Novel Aspects of Genetics, Molecular Biology and Clinical Oncology of Sarcomas.

The Connective Tissue Oncology Group Annual Meeting 2018 (CTOS 2018) took place in Rome from 4 to 17 November 2018, and the 39th Plenary Meeting of the Scandinavian Sarcoma Group (SSGM 2019) was held in Bergen from 8 to 10 May 2019. These two large international conferences brought together an overwhelming majority of molecular and clinical specialists in the sarcoma field, especially those working on soft tissue sarcoma. Topics discussed on the conferences included, among others, sarcoma genetics, clinical and molecular subclassification, targeted therapy, clinical prognostication, and new experimental sarcoma models. A large ongoing international study on germinal sarcoma genetics was presented, the interim results of which revealed the extremely complex nature of genetic disposition to sarcoma, and, surprisingly, a rather prominent place among predisposing genes for those coding for structural telomere constituents. Fusion oncogenes dominate somatic sarcoma genetics, especially because of their origin and impact on sarcoma clinical behaviour, and are especially relevant for karyotypically simple paediatric sarcomas. A crucial issue in karyotypically complex sarcomas are the efforts being made to obtain a subclassification of sarcoma, other than those based on pathology, using either the clinical characteristics of sarcomas (uterine leiomyosarcoma vs. soft tissue leiomyosarcoma) or specific gene expression profiles (molecular subtypes in undifferentiated pleiomorphic sarcoma), which showed that molecular characterization can open the way for subtype specific therapies. Other examples of where this type of strategy can be applied include gastrointestinal stromal tumours, infantile fibrosarcoma, and inflammatory myofibroblastic tumours, where targeted therapy could be conceived based on the actionable mutations identified. Attempts in this direction have been made also for clear cell sarcoma and dedifferentiated liposarcoma, albeit the effectiveness of molecular-targeted treatments for these sarcomas is still poor, and progress in the treatment of osteosarcoma is still rather slow. Actually, the platelet-derived growth factor signalling system holds a prominent position in searches for targeted therapies, not only against rare sarcoma types, where are activated by mutations (some gastrointestinal stromal tumours, infantile hereditary myofibromatosis, and dermatofibrosarcoma protuberans), but also against other more usual sarcoma types, where the blocking anti-PDGFRα-antibody olaratumab has been successfully integrated into combinatorial chemotherapeutic regimens. In the field of clinical prognostication, remarkable progress in sarcoma nomograms was reported. Interesting results were also presented in the area of new experimental sarcoma models. Participation on both scientifi c conferences and all the experimental work leading to the presented sarcoma models were supported by the Czech Science Foundation project No. 17-17636S. The authors declare they have no potential conflicts of interest concerning drugs, products, or services used in the study. The Editorial Board declares that the manuscript met the ICMJE recommendation for biomedical papers. Submitted: 20. 9. 2019 Accepted: 6. 11. 2019.

Simultaneous Targeting of Bladder Tumor Growth, Survival, and Epithelial-to-Mesenchymal Transition with a Novel Therapeutic Combination of Acetazolamide (AZ) and Sulforaphane (SFN).

BACKGROUND: Current chemotherapies for advanced stage metastatic bladder cancer often result in severe side effects, and most patients become drug resistant over time. Thus, there is a need for more effective therapies with minimal side effects. OBJECTIVE: The acid/base balance in tumor cells is essential for tumor cell functioning. We reasoned that simultaneous targeting of pH homeostasis and survival pathways would improve therapeutic efficacy. We evaluated the effectiveness of targeting pH homeostasis with the carbonic anhydrase inhibitor acetazolamide (AZ) in combination with the survival pathway targeting isothiocyanate sulforaphane (SFN) on the HTB-9 and RT112(H) human bladder tumor cell lines. MATERIALS AND METHODS: We assessed viability, proliferation, and survival in vitro and effect on xenografts in vivo. RESULTS: Combination AZ + SFN treatment induced dose-dependent suppression of growth, produced a potent anti-proliferative and anti-clonogenic effect, and induced apoptosis through caspase-3 and PARP activation. The anti-proliferative effect was corroborated by significant reductions in Ki-67, pHH3, cyclin D1, and sustained induction of the cell cycle inhibitors, p21 and p27. Both active p-Akt (Ser473) and p-S6 were significantly downregulated in the AZ + SFN combination treated cells with a concomitant inhibition of Akt kinase activity. The inhibitory effects of the AZ + SFN combination treatment showed similar efficacy as the dual PI3K/mTOR pathway inhibitor NVP-BEZ235, albeit at an expected higher dose. In terms of the effect on the metastatic potential of these bladder cancers, we found downregulated expression of carbonic anhydrase 9 (CA9) concomitant with reductions in both E-cadherin, N-cadherin, and vimentin proteins mitigating the epithelial-to-mesenchymal transition (EMT), suggesting negation of this program. CONCLUSION: We suggest that reductions in these components could be linked with downregulation of the survival mediated Akt pathway and suggested an active role of the Akt pathway in bladder cancer. Altogether, our in vitro and pre-clinical model data support the potential use of an AZ + SFN combination for the treatment of bladder cancer.

[Tumour-stroma interactions in urothelial cancer]. / Tumor-Stroma-Interaktionen im Harnblasenkarzinom.

BACKGROUND: The histopathological structure of malignant tumours involves two essential compartments - the tumour parenchyma with the actual transformed cells, and the supportive tumour stroma. The latter consists of specialized mesenchymal cells, such as fibroblasts, macrophages, lymphocytes and vascular cells, as well as of their secreted products, including components of the extracellular matrix, matrix modifying enzymes and numerous regulatory growth factors and cytokines. In consequence, the tumour stroma has the ability to influence virtually all aspects of tumour development and progression, including therapeutic response. AIM: In this article we review the current knowledge of tumor stroma interactions in urothelial carcinoma and present various experimental systems that are currently in use to unravel the biological basis of these heterotypic cell interactions. RESULTS: For urothelial carcinoma, an extensive tumour stroma is quite typical and markers of activated fibroblasts correlate significantly with clinical parameters of advanced disease. Another clinically important variable is provided by the stromal expression of syndecan-1. CONCLUSION: Integration of markers of activated stroma into clinical risk evaluation could aid to better stratification of urothelial bladder carcinoma patients. Elucidation of biological mechanisms underlying tumour-stroma interactions could provide new therapeutical targets.

The dynamics of cancer stem cells.

With the formulation of the hierarchical model of tumor cell organization, cancer stem cells came to the forefront of cancer biology. As the only self-renewing tumor cells, they were made responsible for continuous tumor growth and their intrinsic self-protection ability was postulated to underlie cancer therapy resistance and/or recurrence. The concept of migrating cancer stem cells extended the relevance of the hierarchical cancer cell model to issue of cancer progression, with the crucial experimental evidence being provided by the demonstration that epithelial mesenchymal transition can convey stemness. Accordingly, cancer stem cells probably represent a highly dynamic cell population continuously differentiating and being continuously replenished by processes like mesenchymal epithelial transition and epithelial mesenchymal transition, respectively. Consequently, from the point of view of therapeutic targeting, cancer stem cells obviously do not represent a fixed target population any longer. Understanding the dynamic nature of cancer stem cells is thus essential not only for the progress in our understanding of basic cancer biology, but also from the therapeutic perspective.

Stem cells in the biology of normal urothelium and urothelial carcinoma.

Urothelium is a special type of stratified epithelium that lines the distal portion of the urinary tract. For a long time, basal urothelial cells have been suspected to include a population of urothelial stem cells. Recent experiments identifying label-retaining cells as well as lineage tracing analyses corroborate this notion. There are striking morphological and antigenic similarities between basal or differentiated urothelial cells and the corresponding cells in some urothelial carcinomas. In this respect, basal cell-specific markers provide good candidates to identify urothelial carcinoma stem cells, e.g. specific cytokeratins (CK5, CK14, CK17) or adhesion molecules (specific integrin subspecies, CD44). Common properties of the stem cells of normal urothelium and urothelial cancer have thus emerged. Both are characterized by a remarkable plasticity and both rely on reciprocal interactions with stromal fibroblasts. However, the stem cells of individual urothelial carcinomas appear to differ considerably and may contribute to the heterogeneity of this disease. The presence, quantity, and particular biological nature of urothelial carcinoma stem cells in each case may thus carry important clinical information that might allow a rationale stratification of urothelial cancer patients for treatment in the near future.

Ovarian cancer stem cells.

Because of its semi-solid character in dissemination and growth, advanced ovarian cancer with its hundreds of peritoneal tumor nodules and plaques appears to be an excellent in vivo model for studying the cancer stem cell hypothesis. The most important obstacle, however, is to adequately define and isolate these tumor-initiating cells endowed with the properties of anoikis-resistance and unlimited self-renewal. Until now, no universal single marker or marker constellation has been found to faithfully isolate (ovarian) cancer stem cells. As these multipotent cells are known to possess highly elaborated efflux systems for cytotoxic agents, these pump systems have been exploited to outline putative stem cells as a side-population (SP) via dye exclusion analysis. Furthermore, the cells in question have been isolated via flow cytometry on the basis of cell surface markers thought to be characteristic for stem cells.In the Vienna variant of the ovarian cancer cell line A2780 a proof-of-principle model with both a stable SP and a stable ALDH1A1+ cell population was established. Double staining clearly revealed that both cell fractions were not identical. Of note, A2780V cells were negative for expression of surface markers CD44 and CD117 (c-kit). When cultured on monolayers of healthy human mesothelial cells, green-fluorescence-protein (GFP)-transfected SP of A2780V exhibited spheroid-formation, whereas non-side-population (NSP) developed a spare monolayer growing over the healthy mesothelium. Furthermore, A2780V SP was found to be partially resistant to platinum. However, this resistance could not be explained by over-expression of the "excision repair cross-complementation group 1" (ERCC1) gene, which is essentially involved in the repair of platinated DNA damage. ERCC1 was, nonetheless, over-expressed in A2780V cells grown as spheres under stem cell-selective conditions as compared to adherent monolayers cultured under differentiating conditions. The same was true for the primary ovarian cancer cells B-57.In summary our investigations indicate that even in multi-passaged cancer cell lines hierarchic government of growth and differentiation is conserved and that the key cancer stem cell population may be composed of small overlapping cell fractions defined by various arbitrary markers.

Comparison of P19-derived neuroprogenitor and naive cell survival after intracerebellar application into B6CBA mice.

Mouse embryonic carcinoma cells (P19 line) were studied for both their survival and developmental potential in the intact cerebellum of B6CBA mice. The P19 cells were cultured and labelled with green fluorescent protein using transfection. Cells were used for transplantation either in the undifferentiated stage or after 3 days of neurodifferentiation induced by retinoic acid. The intracerebellar application was performed in 43 mice: group A (N = 21) received neuroprogenitors and group B (N = 22) received undifferentiated cells. The morphology of transplanted cells within the context of the surrounding cerebellar tissue was evaluated after 3 weeks. Naive P19 cells engrafted and survived in the cerebellum of 7 of the 22 adult mice (survival rate 31.8 %). Neuroprogenitors survived in 13 of the 21 mice (survival rate was 61.9 %). Since the cut-off is P < 0.05, the difference is not statistically significant (P = 0.069). An expansive appearance of the graft was significantly more frequent (P = 0.0047) in naive P19 cells than in neuroprogenitors. In mice in which the grafts did not survive, no marks of grafted cells or only fluorescing detritus were found. In conclusion, this is the first study to track the fate and morphology of embryonic carcinoma cells transplanted into the cerebellum, confirming that neuroprogenitors derived from embryonic carcinoma cells can settle in the host tissue and differentiate according to the surrounding conditions. With further validation, the embryonic carcinoma cells could become a valuable model with which to study the impact of cell therapy on neurodegenerative diseases.

DKC1 overexpression associated with prostate cancer progression.

BACKGROUND: Dyskerin encoded by the DKC1 gene is a predominantly nucleolar protein essential for the formation of pseudouridine in RNA and the telomerase RNA subunit hTR. Inherited mutations inactivating dyskerin cause dyskeratosis congenita, a syndrome with progeroid features characterised by skin defects and haematopoiesis failure, as well as cancer susceptibility. In this study, we report DKC1 overexpression in prostate cancers. METHODS: Expression of DKC1 was measured by quantitative RT-PCR in prostate cancer tissues in relation to hTR and the proliferation marker MKI67. Effects of dyskerin downregulation on proliferation, apoptosis and senescence of prostate cancer cell lines were determined. RESULTS: DKC1 was significantly overexpressed in prostate cancers, particularly in high-stage and recurring cases, correlating moderately with hTR and MKI67. Dyskerin downregulation in prostate carcinoma cell lines by siRNA diminished cell proliferation, but elicited neither apoptosis nor senescence. Apoptosis induction by TNF-alpha or tunicamycin was not enhanced. Long-term downregulation led predominantly to cell shrinking and loss of adhesion. INTERPRETATION: DKC1 upregulation in prostate cancers is common and likely to be necessary for extensive tumour growth. The phenotype of prostate carcinoma cell lines after dyskerin downregulation suggests that its most critical function is sustaining protein biosynthesis. Intriguingly, compromised function and overexpression of dyskerin can both contribute to cancer development.

[Bladder carcinoma cell lines as models of the pathobiology of bladder cancer. Review of the literature and establishment of a new progression series]. / Harnblasenkarzinomzelllinien als Modellsysteme zur Pathobiologie des Harnblasenkarzinoms. Uberblick und Etablierung einer neuen Progressionsserie.

BACKGROUND: Tumour cell lines represent valuable preclinical models to decipher underlying biology and identify potential therapy targets and pharmacologically useful compounds. Approximately 50 human bladder cancer cell lines have been established to date, mainly from invasive and metastatic tumours. Two of these, namely T24 and 253J, were experimentally further developed into progression series. These models have provided important insights into later tumour progression events and metastatic dissemination. Only a few cell lines are available as models of non-invasive papillary bladder cancer and no progression series have yet been established. MATERIAL AND METHODS: During the course of establishing a doxorubicin-resistant variant cell line of the human papillary bladder carcinoma cell line BFTC-905, a unique cell colony was identified, apparently involving cells with divergent growth patterns. Subsequent subculturing yielded three daughter cell lines, BFTC-905-compact, BFTC-905-diffuse und BFTC-905-diffuse M. Their fundamental characterization included basic cell morphology, cell membrane expression of E-Cadherin, karyotype analysis, invasion and colony forming capacity in soft agar. The clonal origin of the newly established daughter cell lines was assessed by means of molecular genetic methods. RESULTS: We could identify important differences in multiple transformation related traits among the cell lines of the BFTC-905 progression series. Both diffuse cell lines (BFTC-905-diffuse und BFTC-905-diffuse M) differed from the BFTC-905-compact cell line by growing in a less organized,"diffuse" manner, which involved colonies of cells exhibiting apparently normal cell-to-cell adhesion as well as individual cells outside of them. This diminution of the cell-to-cell adhesion was accompanied by a corresponding decrease of membranous E-Cadherin. The BFTC-905-diffuse M cell line displayed a dramatic increase in the overall chromosome number, resulting in a hypertetraploid karyotype. At the same time, this cell line, as the only one in the progression series, acquired the ability to grow independent of anchorage in soft agar. All three cell lines remained noninvasive. Allelic distribution of highly polymorphic DNA-markers in the cell lines of the BFTC-905 progression series provided unequivocal evidence of their common origin. CONCLUSION: The newly established BFTC-905 progression series manifests two aspects of the early progression of non-invasive bladder carcinoma, not exhibited by any other progression series published so far, namely dynamic changes in the expression of E-Cadherin and a complex karyotypic evolution. It may thus contribute important insights into further understanding of the pathobiology of bladder cancer.

[Relevance of cell culture models in cutaneous tumour biology: part II: complex culture systems]. / Stellenwert der Zellkulturmodelle in kutaner Tumorbiologie: Teil II: Komplexe Kultivierungssysteme.

There are several limitations to the use of the classical monolayer cell culture and the results obtained by means of it. The two-dimensional architecture and the analysis of pure cell populations of individual cell lines are the most several deviations from the situation prevailing in tissues in vivo, with inevitable consequences for the phenotypic traits displayed on the one hand, and for the genome structure and expression on the other hand. Newer developments in cell culture methodology seek approaches to mimic the in vivo situation in the cell culture as closely as possible. Remarkable variety of such approaches can be noticed, ranging from relative simple three-dimensional conditions of culturing pure cell lines on collagen gels or in form of multicell tumor spheroids. More complex forms try to combine multiple cell types in a single co-culture, e.g. of tumour cells and stromal fibroblasts. The most complex and most revealing among the three-dimensional culture arrangements is unquestionably the organotypic skin culture, in which all the relevant skin cell types are combined in a tissue-resembling construct, with resulting marked similarity to the anatomical structure of normal human skin. Several crucial results were obtained thereby, among others an intrinsic difference in the development of invasive squamous cell carcinoma and melanoma could be demonstrated. Just another experimental direction aims at direct tumourigenic transformation of normal human keratinocytes and melanocytes using highly efficient retroviral vectors. Immediately after establishing of the organotypic skin culture are such directly transformed primary cells transplanted on a nude mouse and the whole tumourigenic process is then essentially followed in vivo. This example illustrates finally the various possibilities of combination of in vitro and in vivo experimental approaches.

[Relevance of cell culture models in cutaneous tumour biology. Part I: tumour cell lines]. / Stellenwert der Zellkulturmodelle in kutaner Tumorbiologie. Teil I: Zelllinien tumorigen transformierter Zellen.

Cutaneous squamous cell carcinoma, basal cell carcinoma and melanoma, much like all other human solid tumors, result from a multi-step process in which genetic and epigenetic changes accumulate in the affected cells. Insight into the biology of human tumors is a requirement for developing effective therapies. Cell culture models are a very valuable experimental system. The degree of tumorigenic transformation can be precisely defined. Tumor cell lines display similar functional hierarchy as tumors or tissues in vivo and can, consequently, provide a crucial source of minor cell subsets, like tumor stem cells. Progression series of clonally related cell lines offer the opportunity to follow the process of sequential acquisition of transformation-related traits up to the development of properties with direct clinical equivalents, like tumorigenicity and metastatic competence. These phenotypical changes can be directly correlated with changes at the genome level, concerning both gene structure and expression. While for most studies, human transformed cell lines are the model of choice, there are questions for which animal cell lines are strongly preferred, such as interactions between the tumor and the immune system. To properly interpret the results of all experiments with classical two-dimensional cell culture, a possible danger of artifacts due to grossly unnatural environment must be constantly taken into account. It is thus obligatory to confirm any such result with other experimental models like complex three-dimensional culture models or experimental animals, and with clinical samples.

[Tumour stem cells--a new concept in tumour biology]. / Tumorstammzellen--ein neues Konzept in der Tumorbiologie.

There is growing evidence that tumours display a hierarchy similar to normal tissues. Accordingly, a small population of tumour stem cells is supposed to perpetuate tumour growth. These cells renew themselves and are highly tumourigenic upon injection into immunocompromised animals, yielding tumours largely identical to those from which they were derived. Tumour stem cells exhibit a high degree of chemoresistance due to slow cycling and constitutive expression of multiple members of the ATP-BINDING CASSETTE (ABC) family of transporters. An increasing number of tumours with tumour stem cells has been identified by now, including breast, brain, colon and prostate cancers, as well as leukemias. Detailed characterization of tumour stem cells may contribute to a better understanding of the underlying biology of the respective tumours and lead to novel curative therapies.

Epigenetics of prostate cancer: beyond DNA methylation.

Epigenetic mechanisms permit the stable inheritance of cellular properties without changes in DNA sequence or amount. In prostate carcinoma, epigenetic mechanisms are essential for development and progression, complementing, amplifying and diversifying genetic alterations. DNA hypermethylation affects at least 30 individual genes, while repetitive sequences including retrotransposons and selected genes become hypomethylated. Hypermethylation of several genes occurs in a coordinate manner early in carcinogenesis and can be exploited for cancer detection, whereas hypomethylation and further hypermethylation events are associated with progression. DNA methylation alterations interact with changes in chromatin proteins. Prominent alterations at this level include altered patterns of histone modification, increased expression of the EZH2 polycomb histone methyltransferase, and changes in transcriptional corepressors and coactivators. These changes may make prostate carcinoma particularly susceptible to drugs targeting chromatin and DNA modifications. They relate to crucial alterations in a network of transcription factors comprising ETS family proteins, the androgen receptor, NKX3.1, KLF, and HOXB13 homeobox proteins. This network controls differentiation and proliferation of prostate epithelial cells integrating signals from hormones, growth factors and cell adhesion proteins that are likewise distorted in prostate cancer. As a consequence, prostate carcinoma cells appear to be locked into an aberrant state, characterized by continued proliferation of largely differentiated cells. Accordingly, stem cell characteristics of prostate cancer cells appear to be secondarily acquired. The aberrant differentiation state of prostate carcinoma cells also results in distorted mutual interactions between epithelial and stromal cells in the tumor that promote tumor growth, invasion, and metastasis.

Jun oncoproteins do not function as primary transcription factors for the mouse major histocompatibility complex class I H-2 genes in fibroblasts.

There are several reports in the literature focusing on regulation of major histocompatibility complex (MHC) class I genes by transcription factors of the jun family. The methods employed in these reports differed in various respects, and their results are inconsistent. In mouse Lewis lung carcinoma, B16-melanoma and F9-teratocarcinoma cell lines, c-jun was characterized as a transcriptional activator of the murine MHC class I H2-Kb gene, while c-jun was identified as a direct transcriptional repressor of the swine class I PD1 gene, and c-jun stably transfected clones of mouse L-fibroblasts markedly reduced their H-2 class I gene expression. In this study, we attempted to reproduce this last effect by means of transient transfection coupled to Northern hybridization, upon transfecting L-fibroblasts with expression vectors for all jun family members as well as with an array of c-jun-derived dominant negative mutants. No change in H-2 class I expression could be identified. Next, we derived two additional fibroblastic cell lines from the fibrosarcoma of the H2-Kk/v-jun transgenic mouse and transfected them with the two most potent c-jun dominant negative mutants, again without eliciting any change in H-2 class I mRNA level. We conclude that the negative regulation of H-2 class I genes by c-jun in cells of the fibroblastic lineage is not a primary effect.

Tracing European founder lineages in the Near Eastern mtDNA pool.

Founder analysis is a method for analysis of nonrecombining DNA sequence data, with the aim of identification and dating of migrations into new territory. The method picks out founder sequence types in potential source populations and dates lineage clusters deriving from them in the settlement zone of interest. Here, using mtDNA, we apply the approach to the colonization of Europe, to estimate the proportion of modern lineages whose ancestors arrived during each major phase of settlement. To estimate the Palaeolithic and Neolithic contributions to European mtDNA diversity more accurately than was previously achievable, we have now extended the Near Eastern, European, and northern-Caucasus databases to 1,234, 2, 804, and 208 samples, respectively. Both back-migration into the source population and recurrent mutation in the source and derived populations represent major obstacles to this approach. We have developed phylogenetic criteria to take account of both these factors, and we suggest a way to account for multiple dispersals of common sequence types. We conclude that (i) there has been substantial back-migration into the Near East, (ii) the majority of extant mtDNA lineages entered Europe in several waves during the Upper Palaeolithic, (iii) there was a founder effect or bottleneck associated with the Last Glacial Maximum, 20,000 years ago, from which derives the largest fraction of surviving lineages, and (iv) the immigrant Neolithic component is likely to comprise less than one-quarter of the mtDNA pool of modern Europeans.

[Hormonal regulation of gene transcription--specificity and regulation of the hormonal response and results of its dysfunction]. / Hormonální regulace genové transkripce--specificita a regulace hormonální odpovedi a dusledky jejího porusení.

Nuclear hormone receptors act as ligand activated transcription factors. Hormonal response in vivo is, however, modulated at various levels beyond the mere activation of hormone receptors by their respective ligands. There is a great variability of particular arrangements of HRE and receptors of the TR/RAR subfamily use extensive dimerization to produce a particular dimer with optimal affinity for HRE in question. Hormonal response is mediated by complex hormone responsive units (HRU), which integrate HRE together with other regulatory sequences. Some hormone responsive genes may themselves code transcription factors or other regulatory proteins, underlying delayed primary and secondary responses. In addition to transcription activation, nuclear receptors enter repressive interactions with unrelated transcription factors like AP-1 or NF-kappa B. Genes for coding nuclear receptors are subjected to homologous downregulation. Functioning of nuclear receptors might be modulated by phosphorylation, which can underly ligand-independent activation. Nuclear receptors are mutated and/or disregulated in various pathologies, like hormone resistance syndromes of cancer.

[Hormonal regulation of gene transcription--nuclear hormone receptors as ligand-activated transcription factors]. / Hormonální regulace genové transkripce--jaderné receptory jako ligandem-aktivované transkripcní faktory.

Nuclear hormone receptors regulate gene transcription upon recognizing specific regulatory sequences--hormone responsive elements (HRE) in gene promoters, enhancers, and silencers. Receptors for sexual and adrenal steroid hormones, thyroid hormone, retinoic acid and vitamin D, as well as an extensive group of orphan receptors, all exhibit strong homology in structural and functional organization of the molecule and they form together the nuclear receptor superfamily. While classical steroid receptors dissociate upon activation an inhibitory hsp-complex, dimerize and then are able to bind cognate HRE and activate transcription, thyroid and retinoid receptors as well as the vitamin D receptor bind the HRE in question constitutively and activation is represented by dissociating a corepressor and recruiting a coactivator. Similar mode of action applies also for orphan receptors. Some of recently resolved orphan receptors are activated by immediate products of metabolism; the metabolism of cholesterol relies in great part on selective activation of a particular orphan receptors by a particular cholesterol metabolite.

Lymphoid specificity of a copy number-related expression of the H2-Kb transgene.

Transcriptional regulation of the MHC class I genes leading to their developmental and tissue specific expression is still poorly understood in spite of the recovery of a large variety of cis-controlling sequences and trans-acting factors pertaining to the 5' enhancer and the downstream regulatory element. Here we produced a series transgenic lines of mice with a genomic subclone of the H2-Kb gene consisting of 367 bp of the 5' upstream region, the coding region and 1.5 kb of the 3' downstream region and carrying all hitherto known regulatory sequences. The comparison of nine transgenic lines carrying the same H2-Kb transgene made it possible to ask whether the cis-information present in the transgene was sufficient for the tissue- and developmental-specific expression and its copy number dependence. We found the proper developmental onset of expression of the transgene at day 13 p.c. and correct tissue specific mRNA levels in adult mice. While in lymphoid tissues and in lung the number of transgene copies still correlated with RNA levels, the copy number dependence was completely lost in liver, kidney and embryonic tissues. Comparison with previously published H2-Kb transgenes indicates that the H2-Kb locus-controlling region is composed of more than one element.