2 under the transcriptional control of its endogenous promoter (ErbB-2(KI)) were interbred with mice carrying conditional PTEN alleles and an MMTV/Cre transgene. Loss of one or both PTEN alleles resulted in a dramatic acceleration of mammary tumor onset and an increased occurrence of lung metastases in the ErbB-2(KI) strain. Tumor progression in PTEN-deficient/ErbB-2(KI) strains was associated with elevated ErbB-2 protein levels, which were not due to ErbB-2 amplification or to a dramatic increase in ErbB-2 transcripts. Moreover, the PTEN-deficient/ErbB-2(KI)-derived mouse mammary tumors display striking morphologic heterogeneity in comparison with the homogeneous pathology of the ErbB-2(KI) parental strain. Therefore, inactivation of PTEN would not only have a dramatic effect on ErbB-2-induced mammary tumorigenesis but would also lead to the formation of mammary tumors that, in part, display pathologic and molecular features associated with the basal-like subtype of primary human breast cancer.

Taming the PI3K team to hold inflammation and cancer at bay

Recent progress in understanding the molecular mechanisms of receptor signal transduction is continuously highlighting new unforeseen potential drug targets for yet unmet therapeutic needs. While the large number of different cell surface receptors challenge the concept of antagonists development, the finding of signal transduction platforms common to multiple receptor families has boosted the development of new therapeutic approaches. The identification of the role of phosphoinositide 3-kinase family members downstream receptors as directors of multiple cellular responses ranging from cell proliferation and survival to immunity and cardiovascular control, is an example of successful drug target validation studies. This review will focus on these findings and on the ongoing efforts to tame this family of enzymes to beat inflammation and cancer.

Portrait of PTEN: messages from mutant mice

PTEN is a tumor suppressor gene mutated in many human sporadic cancers and in hereditary cancer syndromes such as Cowden disease. The major substrate of PTEN is phosphatidylinositol-3,4,5-trisphosphate (PI(3,4,5)P3), a second messenger molecule produced following PI3K activation induced by a variety of stimuli. PI(3,4,5)P3 activates the serine-threonine kinase Akt, which is involved in antiapoptosis, proliferation and oncogenesis. In mice, heterozygosity for a null mutation of Pten (Pten(+/-)mice) frequently leads to the development of a variety of cancers and autoimmune disease. Homozygosity for the null mutation (Pten(-/-) mice) results in early embryonic lethality, precluding the functional analysis of Pten in adult tissues and organs. To investigate the physiological functions of Pten in viable mice, we and other groups have used the Cre-loxP system to generate various tissue-specific Pten mutations. The present review will summarize results obtained from the study of conditional mutant mice lacking Pten in specific tissues, and discuss the possible biological and molecular explanations for why Pten deficiency leads to tumorigenesis.

Targeting the RNA-binding protein Sam68 as a treatment for cancer?

The contradictory properties of RNA-binding proteins (RBPs) have mystified their roles in human diseases including cancer. Are certain RBPs oncogenes or tumor suppressors? In the case of the signal transduction activator of RNA metabolism (STAR) family of hnRNP K homology (KH)-domain-containing RBPs, the dominant view with loose experimental evidence is that these proteins are tumor suppressors. However, recent developments support a pro-oncogenic role for archetypical STAR protein Sam68. Sam68-null mice are not prone to cancer, but instead display pronounced defects in mammary gland ductal development, and haploinsufficiency of Sam68 impedes mammary tumor onset and tumor multiplicity in mouse models expressing the mammary-targeted polyoma middle T antigen oncogene. These advances have increased the interest in the role of Sam68 as a positive regulator of cancer progression and position Sam68 as a viable therapeutic target. Retrospective and perspective implications of Sam68 in cancer are discussed.

Phosphatase and tensin homolog, deleted on chromosome 10 deficiency in brain causes defects in synaptic structure, transmission and plasticity, and myelination abnormalities

The phosphatidylinositol 3-kinase (PI3K) signaling pathway modulates growth, proliferation and cell survival in diverse tissue types and plays specialized roles in the nervous system including influences on neuronal polarity, dendritic branching and synaptic plasticity. The tumor-suppressor phosphatase with tensin homology (PTEN) is the central negative regulator of the PI3K pathway. Germline PTEN mutations result in cancer predisposition, macrocephaly and benign hamartomas in many tissues, including Lhermitte-Duclos disease, a cerebellar growth disorder. Neurological abnormalities including autism, seizures and ataxia have been observed in association with inherited PTEN mutation with variable penetrance. It remains unclear how loss of PTEN activity contributes to neurological dysfunction. To explore the effects of Pten deficiency on neuronal structure and function, we analyzed several ultra-structural features of Pten-deficient neurons in Pten conditional knockout mice. Using Golgi stain to visualize full neuronal morphology, we observed that increased size of nuclei and somata in Pten-deficient neurons was accompanied by enlarged caliber of neuronal projections and increased dendritic spine density. Electron microscopic evaluation revealed enlarged abnormal synaptic structures in the cerebral cortex and cerebellum. Severe myelination defects included thickening and unraveling of the myelin sheath surrounding hypertrophic axons in the corpus callosum. Defects in myelination of axons of normal caliber were observed in the cerebellum, suggesting intrinsic abnormalities in Pten-deficient oligodendrocytes. We did not observe these abnormalities in wild-type or conditional Pten heterozygous mice. Moreover, conditional deletion of Pten drastically weakened synaptic transmission and synaptic plasticity at excitatory synapses between CA3 and CA1 pyramidal neurons in the hippocampus. These data suggest that Pten is involved in mechanisms that control development of neuronal and synaptic structures and subsequently synaptic function.

High-resolution mapping of DNA breakpoints to define true recurrences among ipsilateral breast cancers

BACKGROUND

To distinguish new primary breast cancers from true recurrences, pangenomic analyses of DNA copy number alterations (CNAs) using single-nucleotide polymorphism arrays have proven useful.

METHODS

The pangenomic profiles of 22 pairs of primary breast carcinoma (ductal or lobular) and ipsilateral breast cancers from the same patients were analyzed. Hierarchical clustering was performed using CNAs and DNA breakpoint information. A partial identity score developed using DNA breakpoint information was used to quantify partial identities between two tumors. The nature of ipsilateral breast cancers (true recurrence vs new primary tumor) as defined using the clustering methods and the partial identity score was compared with that based on clinical characteristics. Metastasis-free survival was compared among patients with primary tumors and true recurrences as defined using the partial identity score and by clinical characteristics. All statistical tests were two-sided.

RESULTS

All methods agreed on the nature of ipsilateral breast cancers for 14 pairs of samples. For five pairs, the clinical definition disagreed with both clustering methods. For three pairs, the two clustering methods were discordant and the one using DNA breakpoints agreed with the clinical definition. The partial identity score confirmed the nature of ipsilateral breast cancers as defined by clustering of DNA breakpoints in 21 of 22 pairs. The difference in metastasis-free survival of patients with new primary tumors and those with true recurrences was not statistically significant when tumors were defined based on clinical and histologic characteristics (5-year metastasis-free survival: 76%, 95% confidence interval [CI] = 52% to 100% for new primary tumors and 38%, 95% CI = 17% to 83% for true recurrences; P = .18; new primary tumor vs true recurrence, hazard ratio = 2.8, 95% CI = 0.6 to 13.7), but the difference was statistically significant when tumors were defined using the partial identity score (5-year metastasis-free survival: 100% for new primary tumors and 29%, 95% CI = 11% to 78% for true recurrences; P = .01).

CONCLUSIONS

DNA breakpoint information more often agreed with the clinical determination than CNAs in this population. The partial identity score, which was calculated based on DNA breakpoints, allows statistical discrimination between new primary tumors and true recurrences that could outperform the clinical determination in terms of prognosis.