Cellular senescence in naevi and immortalisation in melanoma: a role for p16?

Cellular senescence, the irreversible proliferative arrest seen in somatic cells after a limited number of divisions, is considered a crucial barrier to cancer, but direct evidence for this in vivo was lacking until recently. The best-known form of human cell senescence is attributed to telomere shortening and a DNA-damage response through p53 and p21. There is also a more rapid form of senescence, dependent on the p16-retinoblastoma pathway. p16 (CDKN2A) is a known melanoma susceptibility gene. Here, we use retrovirally mediated gene transfer to confirm that the normal form of senescence in cultured human melanocytes involves p16, since disruption of the p16/retinoblastoma pathway is required as well as telomerase activation for immortalisation. Expression (immunostaining) patterns of senescence mediators and markers in melanocytic lesions provide strong evidence that cell senescence occurs in benign melanocytic naevi (moles) in vivo and does not involve p53 or p21 upregulation, although p16 is widely expressed. In comparison, dysplastic naevi and early (radial growth-phase, RGP) melanomas show less p16 and some p53 and p21 immunostaining. All RGP melanomas expressed p21, suggesting areas of p53-mediated senescence, while most areas of advanced (vertical growth-phase) melanomas lacked both p16 and p21, implying escape from both forms of senescence (immortalisation). Moreover, nuclear p16 but not p21 expression can be induced in human melanocytes by oncogenic BRAF, as found in around 80% of naevi. We conclude that cell senescence can form a barrier to melanoma development. This also provides a potential explanation of why p16 is a melanoma suppressor gene.

A housekeeper with power of attorney: the rRNA genes in ribosome biogenesis

Ribosome biogenesis centres both physically and functionally on the activity of the ribosomal RNA (rRNA) genes. Ribosome assembly occurs co-transcriptionally on these genes, requires the coordinated expression and assembly of many hundreds of proteins and is finely tuned to cell and organism growth. This review presents contemporary understanding of the mode and the means of rRNA gene transcription and how growth factors, oncogenes and tumour suppressors regulate this transcription. It is argued that transcription elongation is a key mechanism regulating rRNA gene transcription. This unorthodox view provides a logical framework to explain the co-transcriptional phase of ribosome biogenesis.

A transcriptional function for the repetitive ribosomal spacer in Xenopus laevis

The function of the Xenopus laevis ribosomal spacer has been studied in vivo and by microinjection of in vitro mutants into Xenopus oocytes. It is shown that the spacer directs specific RNA transcripts which most probably terminate upstream of the ribosomal genes and that it is able to modulate transcription of these genes. The data lead to a model in which the ribosomal spacer is a loading site for RNA polymerase I and spacer transcription is the driving force by which polymerase is delivered to the ribosomal gene promoter.

Studies on the role and mode of operation of the very-lysine-rich histone H1 in eukaryote chromatin. The three structural regions of the histone H1 molecule

Limited digestion with trypsin of both calf thymus H1 histone and the fragment 1--120 of the H1 molecule has resulted in the isolation of the fragment 35--120. This fragment assumes a globular structure under physiological conditions of pH and ionic strength. The variable N-terminal portion of the molecule, up to residue 34, is not required for the formation of the H1 globular structure. Proton nuclear magnetic resonance (NMR) and ultracentrifugation studies show that the H1 histone molecule consists of three distinct structural domains under structuring conditions: a random coil 'nose' consisting of 35 to 40 residues from the N-terminal end; a globular 'head' involving the next approximately 80 residues; and a random-coil 'tail' of the remainder of the molecule.

Sequence organization of the spacer DNA in a ribosomal gene unit of Xenopus laevis

A detailed restriction map was constructed for a cloned Xenopus laevis rDNA fragment containing the nontranscribed spacer (NTS) and external transcribed spacer (ETS) together with a portion of both the 18S and 28S rRNA genes. The NTS was found to contain at least three distinct repetitious areas. Region 1 has a repeating unit of approximately 100 bp. The primary structure of this unit has been determined by DNA sequencing. Region 2 is very similar in organization to region 3, and both have an alternating 81/60 bp arrangement as revealed by restriction with Alu I and DNA sequencing. It can be shown that the 81 and 60 bp canons are virtually identical to one another excepting a deletion/insertion of a 21 bp segment. Region 3 differs from region 2 in having sites for Sma I with its 81 bp units. Between these repeated DNA sequences there are two identical, nonrepetitive DNA sequences, each of which is centered around a Bam Hl site. Most of the ETS has been sequenced. It was found to be nonrepetitive and extremely rich in Cs. Close to the 5' end of the 18S coding sequence there is a DNA stretch very rich in purines. About 2.25 kb upstream from the Eco Rl restriction site bisecting the 18S structural gene there is a unique sequence which may be homologous to the 5' end of the 40S precursor RNA. Present evidence suggests that the boundaries between NTS and ETS occur farther downstream than was suggested by electron microscopic data. Sequencing has revealed that the spacer DNA of X. laevis contains different kinds of simple DNA sequences, but no evidence has been found that spacer DNA once arose by saltation of a 15 bp segment. The most surprising finding was that the spacer sequences around the Bam restriction sites (the Bam islands) show high homology with a sequence near the NTS/ETS interface. From the restriction and sequencing analyses it can be deduced that in recent evolutionary times the DNA sequences near the 5' end of the ribosomal transcription unit were reduplicated twice and displaced into spacer by saltation of an intervening short DNA sequence (the 60/81 bp canons). Possible implications of these evolutionary events for spacer functions are consisdered. The sequencing has also provided a molecular basis for a whole range of conclusions arrived at previously by indirect approaches, and these are discussed.

An immediate response of ribosomal transcription to growth factor stimulation in mammals is mediated by ERK phosphorylation of UBF

Ribosomal transcription in mammals is regulated in response to growth, differentiation, disease, and aging, but the mechanisms of this regulation have remained unresolved. We show that epidermal growth factor induces immediate, ERK1/2-dependent activation of endogenous ribosomal transcription, while inactivation of ERK1/2 causes an equally immediate reversion to the basal transcription level. ERK1/2 was found to phosphorylate the architectural transcription factor UBF at amino acids 117 and 201 within HMG boxes 1 and 2, preventing their interaction with DNA. Mutation of these sites inhibited transcription activation and abrogated the transcriptional response to ERK1/2. Thus, growth factor regulation of ribosomal transcription likely acts by a cyclic modulation of DNA architecture. The data suggest a central role for ribosome biogenesis in growth regulation.

Short-range DNA looping by the Xenopus HMG-box transcription factor, xUBF

Xenopus UBF (xUBF) interacts with DNA by way of multiple HMG-box domains. When xUBF binds to the ribosomal promoter, the carboxyl-terminal acidic tail and amino-terminal HMG-box interact. Binding also leads to negative DNA supercoiling and the formation of a disk-like structure, the enhancesome. Within the enhancesome, an xUBF dimer makes a low-density protein core around which DNA is looped into a single 180-base pair turn, probably by in-phase bending. The enhancesome structure suggests a mechanism for xUBF-dependent recruitment of the TATA box-binding protein complex without direct interaction between the two factors.

Percutaneous penetration and dermal metabolism of triclosan (2,4, 4'-trichloro-2'-hydroxydiphenyl ether)

triclosan is widely used in many products that contact the skin of consumers. This study compares in vivo and in vitro skin absorption of triclosan and determines the potential of skin to metobolize it prior to entering the blood stream. After in vivo topical application of a 64.5mM alcoholic solution of [(3)H]triclosan to rat skin, 12% radioactivity was recovered in the faeces, 8% in the carcass 1% in the urine, 30% in the stratum corneum and 26% was rinsed from the skin surface at 24 hours after application. Free triclosan and the glucuronide and sulfate conjugates of triclosan were found in urine and faeces. triclosan penetrated rat skin more rapidly and extensively than human skin in vitro. 23% of the dose had penetrated completely through rat skin into the receptor fluid by 24 hours, whereas penetration through human skin was only 6.3% of the dose. Chromatographic analysis of the receptor solutions showed that triclosan was metabolized to the glucuronide, and to a lesser extent to the sulfate, during passage through the skin. triclosan glucuronide appeared rapidly in the receptor fluid whereas triclosan sulfate remained in the skin. Although the major site of metabolism was the liver, conjugation of triclosan in skin was also demonstrated in vitro and in vivo, particularly to the glucuronide conjugate which was more readily removed from the skin. The in vitro system provides a reasonable estimate of dermal absorption in vivo for the rat. Therefore by extrapolation of the comparative in vitro data for human and rat skin it is reasonable to deduce that dermal absorption in human of triclosan applied at the same dose is about one-third of that in the rat in vivo.

Event-related potentials--neurophysiological tools for predicting emergence and early outcome from traumatic coma

OBJECTIVE

To determine the prognostic value of multimodal evoked potentials (EPs) and event-related (ERPs) potentials in coma (Glasgow Coma Score <8), after severe traumatic brain injury (TBI).

DESIGN

Prospective, longitudinal study of neurophysiological responses recorded during traumatic coma.

SETTING

Intensive Care Unit, Frenchay Hospital, Bristol, UK.

PARTICIPANTS

Fifty-four comatose TBI patients (age range 1-80 years, mean 36.4).

METHODS

Neurophysiological responses were recorded from 11 scalp electrodes with earlobe reference. Conduction times were measured for brainstem auditory, flash visual and somatosensory, short-latency EPs. Peak latencies and amplitudes were determined for long-latency components of visual and auditory ERPs, generated by passive "oddball" paradigms. These neurophysiological and various clinical parameters were correlated with patient outcome using Pearson's coefficient.

MAIN OUTCOME MEASURE

Three month Glasgow Outcome Scale (GOS).

RESULTS AND CONCLUSION

Highly significant (P <0.001) correlations exist between long-latency ERP components and 3-month outcome. Short-latency EPs, brainstem (wave I-V) and somatosensory conduction times also correlate significantly with the GOS (P <0.01). Of the clinical measurements, pupillary response patterns, APACHE II and Glasgow Coma Scores (GCS) correlate significantly with outcome, as do the retrospective measures of duration of coma and post-traumatic amnesia (PTA) in survivors. Unfortunately, due to variance of long-latency responses, even in controls, absolute values cannot be relied upon as prognosticators. The presence of "mismatch negativity" predicted the return of consciousness (89.7% sensitivity and 100% specificity) and preceded changes in GCS. Its latency was the single best indicator of 90-day outcome from coma (r = -0.641).

Gadolinium-labeled liposomes: targeted MR contrast agents for the liver and spleen

A contrast agent for use in magnetic resonance (MR) imaging of the liver and spleen has been designed in which gadolinium-DTPA is chemically incorporated into the lamellar phase of liposome particles. This agent has excellent in vivo stability and is taken up by liver and spleen of normal mice after intravenous administration. The T1 increased by 110% in liver and 66% in spleen at 4 degrees C. At 37 degrees, the relaxation rate in the liver increased by 180%. This method is an attractive concept for the development of various organ-specific liposomal contrast agents that may be used for either MR imaging or nuclear medicine.

High-resolution proton-magnetic-resonance studies of chromatin core particles

The binding of histones in chromatin core particles and in core particles depleted of histones H2A and H2B has been studied by high-resolution proton nuclear magnetic resonance (NMR) at 270 MHZ. At low ionic strengths it is shown that histones H3 and H4 are bound in the core particle. Further, whereas the apolar regions of H2A and H2B are also bound to the core particle, the basic N-terminal and C-terminal regions are more mobile and give rise to sharp resonances in the NMR spectrum of the core particle. Between 0.3 and 0.6 M NaCl there is further release of basic regions of histones H3 and H4 from the complex. The dissociation of the core particle between 0.6 and 2.0 M NaCl is accompanied by the release of the structured apolar regions of the histones as evidenced by the appearance of a complex aromatic spectrum and perturbed upfield ring-current-shifted methyl resonances. Arginine residues are implicated in the binding between histones and DNA and 69% of these residues are found in the apolar regions of the histones. The interactions between histones and DNA in the core particle thus involves H3 and H4 and the apolar regions of H2A and H2B. It is suggested that these basic regions of H2A and H2B have binding sites outside the core particle.

Competitive recruitment of CBP and Rb-HDAC regulates UBF acetylation and ribosomal transcription

RNA polymerase I (PolI) transcription is activated by the HMG box architectural factor UBF, which loops approximately 140 bp of DNA into the enhancesome, necessitating major chromatin remodeling. Here we show that the acetyltransferase CBP is recruited to and acetylates UBF both in vitro and in vivo. CBP activates PolI transcription in vivo through its acetyltransferase domain and acetylation of UBF facilitates transcription derepression and activation in vitro. CBP activation and Rb suppression of ribosomal transcription by recruitment to UBF are mutually exclusive, regulating in vivo PolI transcription through an acetylation-deacetylation "flip-flop." Thus, PolI transcription is regulated by protein acetylation, and the competitive recruitment of CBP and Rb.

Neuropathological correlates of temporal pole white matter hyperintensities in CADASIL

BACKGROUND AND PURPOSE

White matter (WM) hyperintensities on MRI or leukoaraiosis is characteristic of stroke syndromes. Increased MRI signals in the anterior temporal pole are suggested to be diagnostic for cerebral autosomal-dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL), with 90% sensitivity and 100% specificity. The structural correlates of these specific WM hyperintensities seen on T2-weighted and FLAIR sequences in the temporal pole of CADASIL are unclear. We assessed pathological changes in postmortem tissue from the temporal pole to reveal the cause of CADASIL-specific WM hyperintensities.

METHODS

A combination of tinctorial and immunostaining approaches and in vitro imaging methods were used to quantify the extent of perivascular space (PVS), arteriosclerosis determined as the sclerotic index, WM myelination as the myelin index, and damage within the WM as accumulated degraded myelin basic protein in samples of the anterior temporal pole from 9 CADASIL and 8 sporadic subcortical ischemic vascular dementia cases, and 5 similarly aged (young) and 5 older controls. Luxol fast blue-stained serial sections from a CADASIL case were also used to reconstruct the temporal pole, which was then compared to the MR images.

RESULTS

Luxol fast blue sections used to reconstruct the temporal pole revealed an abundance of enlarged PVS in the WM that topographically appeared as indistinct opaque regions. The mean and total areas of the PVS per WM area (%PVS) were significantly greater in CADASIL compared to the controls. The myelin index was severely reduced in CADASIL in relation to the subcortical ischemic vascular dementia and control sample that was consistent with increased immunoreactivity of degraded myelin basic protein, indicating myelin degeneration. Cerebral microvessels associated with the PVS exhibited a 4.5-fold greater number of basophilic (hyalinized) vessels and a 57% increase in the sclerotic index values in CADASIL subjects compared to young controls. A significant correlation between the quantity of hyalinized vessels and sclerotic index values was also apparent (P<0.05).

CONCLUSIONS

Our findings suggest that MRI hyperintensities in the temporal pole of CADASIL patients are explained by enlarged PVS and degeneration of myelin accompanied by lack of drainage of the interstitial fluid rather than lacunar infarcts. Consistent with the lack of MR hypersignals in the temporal pole of older subcortical ischemic vascular dementia subjects, our observations imply greater progression of pathological changes in CADASIL patients.

More ribosomal spacer sequences from Xenopus laevis

The base sequence analysis of a Xenopus laevis ribosomal DNA repeat (7) has been extended to cover almost the entire non-transcribed and external transcribed spacer. A compilation of these sequences is presented. All the repetitive and non-repetitive sequence elements of the spacer are identified and their evolution discussed. Comparison of the X.laevis and S.cerevisiae (25,26) ribosomal DNAs shows about 80% sequence conservation in the 18S gene but no sequence conservation, from the available data, in the external transcribed spacer. The sequence coding for the 3' terminus of the X.laevis 40S ribosomal precursor RNA is presented and its structural features analyzed.

A pH-dependent interaction between histones H2A and H2B involving secondary and tertiary folding

It has been shown by high-resolution proton magnetic resonance (PMR) spectroscopy and circular dichroism (CD) that an H2A/H2B histone complex exists after salt extraction of these histones from chromatin and that this complex can be fully renatured from both urea-denatured acid-extracted and from urea-denatured salt-extracted histones. The histone complex is shown to involve specific secondary and tertiary structure. Formation of this complex is observed to be critically dependent on pH, occurring at and above pH 5. It cannot be induced below pH 5 by increase in ionic strength. From CD spectra the H2A/H2B complex is shown to contain about 37% alpha helix but no beta structure, the latter being confirmed by infrared spectroscopy in the 6-mum region. The PMR spectra show that the structured region includes most of the aromatic residues of both histones, at least two histidine residues of H2B and probably histidines 31 and 82 of histone H2A. The secondary structure of histones H2A and H2B is predicted using the Chou and Fasman procedure and comparisons are made between the predictions for histones of different species. These results in conjunction with the experimental evidence lead to the conclusion that at least residues 31-95 of H2A and residues 37-114 of H2B, i.e. the more apolar regions of the molecules, are involved in the tertiary structure of the H2A/H2B complex.

The RNA polymerase I transcription factor xUBF contains 5 tandemly repeated HMG homology boxes

The RNA polymerase I transcription factor UBF has been identified in human, mouse, rat and Xenopus and the primary structure of the human protein has been determined. Human UBF was shown to contain four tandem homologies to the folding domains of the HMG1 and 2 proteins and hence to belong to a previously unrecognised family of 'HMG-box' transcription factors. Here, cDNA clones encoding the Xenopus laevis UBF (xUBF) have been isolated and sequenced. Northern and Southern blots revealed that in tissue culture cells, xUBF is coded on a single major mRNA size species by a small number of genes. The deduced primary structure of xUBF is highly homologous with the human protein except for a central deletion which removes most of one HMG-box. This explains the major size difference between the X. laevis and human proteins and may well explain their different transcriptional specificities. It is shown that xUBF contains 5 tandemly repeated HMG-boxes and that by analogy the human protein contains 6.

Spacer promoters are essential for efficient enhancement of X. laevis ribosomal transcription

The X. laevis ribosomal DNA spacer contains duplicated RNA polymerase I "spacer promoters" and an array of repeated 60/81 bp promoter-related sequences. The latter have been shown to enhance transcription from a 40S preribosomal RNA promoter in cis. Here we present evidence that at least one spacer promoter is also necessary for efficient enhancement. Deletion of the spacer promoter sequences in a construct carrying only one such promoter reduces 40S RNA transcription to approximately 10% of wild type. This effect apparently is caused by inactivation of the spacer promoter, since mutants in which 4-177 bp of the spacer promoter and adjacent sequences are deleted are functionally equivalent. Spacer promoters and 60/80 bp arrays therefore probably act together to enhance 40S pre-RNA transcription in X. laevis.

The putative promoter of a Xenopus laevis ribosomal gene is reduplicated

With the aid of a novel poly-dA tailing-partial restriction technique and S1-protection mapping, the 5' terminal coding sequence for the 40S precursor ribosomal RNA of Xenopus laevis has been exactly identified. Since the promoter sequence for the 40S RNA should lie close to its 5' terminal coding sequence, we are able to conclude that the "Bam-Island" sequence reduplication (1) almost certainly represents a promoter reduplication.

Transcription of cloned Xenopus laevis ribosomal DNA microinjected into Xenopus oocytes, and the identification of an RNA polymerase I promoter

Transcription of a cloned Xenopus laevis ribosomal DNA (rDNA) fragment, microinjected into Xenopus oocytes, is initiated at the in vivo 40S pre-ribosomal RNA (pre-rRNA) site (+/- 2 bp) by RNA polymerase I. An X. laevis RNA polymerase I promoter has been mapped by studying the transcription of in vitro rDNA mutants in the oocyte system. The active promoter lies within the DNA segment beginning 145 bp upstream, and most probably ending 16 bp downstream, from the 40S pre-rRNA initiation site (-145 bp to +16 bp). Furthermore, active promoter elements lie more than 35 bp upstream from the initiation site (-35 bp). The X. laevis RNA polymerase I promoter therefore lies mainly upstream from the 40S pre-rRNA initiation site. Independent deletion of three adjacent rDNA segments lying between -61 and +16 bp reduces promoter activity by a factor of more than 16. The central of these "null" deletions removes an oligo(T)6 motif at -27 bp that is in an analogous position to the Goldberg-Hogness (TATA) box of RNA polymerase II promoters.

Physical studies on the H3/H4 histone tetramer

High-resolution proton magnetic resonance spectroscopy (270 MHz), circular dichroism, and infrared spectroscopies and ultracentrifugation studies have been carried out on the salt-extracted (H3/H4)2 tetramer from calf thymus. The tetramer contains about 29% alpha helix and no beta structure. It is denatured in 6 M urea but can be renatured simply by dialysis to water. The proton spectrum shows a number of perturbed resonances which are not observed in the spectra of either H3 or H4 alone. The observation of these resonances demonstrates that the tetramer contains some elements of tertiary structure. The overall appearance of the spectrum however is close to that of a partially denatured protein. Sedimentation velocity studies show the tetramer to have a frictional ratio of 1.99 in 50 mM acetate/50 mM bisulfite and thus to be hydrodynamically quite different from a globular protein. Two possible structural models compatible with the data are discussed.

DNA looping in the RNA polymerase I enhancesome is the result of non-cooperative in-phase bending by two UBF molecules

The so-called upstream binding factor (UBF) is required for the initial step in formation of an RNA polymerase I initiation complex. This function of UBF correlates with its ability to induce the ribosomal enhancesome, a structure which resembles in its mass and DNA content the nucleosome of chromatin. DNA looping in the enhancesome is probably the result of six in-phase bends induced by the HMG boxes of a UBF dimer. Here we show that insertion/deletion mutations in the basic peptide linker lying between the N-terminal dimerisation domain and the first HMG box of Xenopus UBF prevent the DNA looping characteristic of the enhancesome. Using these mutants we demonstrate that (i) the enhancesome structure does not depend on tethering of the entering and exiting DNA duplexes, (ii) UBF monomers induce hemi-enhancesomes, bending the DNA by 175 +/- 24 degrees and (iii) two hemi-enhancesomes are precisely phased by UBF dimerisation. We use this and previous data to refine the existing enhancesome model and show that HMG boxes 1 and 2 of UBF lie head-to-head along the DNA.

Synthetic glucocorticoids: antenatal administration and long-term implications

A clear relationship between intrauterine development and later life predisposition to long-term disease is well established. Weight at birth provides a surrogate measure for fetal development and low birth weight predicts changes in most endocrine axes in adulthood. The exposure of the fetus to elevated levels of either endogenous or synthetic glucocorticoids, pre and periconceptional nutritional status and immediate postnatal development including catch-up growth all contribute substantially to the development of adult onset disease. Fetal exposure to high levels of glucocorticoids has direct clinical relevance. Synthetic glucocorticoids (betamethasone/ dexamethasone) are administered to women at risk of preterm delivery to advance fetal maturation and reduce neonatal morbidity and mortality. However, in human pregnancy, evidence suggests that fetal exposure to synthetic glucocorticoids has detrimental effects on birth outcome, childhood cognition and long-term behavior. Studies in animals have established a link between prenatal exposure to synthetic glucocorticoids and alterations in fetal development as well as changes in placental function. These developmental alterations appear to be permanent. Whether this is the case in humans awaits long-term follow-up of children enrolled in randomized controlled trials of prenatal glucocorticoid therapy. The research challenges in this field are now centered on uncovering the mechanisms by which glucocorticoids are involved in programming the fetus for its future life, and discovering ways in which the effectiveness and safety of antenatal glucocorticoids can be enhanced. The purpose of this mini-review is to provide a background into the use of antenatal synthetic corticosteroids and to highlight and summarize recently published clinical and animal-based studies.