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Biomedical subjects

Robert A Weinberg

Publications and source records attributed to Robert A Weinberg.

At least 19 recordsLinked to original sources
PubMed2006

The Spemann organizer gene, Goosecoid, promotes tumor metastasis.

The process of invasion and metastasis during tumor progression is often reminiscent of cell migration events occurring during embryonic development. We hypothesized that genes controlling cellular changes in the Spemann organizer at gastrulation might be reactivated in tumors. The Goosecoid homeobox transcription factor is a known executer of cell migration from the Spemann organizer. We found that indeed Goosecoid is overexpressed in a majority of human breast tumors. Ectopic expression of Goosecoid in human breast cells generated invasion-associated cellular changes, including an epithelial-mesenchymal transition. TGF-beta signaling, known to promote metastasis, induced Goosecoid expression in human breast cells. Moreover, Goosecoid significantly enhanced the ability of breast cancer cells to form pulmonary metastases in mice. These results demonstrate that Goosecoid promotes tumor cell malignancy and suggest that other conserved organizer genes may function similarly in human cancer.

Aging↗
PubMed2006

Stromal fibroblasts in cancer: a novel tumor-promoting cell type.

Tumors are highly complex tissues composed of neoplastic cells and, in the case of carcinomas, stromal cell compartments containing a variety of mesenchymal cells, notably fibroblasts, myofibroblasts, endothelial cells, pericytes, and a variety of inflammatory cells associated with the immune system. Fibroblasts and myofibroblasts often represent the majority of the stromal cells within various types of human carcinomas, yet the specific contributions of these cells to tumor growth are poorly understood. Recent work has demonstrated that stromal fibroblast fractions, named carcinoma-associated fibroblasts (CAFs), that have been extracted from a number of invasive human breast carcinomas are more competent to promote the growth of mammary carcinoma cells and to enhance tumor angiogenesis than are comparable cells derived from outside of these tumor masses. CAFs include large populations of myofibroblasts that secrete elevated levels of stromal cell-derived factor 1 (SDF-1), also called CXCL12, which plays a central role in the promotion of tumor growth and angiogenesis; CAF-derived SDF-1 not only stimulates carcinoma cell growth directly through the CXCR4 receptor displayed on tumor cells but also serves to recruit endothelial progenitor cells (EPCs) into tumors, thereby furthering neoangiogenesis. In this review, we highlight the importance of this SDF-1-CXCR4 signaling pathway in the tumor microenvironment and discuss the mechanisms by which stromal fibroblasts within mammary carcinomas enhance tumor growth.

Animals↗
PubMed2006

A lost generation.

The funding policies of the NIH have made it increasingly difficult for young researchers to procure research funds. This threatens to drive a whole generation of young people away from careers in basic biomedical research.

Age Factors↗
PubMed2006

Exploring a new twist on tumor metastasis.

Unraveling the genetic programs that drive -metastasis may offer insights into how to limit or prevent this deadly aspect of cancer progression. Our recent studies indicate that tumor cell metastasis involves the activity of the transcription factor, Twist, which regulates epithelial-mesenchymal transition and early embryonic morphogenesis. Here, we review the Twist signaling pathway during normal development and discuss how the transcription factor Twist and the epithelial-mesenchymal transition program impinge their biological functions during tumor metastasis.

Animals↗
PubMed2006

Telomeres: cancer to human aging.

The cell phenotypes of senescence and crisis operate to circumscribe the proliferative potential of mammalian cells, suggesting that both are capable of operating in vivo to suppress the formation of tumors. The key regulators of these phenotypes are the telomeres, which are located at the ends of chromosomes and operate to protect the chromosomes from end-to-end fusions. Telomere erosion below a certain length can trigger crisis. The relationship between senescence and telomere function is more complex, however: Cell-physiological stresses as well as dysfunction of the complex molecular structures at the ends of telomeric DNA can trigger senescence. Cells can escape senescence by inactivating the Rb and p53 tumor suppressor proteins and can surmount crisis by activating a telomere maintenance mechanism. The resulting cell immortalization is an essential component of the tumorigenic phenotype of human cancer cells. Here we discuss how telomeres are monitored and maintained and how loss of a functional telomere influences biological functions as diverse as aging and carcinogenesis.

Aging↗
PubMed2005

The melanocyte differentiation program predisposes to metastasis after neoplastic transformation.

The aggressive clinical behavior of melanoma suggests that the developmental origins of melanocytes in the neural crest might be relevant to their metastatic propensity. Here we show that primary human melanocytes, transformed using a specific set of introduced genes, form melanomas that frequently metastasize to multiple secondary sites, whereas human fibroblasts and epithelial cells transformed using an identical set of genes generate primary tumors that rarely do so. Notably, these melanomas have a metastasis spectrum similar to that observed in humans with melanoma. These observations indicate that part of the metastatic proclivity of melanoma is attributable to lineage-specific factors expressed in melanocytes and not in other cell types analyzed. Analysis of microarray data from human nevi shows that the expression pattern of Slug, a master regulator of neural crest cell specification and migration, correlates with those of other genes that are important for neural crest cell migrations during development. Moreover, Slug is required for the metastasis of the transformed melanoma cells. These findings indicate that melanocyte-specific factors present before neoplastic transformation can have a pivotal role in governing melanoma progression.

Animals↗
PubMed2005

A mouse model of human breast cancer metastasis to human bone.

Currently, an in vivo model of human breast cancer metastasizing from the orthotopic site to bone does not exist, making it difficult to study the many steps of skeletal metastasis. Moreover, models used to identify the mechanisms by which breast cancer metastasizes to bone are limited to intracardiac injection, which seeds the cancer cells directly into the circulation, thus bypassing the early steps in the metastatic process. Such models do not reflect the full process of metastasis occurring in patients. We have developed an animal model of breast cancer metastasis in which the breast cancer cells and the bone target of osteotropic metastasis are both of human origin. The engrafted human bone is functional, based on finding human IgG in the mouse bloodstream, human B cells in the mouse spleen, and normal bone histology. Furthermore, orthotopic injection of a specific human breast cancer cell line, SUM1315 (derived from a metastatic nodule in a patient), later resulted in both bone and lung metastases. In the case of bone, metastasis was to the human implant and not the mouse skeleton, indicating a species-specific osteotropism. This model replicates the events observed in patients with breast cancer skeletal metastases and serves as a useful and relevant model for studying the disease.

Animals↗
PubMed2005

Stromal fibroblasts present in invasive human breast carcinomas promote tumor growth and angiogenesis through elevated SDF-1/CXCL12 secretion.

Fibroblasts often constitute the majority of the stromal cells within a breast carcinoma, yet the functional contributions of these cells to tumorigenesis are poorly understood. Using a coimplantation tumor xenograft model, we demonstrate that carcinoma-associated fibroblasts (CAFs) extracted from human breast carcinomas promote the growth of admixed breast carcinoma cells significantly more than do normal mammary fibroblasts derived from the same patients. The CAFs, which exhibit the traits of myofibroblasts, play a central role in promoting the growth of tumor cells through their ability to secrete stromal cell-derived factor 1 (SDF-1); CAFs promote angiogenesis by recruiting endothelial progenitor cells (EPCs) into carcinomas, an effect mediated in part by SDF-1. CAF-secreted SDF-1 also stimulates tumor growth directly, acting through the cognate receptor, CXCR4, which is expressed by carcinoma cells. Our findings indicate that fibroblasts within invasive breast carcinomas contribute to tumor promotion in large part through the secretion of SDF-1.

Actins↗
PubMed2004

The signals and pathways activating cellular senescence.

Cellular senescence is a program activated by normal cells in response to various types of stress. These include telomere uncapping, DNA damage, oxidative stress, oncogene activity and others. Senescence can occur following a period of cellular proliferation or in a rapid manner in response to acute stress. Once cells have entered senescence, they cease to divide and undergo a series of dramatic morphologic and metabolic changes. Cellular senescence is thought to play an important role in tumor suppression and to contribute to organismal aging, but a detailed description of its physiologic occurrence in vivo is lacking. Recent studies have provided important insights regarding the manner by which different stresses and stimuli activate the signaling pathways leading to senescence. These studies reveal that a population of growing cells may suffer from a combination of different physiologic stresses acting simultaneously. The signaling pathways activated by these stresses are funneled to the p53 and Rb proteins, whose combined levels of activity determine whether cells enter senescence. Here we review recent advances in our understanding of the stimuli that trigger senescence, the molecular pathways activated by these stimuli, and the manner by which these signals determine the entry of a population of cells into senescence.

Cellular Senescence↗
PubMed2004

Twist, a master regulator of morphogenesis, plays an essential role in tumor metastasis.

Metastasis is a multistep process during which cancer cells disseminate from the site of primary tumors and establish secondary tumors in distant organs. In a search for key regulators of metastasis in a murine breast tumor model, we have found that the transcription factor Twist, a master regulator of embryonic morphogenesis, plays an essential role in metastasis. Suppression of Twist expression in highly metastatic mammary carcinoma cells specifically inhibits their ability to metastasize from the mammary gland to the lung. Ectopic expression of Twist results in loss of E-cadherin-mediated cell-cell adhesion, activation of mesenchymal markers, and induction of cell motility, suggesting that Twist contributes to metastasis by promoting an epithelial-mesenchymal transition (EMT). In human breast cancers, high level of Twist expression is correlated with invasive lobular carcinoma, a highly infiltrating tumor type associated with loss of E-cadherin expression. These results establish a mechanistic link between Twist, EMT, and tumor metastasis.

Animals↗
PubMed2004

Reconstruction of functionally normal and malignant human breast tissues in mice.

The study of normal breast epithelial morphogenesis and carcinogenesis in vivo has largely used rodent models. Efforts at studying mammary morphogenesis and cancer with xenotransplanted human epithelial cells have failed to recapitulate the full extent of development seen in the human breast. We have developed an orthotopic xenograft model in which both the stromal and epithelial components of the reconstructed mammary gland are of human origin. Genetic modification of human stromal cells before the implantation of ostensibly normal human mammary epithelial cells resulted in the outgrowth of benign and malignant lesions. This experimental model allows for studies of human epithelial morphogenesis and differentiation in vivo and underscores the critical role of heterotypic interactions in human breast development and carcinogenesis.

Animals↗
PubMed2004

Species- and cell type-specific requirements for cellular transformation.

Recent evidence suggests that human cells require more genetic changes for neoplastic transformation than do their murine counterparts. However, a precise enumeration of these differences has never been undertaken. We have determined that perturbation of two signaling pathways-involving p53 and Raf-suffices for the tumorigenic conversion of normal murine fibroblasts, while perturbation of six pathways-involving p53, pRb, PP2A, telomerase, Raf, and Ral-GEFs-is needed for human fibroblasts. Cell type-specific differences also exist in the requirements for tumorigenic transformation: immortalized human fibroblasts require the activation of Raf and Ral-GEFs, human embryonic kidney cells require the activation of PI3K and Ral-GEFs, and human mammary epithelial cells require the activation of Raf, PI3K, and Ral-GEFs.

Animals↗
PubMed2004

High efficiency creation of human monoclonal antibody-producing hybridomas.

The native human antibody repertoire holds unexplored potential for the development of novel monoclonal antibody therapeutics. Current techniques that fuse immortal cells and primary B-lymphocytes are sub-optimal for the routine production of hybridomas that secrete human monoclonal antibodies. We have found that a murine cell line that ectopically expresses murine interleukin-6 (mIL-6) and human telomerase (hTERT) efficiently forms stable human antibody-secreting heterohybridomas through cell fusion with primary human B-lymphocytes. The hybrid cells maintain secretion of human antibodies derived from the primary B-lymphocytes through multiple rounds of cloning. Using splenic B-lymphocytes from a patient immunized with a Streptococcus pneumoniae capsular polysaccharide vaccine, we have succeeded in creating hybridomas that secrete human monoclonal antibodies specific for S. pneumoniae antigens. Using peripheral blood lymphocytes, we have similarly cloned a human antibody that binds a viral antigen. These experiments establish that SP2/0-derived cell lines ectopically expressing mIL-6 and hTERT will enable the rapid cloning of native human monoclonal antibodies.

Animals↗
PubMed2004

When cells get stressed: an integrative view of cellular senescence.

Cells entering a state of senescence undergo a permanent cell cycle arrest, accompanied by a set of functional and morphological changes. Senescence of cells occurs following an extended period of proliferation in culture or in response to various physiologic stresses, yet little is known about the role this phenomenon plays in vivo. The study of senescence has focused largely on its hypothesized role as a barrier to extended cell division, governed by a division-counting mechanism in the form of telomere length. Here, we discuss the biological functions of cellular senescence and suggest that it should be viewed in terms of its role as a general cellular stress response program, rather than strictly as a barrier to unlimited cycles of cell growth and division. We also discuss the relative roles played by telomere shortening and telomere uncapping in the induction of senescence.

Animals↗
PubMed2003

Telomerase maintains telomere structure in normal human cells.

In normal human cells, telomeres shorten with successive rounds of cell division, and immortalization correlates with stabilization of telomere length. These observations suggest that human cancer cells achieve immortalization in large part through the illegitimate activation of telomerase expression. Here, we demonstrate that the rate-limiting telomerase catalytic subunit hTERT is expressed in cycling primary presenescent human fibroblasts, previously believed to lack hTERT expression and telomerase activity. Disruption of telomerase activity in normal human cells slows cell proliferation, restricts cell lifespan, and alters the maintenance of the 3' single-stranded telomeric overhang without changing the rate of overall telomere shortening. Together, these observations support the view that telomerase and telomere structure are dynamically regulated in normal human cells and that telomere length alone is unlikely to trigger entry into replicative senescence.

Antibodies, Monoclonal↗
PubMed2003

Erosion of the telomeric single-strand overhang at replicative senescence.

Cultured primary human cells inevitably enter a state of replicative senescence for which the specific molecular trigger is unknown. We show that the single-strand telomeric overhang, a key component of telomere structure, is eroded at senescence. Expression of telomerase prevents overhang loss, suggesting that this enzyme prevents senescence by maintaining proper telomere structure. In contrast, progressive overhang loss occurs in cells that avoid senescence through the inactivation of p53 and Rb, indicating that overhang erosion is the result of continuous cell division and not a consequence of senescence. We thus provide evidence for a specific molecular alteration in telomere structure at senescence and suggest that this change, rather than overall telomere length, serves to trigger this state.

Antigens, Viral, Tumor↗
PubMed2003

Ras modulates Myc activity to repress thrombospondin-1 expression and increase tumor angiogenesis.

Tumor angiogenesis is postulated to be regulated by the balance between pro- and anti-angiogenic factors. We demonstrate that the critical step in establishing the angiogenic capability of human cells is the repression of the critical anti-angiogenic factor, thrombospondin-1 (Tsp-1). This repression is essential for tumor formation by mammary epithelial cells and kidney cells engineered to express SV40 early region proteins, hTERT, and H-RasV12. We have uncovered the signaling pathway leading from Ras to Tsp-1 repression. Ras induces the sequential activation of PI3 kinase, Rho, and ROCK, leading to activation of Myc through phosphorylation; phosphorylation of Myc via this mechanism enables it to repress Tsp-1 expression. We thus describe a novel mechanism by which the cooperative activity of the oncogenes, ras and myc, leads directly to angiogenesis and tumor formation.

Cell Line↗