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RESEARCH BRIEFSReceptor's Effect on Polarity May Switch on Breast Tumor GrowthThe epidermal growth factor receptors, ErbB1 and ErbB2, have been implicated in tumorigenesis of breast tissue epithelium. However, since they are activated through homo- and hetero-dimerization, evaluating the contribution of individual receptors to cell growth has proven difficult. Now, in September's Nature Cell Biology, HMS researchers show that homodimers of ErbB2, but not ErbB1, can induce epithelial cell proliferation. HMS research fellow Senthil Muthuswamy and colleagues, working in the lab of Joan Brugge, HMS professor of cell biology, used a unique strategy to selectively homodimerize receptors. Chimeras encoding ErbB proteins linked to the binding protein for the ligand FK506 (FKBP) were transfected into epithelial cells and their dimerization induced by adding the ligand AP1510. This bivalent ligand, which contains two FK506 moieties covalently attached, effectively glues chimeras together through their FKBP domains.
ErbB2 dimerization causes proliferation and morphological changes in cultured breast epithelial cells. Inset shows untreated tissue. Courtesy of Senthil Muthuswamy The study tested this strategy in a 3-D cell culture system that mimics epithelial acini found in breast tissue. MCF-10A cells grown in a laminin- and collagen-IV-rich matrix formed single circular layers with a central lumen. These became growth-arrested after about eight days and were polarized. Addition of the dimerizer to in vitro acini expressing ErbB2 chimeras reinitiated cell growth, as demonstrated by expression of the proliferating cell antigen Ki67. In addition, about 70 percent of the acini lost their polarized organization and their lumina became filled. These new structures were at least 10-fold larger than the original acinus and shared many properties with noninvasive early stage carcinomas. In contrast, ErbB1 dimers had no effect. These changes in morphology and proliferative capability may be directly related to ErbB2's effect on cell polarity. In MDCK cells, for example, ErbB2 caused marked changes in localization of normally polarized cell proteins, whereas ErbB1 had no effect. ErbB2 dimerization failed to induce anchorage-independent growth, however, suggesting that though this receptor may be critical in the early stages of breast cancer development, additional signals are required to induce tumor invasion.
Complementary Therapies Are Here to StayA great number of Americans now use some form of complementary or alternative medical (CAM) therapy. Most medical schools have initiated CAM courses, and some managed care companies have begun insurance coverage for some of these treatments. But are CAM therapies a passing fad or are they here to stay? A report from the HMS Center for Alternative Medicine Research and Education suggests the latter. The study, directed by Ronald Kessler, HMS professor of health care policy, appears in the Aug. 21 Annals of Internal Medicine. It examines trends in the use of 20 different CAM therapies, from acupuncture to yoga, among representative sociodemographic groups across the continental U.S. Data compiled from more than 2,000 interviews showed a trend toward the use of these therapies in younger respondents: by age 33, seven out of 10 post–baby boomers (born 1965–79) had used some type of CAM, compared to five out of 10 baby boomers (born 1945–64) and three out of 10 pre–baby boomers (born before 1945). However, in all age groups the use of CAM therapies has steadily increased since the 1950s. The use of alternative treatments was independent of gender, ethnicity, and level of education. Regional trends and city vs. rural differences were also absent. Most of the 20 therapies have steadily increased in popularity since the 1960s, with the largest overall growth rate occurring during the transition from the 1960s to the 1970s. Some individuals reported using alternative therapies for many years; of those who had tried an alternative therapy, almost 50 percent were still using it 11 to 20 years later. Overall the findings dispel suggestions that CAM therapies are a passing fad, used by a particular generation or certain fringe groups.
Weakness Found in Replication of Epstein–Barr VirusThough Epstein–Barr virus (EBV), a member of the herpesvirus family, is probably best known for causing mononucleosis, it has also been implicated in the etiology of Hodgkins disease, Burkitt's tumor, and nasopharyngeal carcinomas. In immuno- compromised patients, it is responsible for various lymphomas, including those of the central nervous system. To date, there is no treatment available for EBV, and it does not respond to antiviral agents. However, recent data from Brigham and Women's Hospital offers new insights into the virus's reproductive mechanism that may lead to new therapies. Most of the machinery needed for replication of this virus's DNA comes from the host cell. In fact, this process requires only one viral-encoded protein, EBNA-1. In the Aug. 10 Cell, research fellows Suman Dhar and Kenichi Yoshida, working under the direction of Anindya Dutta, HMS associate professor of pathology, show that EBNA-1 most likely interacts with the human origin-recognition-complex protein ORC2. This protein is essential for mitosis in human cells. The researchers were able to introduce an ORC2 mutation into a colon carcinoma cell line that reduces the cellular level of the protein by almost 95 percent. Though these viable cells had a lower proliferation rate than wild type, DNA synthesis proceeded unchanged at two loci known to contain origins of replication, c-myc and beta-globin. In contrast to wild type, the mutant cells failed to replicate plasmids carrying the gene for EBNA-1 and oriP, EBV's only origin of replication. Furthermore, chromatin immunoprecipitation assays indicated that ORC2 binds to oriP, an interaction that is most likely mediated by EBNA-1, which was found in anti-ORC2 immunoprecipitates. The data suggest that although common proteins are used for viral and host cell DNA replication, the former may be more susceptible to interference and thus may provide a target for drug development.
Genome Shows Relative Youth of Deadly Malaria ParasiteThe parasite Plasmodium falciparum is responsible for millions of cases of malaria worldwide. Its success can be attributed, at least partially, to widespread genetic variation, including differences in drug resistance, pathogenicity, and antigenicity, which all too often allow it to evade the immune system of its hosts. Though the extent of this variation suggests that the parasite has been around for eons, recent data from the Harvard School of Public Health suggest that P. falciparum is, in fact, a relative newcomer. Sarah Volkman, working in the lab of Dyann Wirth, HSPH professor of tropical public health, and coworkers in the Harvard–Oxford Malaria Genome Diversity Project, have searched for sequence differences in introns from eight independent isolates of the parasite. Introns are among the most rapidly evolving DNA sequences and can be used to assess mutation rates and the age of an organism. Their study, which appeared in the July 20 Science, surprisingly turned up only one certain single-nucleotide polymorphism (SNP) among more than 3,000 base pairs from 25 introns. When this finding was coupled with previous data on synonymous substitution rates in 10 different genes, the authors could calculate the age of the parasite to be between 3,000 and 7,700 years old, indicating that all extant strains of P. falciparum are derived from a recent common ancestor. This data fits in nicely with the finding that mitochondrial DNA is virtually identical in all strains of the parasite and with the rise of slash-and-burn agriculture in tropical Africa about 6,000 years ago, which is thought to have provided ideal conditions for propagation of the parasite. The age of the parasite seems inconsistent with its genetic variability. This may be explained partly by replication slippage, a phenomenon in which DNA polymerase retraces its steps and duplicates part of the genetic material and partly by extremely strong selection pressure imposed as the organism moves through genetically diverse populations. —Briefs by Tom Fagan |
