Cell cycle-dependent 3D distribution of telomeres and telomere repeat-binding factor 2 (TRF2) in HaCaT and HaCaT-myc cells

How Genomes Emerge, Function, and Evolve: Living Systems Emergence-Genotype-Phenotype-Multilism-Genome/Systems Ecology

What holds together the world in its innermost, what life is, how it emerges, functions, and evolves, has not only been an epic matter of endless romantic sunset poetry and philosophy, but also manifests explicitly in its perhaps most central organization unit-genomes. Their 3D architecture and dynamics, including the interaction networks of regulatory elements, obviously co-evolved as inseparable systems allowing the physical storage, expression, and replication of genetic information. Since we were able to fill finally the much-debated centennial gaps in their 3D architecture and dynamics, now entire new perspectives open beyond epigenetics reaching as far as a general understanding of living systems: besides the previously known DNA double helix and nucleosome structure, the latter compact into a chromatin quasi-fibre folded into stable loops forming stable multi-loop aggregates/rosettes connected by linkers, creating hence the again already known chromosome arms and entire chromosomes forming the cell nucleus. Instantly and for the first time this leads now to a consistent and cross-proven systems statistical mechanics genomics framework elucidating genome intrinsic function and regulation including various components. It balances stability/flexibility ensuring genome integrity, enabling expression/regulation of genetic information, as well as genome replication/spread. Furthermore, genotype and phenotype are multiplisticly entangled being evolutionarily the outcome of both Darwinian natural selection and Lamarckian self-referenced manipulation-all embedded in even broader genome ecology (autopoietic) i(!)n- and environmental scopes. This allows formulating new meta-level functional semantics of genomics, i.e. notions as communication of genes, genomes, and information networks, architectural and dynamic spaces for creativity and innovation, or genomes as central geno-/phenotype entanglements. Beyond and most fundamentally, the paradoxical-seeming local equilibrium substance stability in its entity though far from a universal heat-death-like equilibrium is solved, and system irreversibility, time directionality, and thus the emergence of existence are clarified. Consequently, real deep understandings of genomes, life, and complex systems in general appear in evolutionary perspectives as well as from systems analyses, via system damage/disease (its repair/cure and manipulation) as far as the understanding of extraterrestrial life, the de novo creation and thus artificial life, and even the raison d'etre.

Simulation of Different Three-Dimensional Models of Whole Interphase Nuclei Compared to Experiments - A Consistent Scale-Bridging Simulation Framework for Genome Organization

The three-dimensional architecture of chromosomes, their arrangement, and dynamics within cell nuclei are still subject of debate. Obviously, the function of genomes-the storage, replication, and transcription of genetic information-has closely coevolved with this architecture and its dynamics, and hence are closely connected. In this work a scale-bridging framework investigates how of the 30 nm chromatin fibre organizes into chromosomes including their arrangement and morphology in the simulation of whole nuclei. Therefore, mainly two different topologies were simulated with corresponding parameter variations and comparing them to experiments: The Multi-Loop-Subcompartment (MLS) model, in which (stable) small loops form (stable) rosettes, connected by chromatin linkers, and the Random-Walk/Giant-Loop (RW/GL) model, in which large loops are attached to a flexible non-protein backbone, were simulated for various loop and linker sizes. The 30 nm chromatin fibre was modelled as a polymer chain with stretching, bending and excluded volume interactions. A spherical boundary potential simulated the confinement to nuclei with different radii. Simulated annealing and Brownian Dynamics methods were applied in a four-step decondensation procedure to generate from metaphase decondensated interphase configurations at thermodynamical equilibrium. Both the MLS and the RW/GL models form chromosome territories, with different morphologies: The MLS rosettes result in distinct subchromosomal domains visible in electron and confocal laser scanning microscopic images. In contrast, the big RW/GL loops lead to a mostly homogeneous chromatin distribution. Even small changes of the model parameters induced significant rearrangements of the chromatin morphology. The low overlap of chromosomes, arms, and subchromosomal domains observed in experiments agrees only with the MLS model. The chromatin density distribution in CLSM image stacks reveals a bimodal behaviour in agreement with recent experiments. Combination of these results with a variety of (spatial distance) measurements favour an MLS like model with loops and linkers of 63 to 126 kbp. The predicted large spaces between the chromatin fibres allow typically sized biological molecules to reach nearly every location in the nucleus by moderately obstructed diffusion and is in disagreement with the much simplified assumption that defined channels between territories for molecular transport as in the Interchromosomal Domain (ICD) hypothesis exist and are necessary for transport. All this is also in agreement with recent selective high-resolution chromosome interaction capture (T2C) experiments, the scaling behaviour of the DNA sequence, the dynamics of the chromatin fibre, the diffusion of molecules, and other measurements. Also all other chromosome topologies can in principle be excluded. In summary, polymer simulations of whole nuclei compared to experimental data not only clearly favour only a stable loop aggregate/rosette like genome architecture whose local topology is tightly connected to the global morphology and dynamics of the cell nucleus and hence can be used for understanding genome organization also in respect to diagnosis and treatment. This is in agreement with and also leads to a general novel framework of genome emergence, function, and evolution.

The Role of Non-Coding RNAs in the Regulation of the Proto-Oncogene MYC in Different Types of Cancer

Alterations in the expression level of the MYC gene are often found in the cells of various malignant tumors. Overexpressed MYC has been shown to stimulate the main processes of oncogenesis: uncontrolled growth, unlimited cell divisions, avoidance of apoptosis and immune response, changes in cellular metabolism, genomic instability, metastasis, and angiogenesis. Thus, controlling the expression of MYC is considered as an approach for targeted cancer treatment. Since c-Myc is also a crucial regulator of many cellular processes in healthy cells, it is necessary to find ways for selective regulation of MYC expression in tumor cells. Many recent studies have demonstrated that non-coding RNAs play an important role in the regulation of the transcription and translation of this gene and some RNAs directly interact with the c-Myc protein, affecting its stability. In this review, we summarize current data on the regulation of MYC by various non-coding RNAs that can potentially be targeted in specific tumor types.

3D organization of telomeres in porcine neutrophils and analysis of LPS-activation effect

BACKGROUND

While the essential role of 3D nuclear architecture on nuclear functions has been demonstrated for various cell types, information available for neutrophils, essential components of the immune system, remains limited. In this study, we analysed the spatial arrangements of telomeres which play a central role in cell fate. Our studies were carried out in swine, which is an excellent model organism for both biomedical research and agronomic applications. We isolated bacterial artificial chromosome (BAC)-containing subtelomeric p and q sequences specific to each porcine chromosome. This allowed us to study the behaviour of p and q telomeres of homologous chromosomes for seven pairs chosen for their difference in length and morphology. This was performed using 3D-FISH on structurally preserved neutrophils, and confocal microscopy. Resting and lipopolysaccharide (LPS)-activated states were investigated to ascertain whether a response to a pathogen aggression modifies this organization.

RESULTS

The positions of the p and q telomeres relative to the nuclear outer border were determined in the two states. All p telomeres changed their position significantly during the activation process, although the effect was less pronounced for the q telomeres. The patterns of telomeric associations between homologs and their frequencies were analysed for 7 pairs of chromosomes. This analysis revealed that the distribution of pp, qq and pq associations differs significantly among the 7 chromosomes. This distribution does not fit with the theoretical distribution for each chromosome, suggesting that preferential associations occur between subtelomeres.

CONCLUSIONS

The percentage of nuclei harbouring at least one telomeric association between homologs varies significantly among the chromosomes, the smallest metacentric chromosome SSC12, which is also the richest in gene-density, harbouring the highest value. The distribution of types of telomeric associations is highly dependent on the chromosomes and is not affected by the activation process. The frequencies of telomeric associations are also highly dependent on the type of association and the type of chromosome. Overall, the LPS-activation process induces only minor changes in these patterns of associations. When telomeric associations occur, the associations of p and q arms from the same chromosome are the most frequent, suggesting that "chromosome bending" occurs in neutrophils as previously observed in gametes.

Three-dimensional nuclear telomere architecture changes during endometrial carcinoma development

Endometrioid or type-I endometrial carcinoma (EC) develops from hyperproliferative glandular pathologies. Inactivation of the tumor suppressor gene PTEN is frequently associated with type-I EC. Using a previously characterized Pten heterozygous (Pten+/-) mouse model, this study investigates the three-dimensional (3D) telomere profiles during progression from hyperplastic lesions to EC to test the hypothesis that altered 3D telomere profiles can be detected prior to Pten loss in early hyperproliferative lesions. We used immunohistochemistry and 3D-telomere fluorescent in-situ hybridization to investigate Pten expression, telomere length and signal distribution, average number and spatial distribution of telomeres and formation of telomere aggregates in uterine glandular epithelial cells from wildtype and Pten+/- mice. Pten showed nuclear and cytoplasmic localization in WT, predominantly cytoplasmic staining in simple hyperplasia (SH) and was markedly reduced in atypical hyperplasia (AH). Telomere length in glandular epithelial cells does not shorten with age. The average number of telomeres per nucleus was not different in WT and Pten+/- mice indicating the lack of substantial numeric chromosome aberrations during EC development. We observed telomere aggregates in lesions of AH and EC. SH lesions in Pten+/- mice differed from normal glandular epithelium by an increased relative number of shorter telomeres and by a telomere signal distribution indicative of a heterogeneous cell population. Our study revealed that alterations in the nuclear 3D telomere architecture are present in early proliferative lesions of mouse uterine tissues indicative of EC development. The changes in telomere length distribution and nuclear signal distribution precede the loss of Pten.

Mechanism of the antiproliferative activity of some naphthalene diimide G-quadruplex ligands

G-quadruplexes are higher-order nucleic acid structures that can form in G-rich telomeres and promoter regions of oncogenes. Telomeric quadruplex stabilization by small molecules can lead to telomere uncapping, followed by DNA damage response and senescence, as well as chromosomal fusions leading to deregulation of mitosis, followed by apoptosis and downregulation of oncogene expression. We report here on investigations into the mechanism of action of tetra-substituted naphthalene diimide ligands on the basis of cell biologic data together with a National Cancer Institute COMPARE study. We conclude that four principal mechanisms of action are implicated for these compounds: 1) telomere uncapping with subsequent DNA damage response and senescence; 2) inhibition of transcription/translation of oncogenes; 3) genomic instability through telomeric DNA end fusions, resulting in mitotic catastrophe and apoptosis; and 4) induction of chromosomal instability by telomere aggregate formation.

Characterization of telomeric repeats in metaphase chromosomes and interphase nuclei of Syrian Hamster Fibroblasts

Background

Rodents have been reported to contain large arrays of interstitial telomeric sequences (TTAGGG)n (ITS) located in pericentromeric heterochromatin. The relative sizes of telomeric sequences at the ends of chromosomes (TS) and ITS in Syrian hamster (Mesocricetus auratus) cells have not been evaluated yet, as well as their structural organization in interphase nuclei.

Results

FISH signal distribution analysis was performed on DAPI-banded metaphase chromosomes of Syrian hamster fibroblasts, and relative lengths of telomere signals were estimated. Besides well-distinguished FISH signals from ITS located on chromosomes ##2, 4, 14, 20 and X that we reported earlier, low-intensity FISH signals were visualized with different frequency of detection on all other metacentric chromosomes excluding chromosome #21. The analysis of 3D-distribution of TS in interphase nuclei demonstrated that some TS foci formed clearly distinguished associations (2–3 foci in a cluster) in the nuclei of cells subjected to FISH or transfected with the plasmid expressing telomeric protein TRF1 fused with GFP. In G0 and G1/early S-phase, the average total number of GFP-TRF1 foci per nucleus was less than that of PNA FISH foci in the corresponding cell cycle phases suggesting that TRF1 overexpression might contribute to the fusion of neighboring telomeres. The mean total number of GFP-TRF1 and FISH foci per nucleus was increased during the transition from G0 to G1/early S-phase that might be the consequence of duplication of some TS.

Conclusions

The relative lengths of TS in Syrian hamster cells were found to be moderately variable. All but one metacentric chromosomes contain ITS in pericentromeric heterochromatin indicating that significant rearrangements of ancestral genome occurred in evolution. Visualization of GFP-TRF1 fibrils that formed bridges between distinct telomeric foci allowed suggesting that telomere associations observed in interphase cells are reversible. The data obtained in the study provide the further insight in the structure and dynamics of telomeric sequences in somatic mammalian cells.

Duplicated chromosomal fragments stabilize shortened telomeres in normal human IMR-90 cells before transition to senescence

To assess why during in vitro aging of fibroblasts the maintenance of chromosomal stability is effective or occasionally fails, a detailed cytogenetic analysis was performed in normal human IMR-90 fetal lung fibroblasts. The onset of senescence was inferred from proliferation activity, expression pattern of cell cycle regulating proteins, activity of β-galactosidase, and morphological features. Over the period of proliferation, a moderate increase of non-transmissible structural chromosomal aberrations was observed. In addition, using fluorescence in situ hybridization (mFISH and mBAND) techniques, we detected clonally expanding translocations in up to 70% of the analyzed metaphases, all involving one homolog of chromosome 9 as an acceptor. Notably, chromosomes are randomly involved as donor-chromosomes of the translocated terminal acentric fragments. These fragments result from duplication because the donor chromosomes are apparently unchanged. Interstitial telomeric signals were detectable at fusion sites, most likely belonging to chromosome 9. Quantitative fluorescence in situ hybridization (QFISH) detecting telomere sequences, followed by mFISH technique revealed that already in young cells the respective telomeres of one chromosome 9 were particularly short. For the first time, we have observed dysfunctional telomeres of one specific chromosome in normal human cells that have been stabilized by duplicated terminal sequences.

Controlled light exposure microscopy reveals dynamic telomere microterritories throughout the cell cycle

Telomeres are complex end structures that confer functional integrity and positional stability to human chromosomes. Despite their critical importance, there is no clear view on telomere organization in cycling human cells and their dynamic behavior throughout the cell cycle. We investigated spatiotemporal organization of telomeres in living human ECV-304 cells stably expressing telomere binding proteins TRF1 and TRF2 fused to mCitrine using four dimensional microscopy. We thereby made use of controlled light exposure microscopy (CLEM), a novel technology that strongly reduces photodamage by limiting excitation in parts of the image where full exposure is not needed. We found that telomeres share small territories where they dynamically associate. These territories are preferentially positioned at the interface of chromatin domains. TRF1 and TRF2 are abundantly present in these territories but not firmly bound. At the onset of mitosis, the bulk of TRF protein dissociates from telomere regions, territories disintegrate and individual telomeres become faintly visible. The combination of stable cell lines, CLEM and cytometry proved essential in providing novel insights in compartment-based nuclear organization and may serve as a model approach for investigating telomere-driven genome-instability and studying long-term nuclear dynamics.

Dynamics of telomeres and promyelocytic leukemia nuclear bodies in a telomerase-negative human cell line

Telomerase-negative tumor cells maintain their telomeres via an alternative lengthening of telomeres (ALT) mechanism. This process involves the association of telomeres with promyelocytic leukemia nuclear bodies (PML-NBs). Here, the mobility of both telomeres and PML-NBs as well as their interactions were studied in human U2OS osteosarcoma cells, in which the ALT pathway is active. A U2OS cell line was constructed that had lac operator repeats stably integrated adjacent to the telomeres of chromosomes 6q, 11p, and 12q. By fluorescence microscopy of autofluorescent LacI repressor bound to the lacO arrays the telomere mobility during interphase was traced and correlated with the telomere repeat length. A confined diffusion model was derived that describes telomere dynamics in the nucleus on the time scale from seconds to hours. Two telomere groups were identified that differed with respect to the nuclear space accessible to them. Furthermore, translocations of PML-NBs relative to telomeres and their complexes with telomeres were evaluated. Based on these studies, a model is proposed in which the shortening of telomeres results in an increased mobility that could facilitate the formation of complexes between telomeres and PML-NBs.

Tissue context-activated telomerase in human epidermis correlates with little age-dependent telomere loss

Telomerase- and telomere length regulation in normal human tissues is still poorly understood. We show here that telomerase is expressed in the epidermis in situ independent of age but was repressed upon the passaging of keratinocytes in monolayer culture. However, when keratinocytes were grown in organotypic cultures (OTCs), telomerase was re-established, indicating that telomerase activity is not merely proliferation-associated but is regulated in a tissue context-dependent manner in human keratinocytes. While not inducible by growth factors, treatment with the histone deacetylation inhibitor FK228 restored telomerase activity in keratinocytes grown in monolayer cultures. Accordingly, CHIP analyses demonstrated an acetylated, active hTERT promoter in the epidermis in situ and in the epidermis of OTCs but a deacetylated, silenced hTERT promoter with subsequent propagation in monolayer culture suggesting that histone acetylation is part of the regulatory program to guarantee hTERT expression/telomerase activity in the epidermis. In agreement with the loss of telomerase activity, telomeres shortened during continuous propagation in monolayer culture by an average of approximately 70 base pairs (bp) per population doubling (pd). However, telomere erosion varied strongly between different keratinocyte strains and even between individual cells within the same culture, thereby arguing against a defined rate of telomere loss per replication cycle. In the epidermis in situ, as determined from early-passage keratinocytes and tissue sections from different age donors, we calculated a telomere loss of only approximately 25 bp per year. Since we determined the same rate for the non-regenerating melanocytes and dermal fibroblasts, our data suggest that in human epidermis telomerase is a protective mechanism against excessive telomere loss during the life-long regeneration.

Telomeres rather than telomerase a key target for anti-cancer therapy?

It was in the 1930s that telomeres (from the Greek telos = end and meros = part) were first recognized as essential structures at the ends of the chromosomes and were shown to be important for chromosomal stability (Muller HJ: The remaking of chromosomes. The Collecting Net-Woods Hole 1938: 13: 181-198, McClintock B, The stability of broken ends of chromosomes in Zea mays. Genetics 1041: 26: 234-282). However, it was only in 1978 that the first telomeric sequence was identified -- in the protocoa Tetrahymena, a single cell organism that at a certain stage of development has many identical minichromosomes with twice as many telomeres (Blackburn EH and Gall JG. A tandemly repeated sequence at the termini of the extrachromosomal ribosomal RNA genes in Tetrahymena. J. Mol. Biol. 1978: 120: 33-53.). Today we know that telomeres form specialized, three-dimensional DNA-protein structures and fulfil important capping functions. Besides, telomeric DNA is essential as ''access DNA'' for those cells that are not able to counteract loss of DNA during replication because they do not express telomerase, the enzyme responsible for telomere length maintenance. Since telomerase is mostly found in tumor cells and inhibition correlates with telomere shortening and finally growth inhibition, telomerase and lately also the telomeres themselves have become attractive targets for anti-cancer therapy. This review aims to critically throw light on different therapeutical approaches and comes to the conclusion that telomeres may be the better targets for cancer therapeutics.

Telomeric aggregates and end-to-end chromosomal fusions require myc box II

Telomeres of tumor cells form telomeric aggregates (TAs) within the three-dimensional (3D) interphase nucleus. Some of these TAs represent end-to-end chromosomal fusions and may subsequently initiate breakage-bridge-fusion cycles. Wild-type (wt) and myc box II mutant (mt) Myc induce different types of genomic instability when conditionally expressed in mouse proB cells (Ba/F3). Only wt Myc overexpressing Ba/F3 cells are capable of tumor formation in severe combined immunodeficient mice. In this study, we investigated whether telomere dysfunction leading to TA formation is linked to the genetic changes that permit wt c-Myc-dependent transformation of Ba/F3 cells. To this end, we examined the 3D organization of telomeres after the deregulated expression of deletion myc boxII mutant (Delta106) or wt Myc. Delta106-Myc overexpression did not induce TAs, whereas wt-Myc deregulation did. Instead, Delta106-Myc remodelled the 3D telomeric organization such that telomeres aligned in the center of the 3D interphase nucleus forming a telomeric disk owing to a Delta106-induced G1/S cell cycle arrest. In contrast, wt-Myc overexpression led to distorted telomere distribution and TA formation. Analysis of chromosomal alterations using spectral karyotyping confirmed Delta106-Myc and wt-Myc-associated genomic instability. A significant number of chromosomal end-to-end fusions indicative of telomere dysfunction were noted in wt-Myc-expressing cells only. This study suggests that TAs may play a fundamental role in Myc-induced tumorigenesis and provides a novel way to dissect tumor initiation.

The significance of telomeric aggregates in the interphase nuclei of tumor cells

Telomeres are TTAGGG repetitive motifs found at the ends of vertebrate chromosomes. In humans, telomeres are protected by shelterin, a complex of six proteins (de Lange [2005] Genes Dev. 19: 2100-2110). Since (Müller [1938] Collecting Net. 13: 181-198; McClintock [1941] Genetics 26: 234-282), their function in maintaining chromosome stability has been intensively studied. This interest, especially in cancer biology, stems from the fact that telomere dysfunction is linked to genomic instability and tumorigenesis (Gisselsson et al. [2001] Proc. Natl. Acad. Sci. USA 98: 12683-12688; Deng et al. [2003] Genes Chromosomes Cancer 37: 92-97; DePinho and Polyak [2004] Nat. Genetics 36: 932-934; Meeker et al. [2004] Clin. Cancer Res. 10: 3317-3326). In the present overview, we will discuss the role of telomeres in genome stability, recent findings on three-dimensional (3D) changes of telomeres in tumor interphase nuclei, and outline future avenues of research.

Telomere-dependent chromosomal instability

Telomeres are specialized DNA-protein structures at the ends of the linear chromosomes. In mammalian cells, they are composed of multifold hexameric TTAGGG repeats and a number of associated proteins. The double-stranded telomeric DNA ends in a 3' single stranded overhang of 150 to 300 base pair (bp) which is believed to be required for a higher order structure (reviewed in (Blackburn, 2001)). One important model is that the telomeres form loop structures, the T-loops, and by invasion of the 3' overhang into the duplex region of the double stranded part protect the DNA against degradation and hinder the cellular machinery to recognize the ends as broken DNA, thus providing chromosomal integrity (Griffith et al, 1999). If telomeres become critically short they loose their capping function, become sticky, and are prone to illegitimate chromosome end-to-end fusions. The resulting dicentric chromosomes are highly unusable and because of bridge-fusion-breakage cycles they give rise to chromosomal translocations, deletions, and amplifications. Thus, critically short telomeres are thought to be responsible for the onset of genomic instability. In addition, we provide evidence that in a length-independent manner telomeres can confer to genomic instability by forming telomericaggregates which through chromosomal dys-locations contribute to chromosomal aberrations.

c-Myc induces chromosomal rearrangements through telomere and chromosome remodeling in the interphase nucleus

In previous work, we showed that telomeres of normal cells are organized within the 3D space of the interphase nucleus in a nonoverlapping and cell cycle-dependent manner. This order is distorted in tumor cell nuclei where telomeres are found in close association forming aggregates of various numbers and sizes. Here we show that c-Myc overexpression induces telomeric aggregations in the interphase nucleus. Directly proportional to the duration of c-Myc deregulation, we observe three or five cycles of telomeric aggregate formation in interphase nuclei. These cycles reflect the onset and propagation of breakage-bridge-fusion cycles that are initiated by end-to-end telomeric fusions of chromosomes. Subsequent to initial chromosomal breakages, new fusions follow and the breakage-bridge-fusion cycles continue. During this time, nonreciprocal translocations are generated. c-Myc-dependent remodeling of the organization of telomeres thus precedes the onset of genomic instability and subsequently leads to chromosomal rearrangements. Our findings reveal that c-Myc possesses the ability to structurally modify chromosomes through telomeric fusions, thereby reorganizing the genetic information.

Non-melanoma skin cancer: what drives tumor development and progression?

Non-melanoma skin cancer, i.e. basal cell carcinoma (BCC) and squamous cell carcinoma (SCC) are the most frequent tumors and their number is still increasing worldwide. Furthermore, immunosuppression in organ transplant patients strongly contributes to the increase in skin cancer incidence--being 65-250 times more frequent than in the general population. Often these patients suffer from a second and third lesion and the severity of these tumors is linked to their number. SCCs in transplant recipients also appear to be more aggressive. They tend to grow rapidly, show a higher rate of local recurrences and metastasize in 5-8% of the patients (all reviewed in Ref. 2). This largely differs from BCCs which are more frequent in the general population--at a ratio of 4:1 as compared with SCCs--but the number is only increased by a factor of 10 in transplant recipients. This may suggest that 'dormant' SCC precursor cells/lesions are present at a high frequency in the population but they are well controlled by the immune system. BCC, on the other hand, may be less dependent on immune surveillance thereby underlining its different etiology. While for BCC development the genetic hallmark is abrogation of the ptch-sonic hedgehog pathway, little is known about the causal alterations of SCCs. However, the complexity of the genetic alterations (numerical and structural aberration profiles) in SCCs argues for several levels of genomic instability involved in the generation and progression of skin cancer.

Mechanisms of ultraviolet (UV) B and UVA phototherapy

Ultraviolet (UV) radiation has been used for decades with great success and at a constantly increasing rate in the management of skin diseases, becoming an essential part of modern dermatologic therapy (Krutmann et al, 1999). For phototherapy, irradiation devices emitting either predominantly middlewave UV (UVB, 290-315 nm) or longwave UV (UVA, 315-400 nm) radiation are employed. In former years, patients were treated with broad-band UVB, broad-band UVA, or combination regimens. Broad-band UV phototherapy, however, is being replaced more frequently by the use of irradiation devices that allow treatment of patients' skin with selected emission spectra. Two such modalities which have their origin in European Photodermatology are 311 nm UVB phototherapy (which uses long-wave UVB radiation above 300 nm rather than broadband UVB) and high-dose UVA1 therapy (which selective employs long-wave UVA radiation above 340 nm). In Europe, 311 nm UVB phototherapy has almost replaced classical broad-band UVB phototherapy and has significantly improved therapeutic efficacy and safety of UVB phototherapy (van Welden et al, 1988; Krutmann et al, 1999). The constantly increasing use of UVA-1 phototherapy has not only improved UVA phototherapy for established indications such as atopic dermatitis (Krutmann et al, 1992a, 1998; Krutmann, 1996), but has also provided dermatologists with the opportunity to successfully treat previously untractable skin diseases, e.g., connective tissue diseases (Stege et al, 1997; Krutmann, 1997). These clinical developments have stimulated studies about the mechanisms by which UVB and UVA phototherapy work. The knowledge obtained from this work is an indispensable prerequisite to make treatment decisions on a rationale rather than an empirical basis. Modern dermatologic phototherapy has started to profit from this knowledge, and it is very likely that this development will continue and provide dermatologists with improved phototherapeutic modalities and regimens for established and new indications. This review aims to provide an overview about current concepts of the mode of action of dermatologic phototherapy. Special emphasis will be given on studies that have identified previously unrecognized immunosuppressive/anti-inflammatory principles of UV phototherapy.