|
The Giovanni Armenise-Harvard Foundation Session 3: Control of Cell Proliferation Overview Cancer cells are essentially good cells that have gone bad, and this session examined some of the wrong turns they can take. Speakers described two different ways in which the ubiquitin system, which normally marks cells for destruction when they have outlived their usefulness, can go awry. The ability of human papilloma virus or Epstein-Barr virus to disrupt normal constraints on growth were discussed, as well as the possible tumor-suppressing capacity of various proteins. Another intriguing presentation suggested that plants, like animals, have a mechanism for recognizing their potential enemies.
Many proteins are essential during certain phases of the cell's reproductive cycle, but once they've done their job they must be swiftly eliminated. These proteins are marked for death by the attachment of a small protein called ubiquitin; once tagged, they will be hauled off and destroyed by the cell's internal garbage collectors. Key proteins regulated by ubiquitination include the tumor suppressor p53, the c-jun and c-fos transcription factors, the cyclin A and B proteins, the NFkB transcription factor and its inhibitor IkB, and the p27 cyclin-dependent kinase inhibitor. Now there is preliminary evidence that certain types of ubiquitin isopeptidases can sneak in and remove the polyubiquitin chain from some proteins that have been labeled for timely destruction-perhaps causing them to stay too long and cause abnormal growth. Although scientists have not directly observed this in animals, there is experimental evidence that certain isopeptidases are implicated in growth control. Because ubiquitination regulates the degradation of so many key proteins in humans, Dr. Draetta and his colleagues decided to search for human ubiquitin isopeptidases that might play a role in growth and cell cycle control. They have identified and characterized a novel ubiquitin isopeptidase, called UBPY, that appears to play a critical role in controlling cell cycle progression. Now they are seeking to characterize others, and to figure out which proteins are hanging around too long due to their actions.
Just as ubiquitin isopeptidases may encourage runaway cell growth by removing the tag that rightly identifies proteins for timely destruction, ubiquitin protein ligases appear capable of the opposite effect. By prematurely attaching a "kill me" sign to proteins that are desperately needed for normal control, these ligases may set the stage for uncontrolled cell growth. This appears to happen when people are infected with one of the carcinogenic strains of human papillomavirus (HPV). This finding has important clinical implications, as these dangerous HPVs are responsible for 90% of all cervical cancer as well as with tumors of the vagina, vulva, penis, and perianal region. Cancer-causing strains of HPV encode two viral genes that become integrated into the DNA of cervical cells: E6 produces a protein that teams up with a ubiquitin protein ligase that has been labeled E6AP (E6 associated protein). Together, E6 and E6AP target and destroy the tumor suppressor protein p53. With p53 out of the way, potentially carcinogenic mutations accumulate in the cell's DNA. Another HPV gene, E7, produces a protein that targets a second tumor suppressor, the retinoblastoma protein (pRB), thus permitting the cell to divide uncontrollably.
This presentation provided additional detail about the actions of the E7 ubiquitin protein ligase produced by carcinogenic strains of human papillomavirus (HPV). As mentioned in the previous talk by Dr. Howley, E7 appears to interfere with the tumor-suppressing activity of the retinoblastoma protein (pRB). In normal cells, this protein acts as a brake on the cell division cycle; in many human cancers pRB is inactivated and cells are able to divide non-stop as a result. In the course of this study, healthy human cells were engineered to express normal HPV E7 or a mutant form incapable of marking pRB for premature destruction. The researchers found that pRB levels fell in the cells that expressed active HPV E7, but remained high in cells with the biologically inactive form. When the researchers measured the stability of pRB in cells expressing normal and mutant E7 proteins, they found that exposure to normal E7 quickly destabilized pRB. Once this tumor-suppressing protein is weakened by the action of E7, the researchers believe that other molecules move in for the kill. They are seeking to identify these assassins, and hope that eventually a clearer understanding of these mechanisms could lead to improvements in cervical cancer treatment.
Human papillomavirus is not the only virus, of course, that that has been associated with human cancers. Since the 1960s, Epstein-Barr virus( EBV) has been linked to cancers found in Africa and Asia. In the industrialized world, EBV rarely caused more than a sore throat or mononucleosis-until the number of patients with compromised immune systems began rising due to AIDS to the use of drugs that prevent rejection of organ transplants. Some years ago, Dr Kieff's team found that EBV makes a substance called latent membrane protein (LMP-1) which appears to play a part in triggering EBV-associated cancers such as lymphoproliferative disease, nasopharyngeal cancer, or Hodgkin's disease. The next step was to identify LMP1's cellular accomplice-a human protein that, when altered by LMP1, promotes abnormal cell growth. The researchers used genetic techniques to find a likely candidate, dubbed LAP1, which promotes cell growth when it acts in concert with LMP1. The researchers then considered what else LAP1 might do in human cells. When they compared its DNA sequence with other known genes, they found that it resembles the mouse gene for a protein that helps transmit signals from a growth factor receptor (TNFR). When they tested this idea, LAP1 interacted with three types of TNFR receptors. One of these, called CD40, leads to cell proliferation when activated. It is typically found on malignant cells from patients with Hodgkin's disease or nasopharyngeal carcinoma. As a result of these investigations, Dr. Kieff now regards LAP1 as a protein that contributes to cancerous cell proliferation in two distinct ways: it causes trouble by interacting with the viral protein LMP1 or, alternatively, it can stimulate the CD40 growth factor receptor. A clearer understanding of these pathways may advance the search for novel anti-cancer drugs.
Unlike humans, plants do not have antibodies to protect them against disease-producing organisms. Yet plants are not totally at the mercy of their enemies, because they do have a sophisticated defense system that springs into action at the sites of an infection. Proteins that encode leucine-rich repeats (LRRs) play a central role in the recognition of foreign invaders. Dr. De Lorenzo described how polygalacturonase-inhibiting protein (PGIP), a type of LRR, recognizes polygalacturonases-harmful enzymes that fungi use to damage plant cells. PGIP's ability recognize polygalacturonase secreted by an invader, and to oppose its activity, is a valuable model system for understanding how plants recognize non-self molecules.
Dr. Pelicci described the identification of a promyelocytic leukemia (PML) protein that appears to suppress tumor growth by inducing apoptosis (death) in malignant cells. Researchers studied PML's role in the context of a leukemia-specific fusion protein called RAR, a protein known to regulate cell differentiation. Experiments in transgenic mice revealed that 30% of the PML/RAR mice developed leukemia within 1 year of life. Promyelocytic leukemia arises from a chromosomal translocation that gives patients with this disease a better prognosis than those with many similar leukemias. The genetic abnormality in PML makes cells sensitive to large doses of retinoic acid, and this treatment often results in remission. Researchers are now seeking to learn more about how retinoic acid induces differentiation of leukemic cells and exactly what role the PML and RAR proteins might play.
Previous speakers described three structurally similar receptor tyrosine kinases, members of the hepatocyte growth factor receptor (HGFR) family, that are encoded by the proto-oncogenes MET, RON and SEA. In this study, the researchers set out to determine whether MET, acting on its own, could induce changes leading to a metastatic type of cancer cell-one that can migrate from the primary tumor site and cause malignant growth elsewhere in the body. The investigators deliberately induced a mutation in the multifunctional docking site of the Met protein, which is known to bind any of several intracellular transducers. The study showed that while a single point mutation affecting signal transduction promoted malignant transformation, it resulted in cancer cells with no metastatic potential. These results led the researchers to conclude that MET can't generate metastatic cells on its own, and that this probably requires concomitant activation of one or more other signaling pathways. The protein encoded by RON may or may not be a player in this process.
|
Ubiquitin isopeptidases in growth control
