Retinoblastoma. For our eyes only

Diabetes mellitus and Alzheimer's disease: shared pathology and treatment?

Epidemiological and basic science evidence suggest a possible shared pathophysiology between type 2 diabetes mellitus (T2DM) and Alzheimer's disease (AD). It has even been hypothesized that AD might be 'type 3 diabetes'. The present review summarizes some of the evidence for the possible link, putative biochemical pathways and ongoing clinical trials of antidiabetic drugs in AD patients. The primary and review literature were searched for articles published in peer-reviewed sources that were related to a putative connection between T2DM and AD. In addition, public sources of clinical trials were searched for the relevant information regarding the testing of antidiabetic drugs in AD patients. The evidence for a connection between T2DM and AD is based upon a variety of diverse studies, but definitive biochemical mechanisms remain unknown. Additional study is needed to prove the existence or the extent of a link between T2DM and AD, but sufficient evidence exists to warrant further study. Presently, AD patients might benefit from treatment with pharmacotherapy currently used to treat T2DM and clinical trials of such therapy are currently underway.

Insulin and the brain

Circulating insulin crosses the blood-brain barrier (BBB) into the central nervous system (CNS). There are many insulin receptors in various areas of the brain; they are expressed by both astrocytes and neurons. The two main insulin actions in the brain are (a) control of food intake and (b) effect on cognitive functions. In obesity there is a relative insulin deficiency in the CNS despite increased circulating levels. Insulin plays an important role in cognitive functions as demonstrated by the intranasal administration of insulin bypassing the liver. Brain insulin decreases with aging and may be related to the decrease in cognitive functions, as has also been reported in Alzheimer's disease. Certain brain tumours over-express insulin receptors. Whether the larger insulin analogues pass the BBB is as yet not known.

50 Years of Preclinical Anticancer Drug Screening: Empirical to Target-Driven Approaches

The number of anticancer agents that fail in the clinic far outweighs those considered effective, suggesting that the selection procedure for progression of molecules into the clinic requires improvement. The value of any preclinical model will ultimately depend on its ability to accurately predict clinical response. This review focuses on the major contributions of preclinical screening models to anticancer drug development over the past 50 years. Over time, a general transition has been observed from the empirical drug screening of cytotoxic agents against uncharacterized tumor models to the target-orientated drug screening of agents with defined mechanisms of action. New approaches to anticancer drug development involve the molecular characterization of models along with an appreciation of the pharmacodynamic and pharmacokinetic properties of compounds [e.g., the US National Cancer Institute (NCI) in vitro 60-cell line panel, hollow fiber assay, and s.c. xenograft]. Contributions of other potentially more clinically relevant in vivo tumor models including orthotopic, metastatic, and genetically engineered mouse models are also reviewed. Although this review concentrates on the preclinical screening efforts of the NCI, European efforts are not overlooked. Europe has played a key role in the development of new anticancer agents. The two largest academic drug development groups, the European Organisation for Research and Treatment of Cancer and Cancer Research UK, have been collaborating with the NCI in the acquisition and screening of compounds since the 1970s. As with the drug development process internationally, rational pharmacodynamic approaches have more recently been adopted by these two groups.

Inactivation of p53 and retinoblastoma family pathways in canine osteosarcoma cell lines

Canine osteosarcoma (OS) has been used as a model system for the study of cancer biology and treatment despite the lack of information regarding its pathogenesis. Expression of tumor suppressor genes known to participate in malignant transformation were studied in five different OS cell lines. Each of the cell lines exhibited properties of transformed cells, and those that were tested grew in soft agarose and formed osteoid-containing tumors when injected subcutaneously into nude mice. p53 function was determined to be defective in each cell line as indicated by the lack of induction of p53-responsive genes, p21 and mdm2, following treatment with 5-fluorouracil. p53 mRNA and protein levels were elevated in three cell lines and were extremely low in two cell lines. p53 protein overexpression correlated with the presence of mutations within the DNA binding domain. Four cell lines appeared to contain normal retinoblastoma (Rb) mRNA and Rb protein and no detectable p16 mRNA or protein. In contrast, the remaining cell line contained high levels of p16 mRNA and protein and significantly reduced levels of Rb, p107, and p130 proteins. These results underscore the importance of inactivating p53 and Rb family pathways in canine OS and suggest that unlike human OS, cells derived from canine OS contain mutations that simultaneously inactivate all three Rb family members.

Insights into cancer from transgenic mouse models

The generation of mice designed to overexpress activated forms of oncogenes or carrying targeted mutations in tumour suppressor genes, has allowed scientists to causally link the function of these genes with specific tumour processes, such as proliferation, apoptosis, angiogenesis or metastasis. In addition, these mice have been interbred to assess the extent of cooperativity between different genetic lesions in disease progression, leading to a greater understanding of the multi-stage nature of tumourigenesis. The effect of genetic mutations is often influenced by the genetic background of the mouse and by analysing strain-dependent phenotypes, modifier loci have been identified. Although genetic mutations in mouse and humans do not always lead to the same tumour spectrum, the underlying molecular mechanisms are frequently relevant to both species. Furthermore, new technical approaches creating conditional mouse mutants which develop tumours in a tissue-specific manner, will allow the effect of mutation of certain genes to be studied in specific tissues, free from the fatal effects of the mutation in other clinically less relevant tissues. Several exising mouse strains have already been used to develop and test new therapies and conditional mutagenesis will undoubtedly increase the potential use of transgenic mice in understanding and treating cancer.

Induction of retinoblastoma gene expression during terminal growth arrest of a conditionally immortalized fetal rat lung epithelial cell line and during fetal lung maturation

The process by which fetal lung epithelial cells differentiate into type 1 and type 2 cell is largely unknown. In order to study lung epithelial cell proliferation and differentiation we have infected 20-day fetal lung epithelial cells with a retrovirus carrying a temperature-sensitive SV40 T antigen (T Ag) and isolated several immortalized fetal epithelial cell lines. Cell line 20-3 has characteristics of lung epithelial cells including the presence of distinct lamellar bodies, tight junctions, keratin 8 and 18 mRNA, HFH8, and T1 alpha mRNA and low levels of surfactant protein A mRNA. At 33 degrees C 20-3 grows with a doubling time of 21 h. At 40 degrees C the majority of cells cease to proliferate. Growth arrest is accompanied by significant morphological changes including an increase in cell size, transition to a squamous phenotype that resembles type 1 cells, and an increase in the number of multinucleated cells within the population. Greater than 95% of the cells incorporate [3H]thymidine into DNA at 33 degrees C whereas at 40 degrees C label incorporation drops to less than 20%. When shifted down to 33 degrees C 40% of the cells remain terminally growth arrested. In addition, cells plated at 40 degrees C have a reduced ability to form colonies when replated at 33 degrees C. Treatment with TGF-beta increases the percentage of cells that terminally growth arrest to greater than 80%. Growth arrest is accompanied by an increase in the levels of c-jun, jun D, cyclin D1, C/EBP-beta, transglutaminase type II, and retinoblastoma (Rb) mRNA and an induction of p105, the hypophosphorylated, growth regulatory form of Rb. Evaluation of Rb mRNA in fetal lung indicates that it is induced 2.5-fold between 17 and 21 days of gestation. These studies indicate that 20-3 terminally growth arrests in culture at the nonpermissive temperature and that it may be useful in studying changes in gene expression that accompany terminal growth arrest during lung development.

Retinoblastoma protein family in cell cycle and cancer: a review

Two genes, p107 and Rb2/p130, are strictly related to RB, the most investigated tumor suppressor gene, responsible for susceptibility to retinoblastoma. The products of these three genes, namely pRb, p107, and pRb2/p130 are characterized by a peculiar steric conformation, called "pocket," responsible for most of the functional interactions characterizing the activity of these proteins in the homeostasis of the cell cycle. The interest in these genes and proteins springs from their ability to regulate cell cycle processes negatively, being able, for example, to dramatically slow down neoplastic growth. So far, among these genes, only RB is firmly established to act as a tumor suppressor, because its lack-of-function is clearly involved in tumor onset and progression. It has been found deleted or mutated in most retinoblastomas and sarcomas, but its inactivation is likely to play a crucial role in other types of human cancers. The two other members of the family have been discovered more recently and are currently under extensive investigation. We review analogies and differences among the pocket protein family members, in an attempt to understand their functions in normal and cancer cells.

Loss of heterozygosity on chromosome 17 and mutation of the p53 gene in retinoblastoma

Loss of heterozygosity (LOH) on chromosome 17 and mutations of the p53 gene were examined in 25 retinoblastomas (RB), consisting of three familial tumors, nine hereditary tumors without family history, 11 non-hereditary tumors, one recurrent tumor and one lung-metastatic tumor. LOH on chromosome 17 was detected in only one of the 23 primary RB. No mutations of the p53 gene were detected in the primary tumors. A recurrent tumor showed LOH on the short arm region of chromosome 17. LOH on chromosome 17 and a point mutation of the p53 gene were also detected in a metastatic tumor. These results suggest that LOH on chromosome 17 and mutation of the p53 gene may not be associated with the development of primary RB, but may play a role in the progression of RB.

Activity of the retinoblastoma family proteins, pRB, p107, and p130, during cellular proliferation and differentiation

Genetic evidence from retinoblastoma patients and experiments describing the mechanism of cellular transformation by the DNA tumor viruses have defined a central role for the retinoblastoma protein (pRB) family of tumor suppressors in the normal regulation of the eukaryotic cell cycle. These proteins, pRB, p107, and p130, act in a cell cycle-dependent manner to regulate the activity of a number of important cellular transcription factors, such as the E2F-family, which in turn regulate expression of genes whose products are important for cell cycle progression. In addition, inhibition of E2F activity by the pRB family proteins is required for cell cycle exit after terminal differentiation or nutrient depletion. The loss of functional pRB, due to mutation of both RB1 alleles, results in deregulated E2F activity and a predisposition to specific malignancies. Similarly, inactivation of the pRB family by the transforming proteins of the DNA tumor viruses overcomes cellular quiescence and prevents terminal differentiation by blocking the interaction of pRB, p107, and p130 with the E2F proteins, leading to cell cycle progression and, ultimately, cellular transformation. Together these two lines of evidence implicate the pRB family of negative cell cycle regulators and the E2F family of transcription factors as central components in the cell cycle machinery.

Suppression of growth in vitro and tumorigenicity in vivo of human carcinoma cell lines by transfected p16INK4

The function of p16INK4 as a putative tumor suppressor gene was examined by investigating its ability to inhibit the growth of cancer cell lines in vitro and tumor formation in vivo. A p16INK4 cDNA expression vector was transfected into five human cancer cell lines that varied in their p16INK4 and retinoblastoma (Rb) status. Suppression of colony-forming efficiency was seen in four cell lines. Of two cell lines wild type for p16INK4 but null for Rb protein expression, one (Hep 3B) showed inhibition of colony formation, whereas the other (Saos-2) did not. This observation may demonstrate involvement of p16INK4 independent of Rb. The transfected p16INK4 gene was frequently selected against and lost during continued growth in vitro. When compared to the colon carcinoma cell line (DLD-1),p16INK4-transfected DLD-1 clone 1 cells were less tumorigenic in athymic nude mice. Tumors arising from p16INK4-transfected DLD-1 clones, which were growth suppressed in vitro, either lost the integrated exogenous p16INK4 or expressed reduced amounts of p16INK4 protein. Therefore, p16INK4 was also selected against during tumor formation in vivo. These data are consistent with the hypothesis that p16INK4 is a tumor suppressor gene.

Alterations of the retinoblastoma a susceptibility gene in chronic lymphocytic leukemia

Research in recent years has shown that malignant transformation is a genetic multistep process. This holds true not only for in-vitro model systems, but has also been elegantly shown in-vivo, as in colorectal cancer. Chronic lymphocytic leukemia (CLL) is the most frequent leukemia in Western countries and occurs mainly in elderly patients, suggesting that in this form of leukemia, cumulative molecular lesions may be necessary for transformation. However, the molecular background is unknown in most cases. Cytogenetic aberrations may be used as markers for genes involved in the process of malignant transformation. In CLL, the most frequently observed structural cytogenetic lesion is a deletion/translocation involving the long arm of chromosome 13, a region where the retinoblastoma susceptibility gene (Rb-gene) has been mapped (13q14). Many groups have studied the question as to whether alterations of the Rb-gene play a causal role in the pathogenesis of CLL. This review deals with recent data indicating that i) the Rb-gene may be altered in a minority of CLL cases, and ii) there may be another gene localized on chromosome 13q14 that may be important in the molecular biology of CLL.

Study of the role of retinoblastoma protein in terminal differentiation of murine erythroleukemia cells

Hexamethylenebisacetamide-induced terminal differentiation of Friend virus-transformed murine erythroleukemia (MEL) cells can be inhibited by okadaic acid, an inhibitor of type 1 and type 2A protein phosphatases. The inhibition is shown to be correlated with prevention of dephosphorylation of retinoblastoma protein (pRB) in cells and bypass of G1 prolongation in the cell cycle. These results suggest that pRB-mediated G1 prolongation is necessary for MEL cells to commit to terminal differentiation. However, further experiments demonstrate that the simple cell cycle exit is not sufficient for commitment to terminal differentiation. Induction of dephosphorylation of pRB and subsequent G1 prolongation by forskolin does not lead MEL cells to differentiate. Additional pRB has been expressed in MEL cells by transfection with a neo-resistant plasmid containing RB cDNA under the control of a cytomegalovirus promoter. Exogenously expressed pRB is hyperphosphorylated in logarithmically growing MEL cells without any noticeable change in growth rate between the transfected cell line and the parental cell line. This result suggests that pRB in MEL cells is regulated by protein kinases and protein phosphatases and not by transcription.

Towards understanding the molecular basis of thyroid cancer

Cancer is a multistep phenomenon and multiple genetic lesions are involved in the emergence of the cancerous lesion. This has best been demonstrated in colonic cancer. The authors review their work and that of others highlighting what is known about thyroid cancer. They implicate ras mutations predominantly in follicular carcinoma, rearrangement of the ret proto-oncogene in papillary carcinoma and the tumor suppressor genes p53 and retinoblastoma gene product in all stages of thyroid carcinoma. They find a low rate of ret proto-oncogene rearrangement in the Saudi population (>5%) as compared to elsewhere in the world (20%). They find TSH receptor message abundance to be predictive of prognosis in thyroid cancer patients. Lastly, they examine whether the abundance of the anti-metastatic gene nm23 message abundance negatively correlated with the tendency of thyroid tumors to metastasize and find that not to be the case in thyroid carcinoma. The study of oncogenes and tumor suppressor genes in the pathogenesis of thyroid cancer is in its infancy; however, rapid progress is being made in identifying genes participating in malignant thyroid cell transformation.

Inhibition of ras-induced proliferation and cellular transformation by p16INK4

The cyclin-dependent kinase 4 (CDK4) regulates progression through the G1 phase of the cell cycle. The activity of CDK4 is controlled by the opposing effects of the D-type cyclin, an activating subunit, and p16INK4, an inhibitory subunit. Ectopic expression of p16INK4 blocked entry into S phase of the cell cycle induced by oncogenic Ha-Ras, and this block was relieved by coexpression of a catalytically inactive CDK4 mutant. Expression of p16INK4 suppressed cellular transformation of primary rat embryo fibroblasts by oncogenic Ha-Ras and Myc, but not by Ha-Ras and E1a. Together, these observations provide direct evidence that p16INK4 can inhibit cell growth.

The retinoblastoma protein and BRG1 form a complex and cooperate to induce cell cycle arrest

The retinoblastoma tumor suppressor protein (RB) binds several cellular proteins involved in cell cycle progression. Using the yeast two-hybrid system, we found that RB bound specifically to the protein BRG1. BRG1 shares extensive sequence similarity to Drosophila brahma, an activator of homeotic gene expression, and the yeast transcriptional activator SNF2/SW12. BRG1 contains an RB-binding motif found in viral oncoproteins and bound to the A/B pocket and the hypophosphorylated form of RB. BRG1 did not bind RB in viral oncoprotein-transformed cells. Coimmunoprecipitation experiments suggested BRG1 associates with the RB family in vivo. In the human carcinoma cell line SW13, BRG1 exhibited tumor suppressor activity by inducing formation of flat, growth-arrested cells. This activity depended on the ability of BRG1 to cooperate and complex with RB, as both an RB-nonbinding mutant of BRG1 and the sequestration of RB by adenovirus E1A protein abolished flat cell formation.

A gene map of congenital malformations

Congenital malformations frequently arise sporadically, making it difficult to determine whether or not they are genetic in aetiology, let alone which gene(s) may be involved. Nevertheless, rapid progress has been made over recent years in the localisation and identification of gene mutations in specific malformations. This review draws from Mendelian inheritance in man (Johns Hopkins University Press, 11th ed, 1994) and the online version (OMIM) to catalogue 139 loci (including 65 specifically identified genes) implicated in congenital malformations. Some of the most interesting recent developments are discussed.

The cell cycle and the retinoblastoma protein family

Tumor formation results from alterations in the control of normal cell proliferation. To further our understanding of the molecular mechanisms underlying the deregulation of cell proliferation much attention, over the past decade, has been focused on the function of proto-oncogenes. Cellular oncogenes are thought to be growth promoting. More recently, a class of genes known as tumor suppressors have come under intense study. Tumor suppressors are largely thought to restrain cell proliferation. The retinoblastoma protein (Rb) is one of a growing list of tumor suppressors. Concurrent with the study of tumor suppressor genes has been a rapid increase in our understanding of the cell cycle at the molecular level. Rb and a related protein p107 are involved in the processes of cell proliferation and differentiation. Each functionally interacts with and affects the activity of the transcription factor E2F as well as other transcription factors involved in cell proliferation and differentiation. Additionally, Rb and p107 are modified by, and/or form specific complexes with, several elements of the basic cell cycle machinery. Specifically, Rb and p107 interact with and are modified by various cyclins and cyclin dependent kinases (cdk), some of which have been shown to be essential for cell cycle progression and in some cases their deregulation has been implicated in the development of cancer. This review will attempt to convey our current functional and mechanistic understanding of the biological roles Rb and p107 play in proliferation, development and differentiation. A knowledge of the interplay between these positive and negative regulators of cell proliferation and differentiation, noted above, is central to our understanding of human cancer.

The role of mitotic recombination in carcinogenesis

Genetic recombination systems are present in all living cells and viruses and generally contribute to their hosts' flexibility with respect to changing environmental conditions. Recombination systems not only help highly developed organisms to protect themselves from microbial attack via an elaborate immune system, but conversely, recombination systems also enable microorganisms to escape from such an immune system. Recombination enzymes act with a high specificity on DNA sequences that either exhibit extended stretches of homology or contain characteristic signal sequences. However, recombination enzymes may rarely act on incorrect alternative target sequences, which may result in the formation of chromosomal deletions, inversions, translocations, or amplifications of defined DNA regions. This review describes the characteristics of several recombination systems and focuses on the implication of aberrant recombination in carcinogenesis. The consequences of mitotic recombination on the inappropriate activation of protooncogenes and on the loss of tumor suppressor genes is discussed. Cases are reported where mitotic recombination clearly has been associated with carcinogenesis in rodents as well as humans. Several test systems able to detect recombinagenic activities of chemical compounds are described.

Altered expression of the retinoblastoma susceptibility gene in chronic lymphocytic leukaemia

The pathogenesis of chronic lymphocytic leukaemia (CLL) is unknown. One of the most frequent cytogenetic abnormalities in CLL is a deletion within the long arm of chromosome 13, the region to which the retinoblastoma (Rb) gene has been mapped. Lack of Rb expression has been linked to the carcinogenic process in many human tumours. We therefore sought to investigate the role of Rb gene inactivation in CLL using differential polymerase chain reaction on reverse transcribed RNA. The result of the PCR was quantitated using HPLC. 5/39 patients revealed a lack or significantly impaired expression of the Rb gene upon differential PCR analysis. In addition, immunocytochemical studies were performed using the Rb-specific monoclonal antibody PMG245. 10/56 patients showed a weak or absent expression upon immunocytochemical analysis compared to monocytes or granulocytes. The samples lacking Rb were from both early and late stage CLL. Our results indicate that inactivation of the Rb protein occurs in a fraction of CLL cases and can be found in early and late stages of the disease.

Induction of the cell cycle in baby rat kidney cells by adenovirus type 5 E1A in the absence of E1B and a possible influence of p53

From previous studies on the induction of DNA synthesis in quiescent primary baby rat kidney cells by adenovirus type 5 (Ad5) E1A deletion mutants, we concluded that induction is prevented only when cellular proteins p300 and pRb are both uncomplexed with E1A (J.A. Howe, J.S. Mymryk, C. Egan, P.E. Branton, and S.T. Bayley, Proc. Natl. Acad. Sci. USA 87:5883-5887, 1990). We have now examined induction by these same mutants in virus lacking the E1B region, so that cellular p53 was no longer complexed to the E1B 55-kDa protein. E1A mutants that fail to bind pRb induced DNA synthesis at a significantly lower level in Ad5 lacking E1B than in Ad5 containing E1B. Apparently, therefore, uncomplexed p53 can partially replace p300 in cooperating with pRb to suppress DNA synthesis in baby rat kidney cells.

The dichotomy between germ line and somatic line, and the origin of cell mortality

The germ cells of extant animals are potentially immortal, whereas somatic cells are mortal, that is, they are able to carry out only a finite number of divisions. In this article we propose an evolutionary interpretation of these differences. We assume that germ cells of the earliest metazoans inherited immortality from their unicellular ancestor, while somatic cells acquired mortality by gaining new functions. It follows that cell mortality was under genetic control from the beginning of metazoan life.

Toxicological and pathological applications of proliferating cell nuclear antigen (PCNA), a novel endogenous marker for cell proliferation

A major stimulus to study cell proliferation, particularly in rodent carcinogenicity assays and human tumors, has been the belief that the quantification of this fundamental biological process will provide the toxicologist and pathologist with objective data allowing a better understanding of the mechanisms involved in the toxicity and/or carcinogenicity of certain compounds as well as guiding more effective management of patients afflicted with neoplasia. Among the markers used for cell proliferation measurement, PCNA has recently gained much attention and holds much promise as it is intricately involved in the cell replication processes. It not only could allow measurement of the replication rates without necessitating pretreatment of the animal/tissue in prospective studies, but also would allow retrospective assessment of the proliferative rates in archival tissues due to the conservation of this marker in fixed and paraffin-embedded tissues. Finally, knowledge of the function of PCNA in the cell cycle and its regulation by other factors may help us understand the advantages and limitations of PCNA as a cell proliferation marker in its application in toxicology and as a prognostic marker in human tumors.