Regulation of transcription by the retinoblastoma protein

Diffuse anterior retinoblastoma: current concepts

Diffuse anterior retinoblastoma is a rare variant of retinoblastoma seeding in the area of the vitreous base and anterior chamber. Patients with diffuse anterior retinoblastoma are older than those with the classical types, with the mean age being 6.1 years. The original cells of diffuse anterior retinoblastoma are supposed to be cone precursor. Patients most commonly present with pseudouveitis, pseudohypopyon, and increased intraocular pressure. The retina under fundus examination is likely to be normal, and the clinical features mimic the inflammation progress, which can often lead to misdiagnosis. The published diffuse anterior retinoblastoma cases were diagnosed after fine-needle aspiration biopsy running the potential risk of inducing metastasis. The most common treatment for diffuse anterior retinoblastoma is enucleation followed by systematic chemotherapy according to the patient’s presentation and clinical course. This review summarizes the recent advances in etiology (including tumorigenesis and cell origin), pathology, diagnosis, differential diagnosis, and new treatment. The challenges of early diagnosis and prospects are also discussed.

p53 Expression as a Prognostic Factor in Upper Urinary Tract Urothelial Carcinoma: A Systematic Review and Meta-Analysis

Objective: To conduct a meta-analysis examining p53 expression as a potential risk factor in upper urinary tract urothelial carcinoma (UUT-UC) and to systematically review the available data. Methods: A comprehensive literature review was performed from January 1991 to August 2012, using search engines such as PubMed, EMBASE, Cochrane Library and KoreaMed. All retrieved references were manually reviewed, and two authors independently extracted the data. The quality of case-control and cohort studies was assessed using the Scottish Intercollegiate Guidelines Network (SIGN) checklists. Heterogeneity among studies was examined using the Q statistics and Higgins' I2 statistic. Results: Of 302 abstracts of original research studies, nine case-control trials fit our criteria for inclusion in the analysis. Of the nine articles included, four scored ‘low' and five scored ‘modest' in the quality assessment performed according to the SIGN checklists. Analysis of the correlation between different factors and p53 expression in UUT-UC showed that pathologic stage (≥pT3 or <pT3) differed significantly between the p53 and non-p53 groups (OR = 2.720, p < 0.001). Statistically significant correlations were also detected between p53 expression and histologic grade (OR = 4.507, p < 0.001) and female gender (OR = 2.724, p < 0.001). Conclusion: The results of our meta-analysis suggest that p53 expression in UUT-UC was correlated with advanced pathologic stage, high histologic grade and female gender.

Identification of a biomarker panel for colorectal cancer diagnosis

BACKGROUND

Malignancies arising in the large bowel cause the second largest number of deaths from cancer in the Western World. Despite progresses made during the last decades, colorectal cancer remains one of the most frequent and deadly neoplasias in the western countries.

METHODS

A genomic study of human colorectal cancer has been carried out on a total of 31 tumoral samples, corresponding to different stages of the disease, and 33 non-tumoral samples. The study was carried out by hybridisation of the tumour samples against a reference pool of non-tumoral samples using Agilent Human 1A 60-mer oligo microarrays. The results obtained were validated by qRT-PCR. In the subsequent bioinformatics analysis, gene networks by means of Bayesian classifiers, variable selection and bootstrap resampling were built. The consensus among all the induced models produced a hierarchy of dependences and, thus, of variables.

RESULTS

After an exhaustive process of pre-processing to ensure data quality--lost values imputation, probes quality, data smoothing and intraclass variability filtering--the final dataset comprised a total of 8, 104 probes. Next, a supervised classification approach and data analysis was carried out to obtain the most relevant genes. Two of them are directly involved in cancer progression and in particular in colorectal cancer. Finally, a supervised classifier was induced to classify new unseen samples.

CONCLUSIONS

We have developed a tentative model for the diagnosis of colorectal cancer based on a biomarker panel. Our results indicate that the gene profile described herein can discriminate between non-cancerous and cancerous samples with 94.45% accuracy using different supervised classifiers (AUC values in the range of 0.997 and 0.955).

Liver-derived cell lines QSG-7701 and HepG2 support different HBV replication patterns

Hepatitis B virus (HBV) infection is currently still a worldwide heath concern. In our study, we compared HBV replication patterns in two liver-derived cell lines, QSG-7701 and HepG2. Viral markers of HBV replication in culture medium and cells were analyzed after transfection of these cells with plasmid pUC18-HBV1.2 into. We showed that QSG-7701 cells could support more stable and a higher level of HBV replication than HepG2 cells. Gene expression profiles of QSG-7701 and HepG2 cells by microarray analysis showed that many genes were differentially expressed between these two cell lines, including those that are related to the HBV life cycle. The global gene expression profile of these two cell types provides some clues to explain how different HBV replication is achieved. QSG-7701 cells offer a new opportunity for basic research on HBV virus-host interactions.

The cell-type-specificity of inherited predispositions to tumours: review and hypothesis

Most hereditary predispositions to tumours affect only one particular cell type of the body but the genes bearing the relevant germ-line mutation are not cell-type-specific. Some predisposition syndromes include increased risks of lesions (developmental or tumourous) of unrelated cell types, in any individual predisposed to the main lesion (e.g. osteosarcoma in patients predisposed to retinoblastoma). Other predispositions to additional lesions occur only in members of some families with the predisposition to the basic lesion (e.g. Gardner's syndrome in some families suffering familial adenomatous polyposis). In yet other predisposition syndromes, different mutations of the same gene are associated with markedly differing family-specific clinical syndromes. In particular, identical germline mutations (e.g. in APC, RET and PTEN genes), have been found associated with differing clinical syndromes in different families. This paper reviews previously suggested mechanisms of the cell-type specificity of inherited predispositions to tumour. Models of tumour formation in predisposition syndromes are discussed, especially those involving a germline mutation (the first 'hit') of a tumour suppressor gene (TSG) and a second (somatic) hit on the second allele of the same TSG. A modified model is suggested, such that the second hit is a co-mutation of the second allele of the TSG and a regulator which is specific for growth and/or differentiation of the cell type which is susceptible to the tumour predisposition. In some cases of tumour, the second hit may be large enough to be associated with a cytogenetically-demonstrable abnormality of the part of the chromosome carrying the TSG, but in other cases, the co-mutation may be of 'sub-cytogenetic' size (i.e. 10(2)-10(5) bases). For the latter, mutational mechanisms of frameshift and impaired fidelity of replication of DNA by DNA polyerases may sometimes be involved. Candidate cell-type-specific regulators may include microRNAs and perhaps transcription factors. It is suggested that searching the introns within 10(5)-10(6) bases either side of known of exonic mutations of TSGs associated with inherited tumour predisposition might reveal microRNA cell-type-specific regulators. Additional investigations may involve fluorescent in situ hybridisations on interphase tumour nuclei.

Transcriptional regulation of interleukin 2 in SLE T cells

Interleukin-2 (IL-2) is a central cytokine required for the activation of T, B, and NK cells. It propagates the immune response and terminates it by promoting the activation induced cell death of T cells. IL-2 production is altered in T cells of patients with systemic lupus erythematosus (SLE). The consequence of reduced IL-2 production in SLE is decreased immune response to infectious agents. Decreased IL-2 production by SLE T cells is the result of transcriptional repression of the IL-2 gene. This article will review the defective transcription regulation of IL-2 in SLE T cells, which is the result of decreased expression of the enhancers NF-kappa B and AP1 and the increased expression of the transcriptional repressor CREM.

A transcriptionally inactive E2F-1 targets the MDM family of proteins for proteolytic degradation

E2F-1-activated transcription promotes cell cycle progression and apoptosis. These functions are regulated by several factors including the E2F-1-binding protein MDM2 and the retinoblastoma protein pRb. Using a yeast two-hybrid screen we have identified the MDM2-related protein, MDMX, as an E2F-1-binding protein. In these studies we find that coexpression of MDMX with E2F-1 results in degradation of the MDMX protein. Although this proteolytic degradation can be blocked by the protease inhibitors bafilomycin A(1), N-acetyl-Leu-Leu-Norleu-AL, and N-acetyl-Leu-Leu-Met-AL, MDMX degradation is not inhibited by lactacystin, suggesting that degradation occurs by a proteasome-independent mechanism. Using an E2F-1 deletion mutant (E2F-1(180-437)) we show that E2F-1-targeted degradation of MDMX does not require the E2F-1 DNA binding domain and therefore is independent of E2F-1-driven transcription. We also find that this transcriptionally inactive E2F-1 mutant is capable of degrading the MDMX-related protein MDM2 and the MDMX isoform MDMX-S. Mapping of the E2F-1 C terminus reveals that neither a previously characterized C-terminal MDM2 binding domain nor the pRb binding domain on E2F-1 is required for MDMX and MDM2 degradation.

Current aspects of the pathology of soft tissue sarcomas

In soft tissue sarcomas, advances in pathological techniques, including immunohistochemistry, cytogenetics, and molecular genetics, have improved diagnostic accuracy, confirmed or clarified interrelationships between tumor subtypes, and revealed mechanisms of tumorigenesis. Many sarcomas are associated with abnormalities of tumor-suppressor genes, and several types have been found to have specific chromosomal translocations. These data and correlative clinicopathologic studies, although confirming many traditional pathological views, enable refinement or reassessment of terminology and classification of some small round cell, spindle cell, pleomorphic, and lipogenic tumors. New factors are emerging for prediction of tumor behavior, which might ultimately relate to therapy once a wider range of treatment options becomes available. This article reviews these current aspects of sarcoma pathology.

Cyclin A is a functional target of retinoblastoma tumor suppressor protein-mediated cell cycle arrest

Although RB inhibits the G(1)-S transition, the mechanism through which RB prevents cell cycle advancement remains unidentified. To delineate the mechanism(s) utilized by RB to exert its anti-proliferative activity, constitutively active RB proteins (which cannot be inactivated by phosphorylation) or p16ink4a (which prevents RB inactivation) were utilized. Both proteins inhibited the G(1)-S transition, whereas wild-type RB did not. We show that active RB acts to attenuate cyclin A promoter activity, and that overexpression of cyclin E reverses RB-mediated repression of the cyclin A promoter. Although cyclin A is an E2F-regulated gene, and it has been long hypothesized that RB mediates cell cycle advancement through binding to E2F and attenuating its transactivation potential, cyclin E does not reverse dominant negative E2F-mediated repression of the cyclin A promoter. Although active RB repressed both cyclin A and two other paradigm E2F-regulated promoters, only cyclin A transcription was restored upon co-expression of cyclin E. Additionally, we show that RB but not dominant negative E2F regulates the cyclin A promoter through the CCRE element. These data identify cyclin A as a downstream target of RB-mediated arrest. Consistent with this idea, ectopic expression of cyclin A reversed RB-mediated G(1) arrest. The findings presented suggest a pathway wherein cyclin A is a downstream target of RB, and cyclin E functions to antagonize this aspect of RB-mediated G(1)-S inhibition.

Cyclosporine induces the expression of the cyclin inhibitor p21

BACKGROUND

Current immunosuppression strategies involve inhibition of T cell activation and/or lymphocyte proliferation. During T cell cycle progression/activation, the expression of cyclins and cyclin dependent kinases is increased. In this study, we examined whether cyclosporine A (CsA) suppresses the cell cycle progression through the induction of the cell cycle inhibitor p21. Because CsA induces the expression of transforming growth factor (TGF)-beta, and TGF-beta induces p21 expression, we also determined whether CsA's induction of p21 is dependent on or independent of TGF-beta.

METHOD

Using reverse transcription assisted polymerase chain reaction and western blot analysis, we studied the induction of p21 mRNA and protein in human T cells and A-549 cells (human lung adenocarcinoma cells) by CsA. The stimulation of p21 promoter activity was studied by luciferase assay using p21-luc, chimeric plasmid DNA containing a p21 promoter segment, and luciferase reporter gene. The dependence of CsA's induction of p21 was studied using anti-TGF-beta antibody and TGF-beta altered A-549 cells.

RESULTS

CsA induced p21 mRNA protein expression and stimulated its promoter activity in lymphoid (T cells) and nonlymphoid (human lung adenocarcinoma, A-549 cells).CsA's induction of p21 was inhibited both by a neutralizing anti-TGF-beta antibody and in TGF-beta-altered A-549 cells, consistent with its effects on p21 requiring TGF-beta.

CONCLUSION

These data support the hypothesis that at least one component of CsA's antiproliferative effects may occur through the induction of p21 and that this induction is dependent on TGF-beta. Should p21 induction be a viable immunosuppressive strategy, inducing this molecule independent from the fibrogenic cytokine TGF-beta might reduce the toxicity associated with current immunosuppression.

Carcinogenesis and the plasma membrane

Presented is a two-stage hypothesis of carcinogenesis based on: (1) plasma membrane defects that produce abnormal electron and proton efflux; and (2) electrical uncoupling of cells through loss of intercellular communication. These changes can be induced by a wide variety of stimuli including chemical carcinogens, oncoviruses, inherited and/or acquired genetic defects, and epigenetic abnormalities. The resulting loss of electron/proton homeostasis leads to decreased transmembrane potential, electrical microenvironment alterations, decreased extracellular pH, and increased intracellular pH. This produces a positive feedback loop to enhance and sustain the proton/electron efflux and loss of intercellular communication. Low transmembrane potential is functionally related to rapid cell cycling, changes in membrane structure, and malignancy. Intracellular alkalinization affects a variety of pH-sensitive systems including glycolysis, DNA synthesis, DNA transcription and DNA repair, and promotes genetic instability, accounting for the accumulation of genetic defects seen in malignancy. The abnormal microenvironment results in the selective survival and proliferation of malignant cells at the expense of contiguous normal cell populations.

The retinoblastoma susceptibility gene product/Sp1 signalling pathway is modulated by Ca2+/calmodulin kinases II and IV activity

To investigate the possible link between Ca2+ signalling and cell cycle control we analysed Ca2+/calmodulin kinases (CamK) interaction with the retinoblastoma susceptibility gene product/SP1 pathway. CamK II and IV activate c-fos transcription through a short promoter region (-99 to -53) containing the retinoblastoma control element (RCE) and a cAMP response element (CRE) related sequences. Deletion analysis revealed that the RCE is a major CamK responsive element and is sufficient to confer CamK and Ca2+ regulation to a minimal promoter. Direct interactions between SP1 and RCE were confirmed by gel shift experiments. Using transient transfection experiments, we show that CamK-dependent transcription is regulated by the retinoblastoma (Rb) susceptibility gene product and the p107 Rb related protein. However, the stimulatory effects of CamKs and Rb on c-fos are blocked by overexpression of both proteins. These effects appear to be directly mediated by SP1 as shown by the use of a Gal4/SP1 fusion proteins. In conclusion, CamK II and IV, two major Ca2+-dependent intracellular effectors, may represent a molecular link between this second messenger transduction pathway and effectors that control cell cycle progression through Rb/SP1 signalling pathway.

Raf-1 physically interacts with Rb and regulates its function: a link between mitogenic signaling and cell cycle regulation

Cells initiate proliferation in response to growth factor stimulation, but the biochemical mechanisms linking signals received at the cell surface receptors to the cell cycle regulatory molecules are not yet clear. In this study, we show that the signaling molecule Raf-1 can physically interact with Rb and p130 proteins in vitro and in vivo and that this interaction can be detected in mammalian cells without overexpressing any component. The binding of Raf-1 to Rb occurs subsequent to mitogen stimulation, and this interaction can be detected only in proliferating cells. Raf-1 can inactivate Rb function and can reverse Rb-mediated repression of E2F1 transcription and cell proliferation efficiently. The region of Raf-1 involved in Rb binding spanned residues 1 to 28 at the N terminus, and functional inactivation of Rb required a direct interaction. Serum stimulation of quiescent human fibroblast HSF8 cells led to a partial translocation of Raf-1 into the nucleus, where it colocalized with Rb. Further, Raf-1 was able to phosphorylate Rb in vitro quite efficiently. We believe that the physical interaction of Raf-1 with Rb is a vital step in the growth factor-mediated induction of cell proliferation and that Raf-1 acts as a direct link between cell surface signaling cascades and the cell cycle machinery.

Effect of transient cyclic AMP elevation on DNA synthesis in rat hepatocytes at G1 phase

Maintaining high levels of intracellular cyclic AMP (cAMP) is known to inhibit the growth of various proliferating cells including hepatocytes. We show here that transient (30 min) elevations of cAMP induced by addition of 8-bromo-cAMP (1 mmol/L) to rat hepatocytes in primary culture at three time points (12 h, 16 h and 20 h) after seeding stimulated DNA synthesis. Sustained levels of cAMP stimulated DNA synthesis to a lesser degree at a lower concentration (1 mumol/L), but inhibited it at concentrations higher than 100 mumol/L. We also determined cyclin-dependent kinase 2 (cdk2) activity in the hepatocytes during this incubation period. The transient addition of 8-bromo-cAMP at the late G1 phase increased cdk2 activity. This suggests that transient cAMP elevation in hepatocytes at the late G1 phase has a growth stimulation effect. Up-regulation of cdk2 activity may have a role in this process.

Sp3 encodes multiple proteins that differ in their capacity to stimulate or repress transcription

The product of the retinoblastoma (Rb) susceptibility gene ( RB-1 ) regulates expression of a variety of growth control genes via discrete promoter elements termed retinoblastoma control elements (RCEs). We have previously shown that RCEs are bound and regulated by a common set of ubiquitously expressed nuclear proteins of 115, 95 and 80 kDa, termed retinoblastoma control proteins (RCPs). We have also previously determined that Sp3 and Sp1, two members of the Sp family of transcription factors, encode the 115 and 95 kDa RCPs respectively and that Rb stimulates Sp1/Sp3-mediated transcription in vivo. In this report we have extended these results by determining that the 80 kDa RCP arises from Sp3 mRNA via translational initiation at two internal sites located within the Sp3 trans -activation domain. Internally initiated Sp3 proteins readily bind to Sp1 binding sites in vitro yet have little or no capacity to stimulate transcription of Sp-regulated genes in vivo. Instead, these Sp3-derived proteins function as potent inhibitors of Sp1/Sp3- mediated transcription. Since cell cycle- or signal- induced expression of a variety of genes, including p21 waf1/cip1, p15 INK4B, CYP11A, mdr1 and acetyl-CoA carboxylase, have been mapped to GC-rich promoter elements that bind Sp family members, we speculate that alterations of the protein and/or DNA binding activities of internally initiated Sp3 isoforms may account in part for the regulation of such differentially expressed genes.

A novel proximal element mediates the regulation of mouse Ren-1C promoter by retinoblastoma protein in cultured cells

The protein product of the retinoblastoma susceptibility gene, RB, is a nuclear phosphoprotein that modulates transcription of genes involved in growth control via interactions with transcription factors. Renin is a rate-limiting enzyme of the renin-angiotensin system that regulates blood pressure and water-electrolyte balance. Renin gene expression is regulated in a tissue-specific and developmentally linked manner. Similarly, the expression of RB is controlled in a differentiation-linked manner. Thus, to investigate whether RB is involved in the regulation of renin gene expression, we examined the effects of RB on transcriptional activity of the mouse renin (Ren-1C) promoter. The Ren-1C promoter contains two transcriptionally important elements; the RU-1 (-224 to -138) and RP-2 (-75 to -47) elements. RB activated the Ren-1C promoter in human embryonic kidney cells. The promoter element responsible for RB-mediated transcriptional regulation was the RP-2 element. The results of DNA-protein binding experiments showed that RB increased nuclear binding activity to the RP-2 element, and site-directed mutation which disrupted binding of nuclear factors to the RP-2 element markedly reduced RB-mediated activation of Ren-1C promoter in human embryonic kidney cells. These results indicate that the RP-2 element plays an important role in RB-mediated transcriptional regulation of Ren-1C promoter activity in human embryonic kidney cells, thereby suggesting an interesting mechanism by which RB may modulate the renin-angiotensin system.

Clonal osteoblastic cell lines from p53 null mouse calvariae are immortalized and dependent on bone morphogenetic protein 2 for mature osteoblastic phenotype

p53 protein regulates cell cycle progression and its absence will result in unlimited cell divisions required for immortalization of cells. Immortalized osteoblastic cell lines were established from p53 null mouse calvariae of normal phenotype. The clonal murine cell lines demonstrated osteoblastic phenotype as exemplified by alkaline phosphatase enzyme activity. They also express bone morphogenetic protein 2 (BMP2) mRNA. Addition of recombinant BMP2 to these cells dramatically increased the alkaline phosphatase activity in a dose dependent manner. In the absence of BMP2 these cells do not undergo osteoblastic differentiation. Treatment of these cells with recombinant bone morphogenetic protein 2 stimulated differentiated osteoblast formation, as determined by mineralized nodule formation. Thus, these immortalized cells in culture represent osteoblast progenitors that lack p53 protein and respond to osteogenic stimuli. These cell lines offer a model system to study the role of p53 in osteoblastic differentiation and programmed cell death. Also these cells will be useful in studying the effects of p53 on transcriptional regulation of osteoblast specific gene expression.

The retinoblastoma interacting zinc finger gene RIZ produces a PR domain-lacking product through an internal promoter

The PR domain is a newly recognized protein motif that characterizes a subfamily of Krüppel-like zinc finger genes. Members of the PR domain family have been shown to play important roles in cell differentiation and malignant transformation. The RIZ gene is the founding member of this family; it was isolated because its gene products can bind to the retinoblastoma tumor suppressor protein. Here, we have studied the RIZ gene structure and expression. By immunoprecipitation and immunoblot analysis we identified two different RIZ protein products of 280 and 250 kDa, designated RIZ1 and RIZ2, respectively. The 280-kDa RIZ1 product comigrated with the RIZ cDNA-derived polypeptide. The 250-kDa RIZ2 product lacked the NH2-terminal PR domain of RIZ1; it comigrated with a truncated RIZ1 polypeptide that was initiated from an internal ATG codon. Both the full-length and the truncated RIZ1 polypeptide were located in the nucleus as shown by transfection and immunofluorescence analysis. We identified the RIZ2 transcripts and showed that they were produced by an internal promoter located at the 5' boundary of coding exon 5. RNase protection analysis revealed similar ratios of RIZ1 and RIZ2 transcripts in most adult rat tissues except in testis, where RIZ1 was more abundant than RIZ2. These observations were strikingly similar to those described for the MDS1-EVI1 cancer gene, which also normally gives rise to a PR domain-lacking product, EVI1, because of an internal promoter.

Nuclear localization of DP and E2F transcription factors by heterodimeric partners and retinoblastoma protein family members

E2F is a family of transcription factors implicated in the regulation of genes required for progression through G1 and entry into the S phase. The transcriptionally active forms of E2F are heterodimers composed of one polypeptide encoded by the E2F gene family and one polypeptide encoded by the DP gene family. The transcriptional activity of E2F/DP heterodimers is influenced by association with the members of the retinoblastoma tumor suppressor protein family (pRb, p107, and p130). Here the intracellular distribution of E2F and DP proteins was investigated in transiently transfected Chinese hamster and human cells. In transfected cells, DP-1 did not accumulate in the nucleus unless it was coexpressed with the heterodimeric partners E2F-1, E2F-2, or E2F-3. Domain mapping experiments showed that regions of E2F-1 and DP-1 that are required for stable association of the two proteins were also required for nuclear localization of DP-1. Unlike E2F-1, -2, and -3, E2F-4 did not accumulate in the nucleus unless it was coexpressed with DP-2, p107 and p130, but not pRb, stimulated nuclear localization of E2F-4, either alone or in combination with DP-2. These results indicate that DP proteins preferentially associate with specific E2F partners, and suggest that the ability of specific E2F/DP heterodimers to localize in the nucleus contributes to the regulation of E2F activity.

Cell cycle-regulated association of E2F1 and Sp1 is related to their functional interaction

Because of the large number of growth-regulated genes containing binding sites for the transcription factors Sp1 and E2F and the reported ability of E2F to mediate cell cycle (growth) regulation, we studied interactions between E2F1 and Sp1. In transient transfection assays using Drosophila melanogaster SL2 cells, transfection with both Sp1 and E2F1 expression vectors resulted in greater than 85-fold activation of transcription from a hamster dihydrofolate reductase reporter construct, whereas cotransfection with either the Sp1 or E2F1 expression vector resulted in 30- or <2-fold activation, respectively. Therefore, these transcription factors act synergistically in activation of dihydrofolate reductase transcription. Transient transfection studies demonstrated that E2F1 could superactivate Sp1-dependent transcription in a promoter containing only Sp1 sites and that Sp1 could superactivate transcription of promoters through E2F sites, further demonstrating that these physically associated in Drosophila cells transfected with Sp1 and E2F1 expression vectors and in human cells, with maximal interaction detected in mid- to late G1. Additionally, E2F1 and Sp1 interact in vitro through specific domains of each protein, and the physical interaction and functional synergism appear to require the same regions. Taken together, these data demonstrate that E2F1 and Sp1 both functionally and physically interact; therefore this interaction, Sp1 and E2F1 may regulate transcription of genes containing binding sites for either or both factors.

Carcinogenesis of cutaneous malignancies

BACKGROUND

Over the past several years significant progress has been made in identifying the cellular and biochemical mechanisms underlying carcinogenesis.

OBJECTIVE

This review summarizes recent advances that have helped clarify the process of malignant transformation in cutaneous tumors.

RESULTS

Ultraviolet radiation-induced mutations in the p53 tumor suppressor gene and human papilloma virus inhibition of the p53 and retinoblastoma tumor suppressor gene products appear to play significant roles in the development of many cutaneous squamous cell carcinomas. Studies of patients with the nevoid basal cell carcinoma syndrome suggest the existence of an additional regulatory gene that may be involved in the development of basal cell carcinomas.

CONCLUSIONS

Carcinogenesis is multistep process involving genetic and epigenetic alterations to specific proto-oncogene and tumor suppressor gene products that progressively release the cell from normal controlled growth and replication.

Molecular and immunochemical analyses of RB1 and cyclin D1 in human ductal pancreatic carcinomas and cell lines

Somatic mutations in the retinoblastoma-1 gene (RB1) and loss of RB1 protein function have been implicated in a number of human malignancies, but the role of RB1 gene and protein abnormalities in ductal pancreatic cancer (DPCA) is virtually unknown. We therefore analyzed expression of the RB1 protein immunohistochemically and/or by western blotting in a total of 54 sporadic and eight familial cases of archival and frozen DPCA and in 18 pancreatic carcinoma cell lines by using the antibodies RB-WL-1, 84-B3-1, and PMG3-245. Mutations in the RB1 promotor region and exons 13-21 of the RB1 gene were likewise examined by single-strand conformation polymorphism (SSCP) analyses and DNA sequencing of genomic DNA from 30 microdissected primary pancreatic tumors and the pancreatic carcinoma cell lines. Moreover, amplification and expression of a major regulatory component of RB1 function, cyclin D1, were assessed by southern and immunohistochemical analyses, respectively. The DPCAs were heterogeneous in both the intensity of RB1 nuclear staining and the percentage of immunoreactive cells. The tumors often had areas where RB1 staining was weak or absent adjacent to normal pancreatic tissue; however, only two of 32 archival cases and one of 30 frozen cases of DPCA completely lacked RB1 nuclear staining. Immunohistochemical and western blot analyses of 18 pancreatic carcinoma cell lines demonstrated the absence of RB1 expression in only two cell lines, Capan-1 and QGP-1. Analyses of the RB1 gene and promotor region by SSCP and DNA sequencing largely confirmed the immunochemical findings. Three of 30 primary carcinomas had abnormalities revealed by SSCP analyses. In one case a single base-pair deletion was confirmed in exon 18 and resulted in premature termination and the absence of detectable RB1 protein. A second case had TAC-->TTC missense mutation in exon 13. The third primary carcinoma could not be reliably sequenced because it had a low percentage of epithelial cells. The cyclin D1 gene was not amplified in any of the primary pancreatic tumors or cell lines examined. These immunochemical and molecular analyses of the RB1 tumor suppressor gene and cyclin D1 proto-oncogene in a large series of human pancreatic cancers and cell lines indicate that RB1 and cyclin D1 alterations occur during the development of some human DPCAs. Nevertheless, it is probable that alterations in cell-cycle regulation in DPCAs more frequently involve pathways other than those involving RB1 and cyclin D1.

Immunohistochemical expression of retinoblastoma protein in cutaneous melanomas

Retinoblastoma protein (pRB) is the product of a tumour-suppressor gene (rb) mapped to chromosome 13q14. pRB acts as a control checkpoint at the G1 phase of the cell cycle, preventing cells from entering into the S phase. Mutational inactivation of both normal alleles leads to loss of pRB expression and the development of malignant neoplasms. Absence of pRB occurs in retinoblastomas, sarcomas and several other types of tumours. The potential role of pRB in the pathogenesis of cutaneous melanoma is unknown, and was the subject of this investigation. Formalin-fixed, paraffin-embedded sections of four cutaneous melanoma metastases, 17 primary invasive melanomas and 10 predominantly intradermal melanocytic naevi were studied. Monoclonal antibodies directed against pRB and Ki-67 antigen were used after microwave heating of sections to restore antigenicity. pRB was not detected in morphologically normal epidermal melanocytes. In five naevi, only scattered cells (1%) expressed pRB, whereas in the other five naevi, pRB expression was undetectable. In contrast, pRB was detected in all primary and metastatic melanomas (5-70% of cells). Expression was always localized to nuclei. Ki-67 expression was detected only in the melanomas, with both cellular staining and regional localization similar to that shown by pRB in 13 of the 20 melanomas studied with both antibodies. pRB appears to be expressed at higher levels in melanomas than in benign naevi. It therefore seems unlikely that loss of rb expression is an important factor in the pathogenesis of melanoma.

The retinoblastoma susceptibility gene product represses transcription when directly bound to the promoter

Rb represses E2F-mediated transcription in part by blocking the trans-activation domain of E2F. In addition, Rb can convert an E2F binding site from a positive to a negative element. To examine the effect of a Rb-DNA-bound complex on transcription, full-length Rb was fused to the DNA binding domain of GAL4. Here, we report that GAL4-Rb can repress transcription mediated by either Sp1, AP-1, or p53, dependent upon the presence of both the GAL4 DNA binding domain and GAL4 binding sites. Moreover, GAL4-Rb inhibited the activity of the herpes simplex virus tk promoter from GAL4 binding sites located at a distance from the promoter. In contrast, GAL4-Rb was unable to repress basal transcription. Cotransfection of specific cyclins and cyclin-dependent kinases or SV40 T-antigen abolished the repressive activity of GAL4-Rb. The domains of Rb involved in mediating the repression of transcription were mapped to regions that are overlapping, but not identical, to those required for the interaction with E2F. We propose that Rb can function as a general repressor of transcription when bound to the promoter region.

Sp-1 binds promoter elements regulated by the RB protein and Sp-1-mediated transcription is stimulated by RB coexpression

The retinoblastoma (RB) protein is implicated in transcriptional regulation of at least five cellular genes, including c-fos, c-myc, and transforming growth factor beta 1. Cotransfection of RB and truncated promoter constructs has defined a discrete element (retinoblastoma control element; RCE) within the promoters of each of these genes as being necessary for RB-mediated transcription control. Previously, we have shown that RCEs form protein-DNA complexes in vitro with three heretofore unidentified nuclear proteins and mutation of their DNA-binding site within the c-fos RCE results in an abrogation of RCE-dependent transcription in vivo. Here, we demonstrate that one of the nuclear proteins that binds the c-fos, c-myc, and transforming growth factor beta 1 RCEs in vitro is Sp-1 and that Sp-1 stimulates RCE-dependent transcription in vivo. Moreover, we show that Sp-1-mediated transcription is stimulated by the transient coexpression of RB protein. We conclude from these observations that RB may regulate transcription in part by virtue of its ability to functionally interact with Sp-1.