Acceleration of Diabetic Wound Healing with PHD2- and miR-210-Targeting Oligonucleotides

In diabetes-associated chronic wounds, the normal response to hypoxia is impaired and many cellular processes involved in wound healing are hindered. Central to the hypoxia response is hypoxia-inducible factor-1α (HIF-1α), which activates multiple factors that enhance wound healing by promoting cellular motility and proliferation, new vessel formation, and re-epithelialization. Prolyl hydroxylase domain-containing protein 2 (PHD2) regulates HIF-1α activity by targeting it for degradation under normoxia. HIF-1α also upregulates microRNA miR-210, which in turn regulates proteins involved in cell cycle control, DNA repair, and mitochondrial respiration in ways that are antagonistic to wound repair. We have identified a highly potent short synthetic hairpin RNA (sshRNA) that inhibits expression of PHD2 and an antisense oligonucleotide (antimiR) that inhibits miR-210. Both oligonucleotides were chemically modified for improved biostability and to mitigate potential immunostimulatory effects. Using the sshRNA to silence PHD2 transcripts stabilizes HIF-1α and, in combination with the antimiR targeting miR-210, increases proliferation and migration of keratinocytes in vitro. To assess activity and delivery in an impaired wound healing model in diabetic mice, PHD2-targeting sshRNAs and miR-210 antimiRs both alone and in combination were formulated for local delivery to wounds using layer-by-layer (LbL) technology. LbL nanofabrication was applied to incorporate sshRNA into a thin polymer coating on a Tegaderm mesh. This coating gradually degrades under physiological conditions, releasing sshRNA and antimiR for sustained cellular uptake. Formulated treatments were applied directly to splinted full-thickness excisional wounds in db/db mice. Cellular uptake was confirmed using fluorescent sshRNA. Wounds treated with a single application of PHD2 sshRNA or antimiR-210 closed 4 days faster than untreated wounds, and wounds treated with both oligonucleotides closed on average 4.75 days faster. Markers for neovascularization and cell proliferation (CD31 and Ki67, respectively) were increased in the wound area following treatment, and vascular endothelial growth factor (VEGF) was increased in sshRNA-treated wounds. Our results suggest that silencing of PHD2 and miR-210 either together or separately by localized delivery of sshRNAs and antimiRs is a promising approach for the treatment of chronic wounds, with the potential for rapid clinical translation.

Skin tissue engineering

The major applications of tissue-engineered skin substitutes are in promoting the healing of acute and chronic wounds. Several approaches have been taken by commercial companies to develop products to address these conditions. Skin substitutes include both acellular and cellular devices. While acellular skin substitutes act as a template for dermal formation, this discussion mainly covers cellular devices. In addressing therapeutic applications in tissue engineering generally, a valuable precursor is an understanding of the mechanism of the underlying pathology. While this is straightforward in many cases, it has not been available for wound healing. Investigation of the mode of action of the tissue-engineered skin substitutes has led to considerable insight into the mechanism of formation, maintenance and treatment of chronic wounds. Four aspects mediating healing are considered here for their mechanism of action: (i) colonization of the wound bed by live fibroblasts in the implant, (ii) the secretion of growth factors, (iii) provision of a suitable substrate for cell migration, particularly keratinocytes and immune cells, and (iv) modification of the immune system by secretion of neutrophil recruiting chemokines. An early event in acute wound healing is an influx of neutrophils that destroy planktonic bacteria. However, if the bacteria are able to form biofilm, they become resistant to neutrophil action and prevent reepithelialization. In this situation the wound becomes chronic. In chronic wounds, fibroblasts show a senescence-like phenotype with decreased secretion of neutrophil chemoattractants that make it more likely that biofilms become established. Treatment of the chronic wounds involves debridement to eliminate biofilm, and the use of antimicrobials. A role of skin substitutes is to provide non-senescent fibroblasts that attract and activate neutrophils to prevent biofilm re-establishment. The emphasis of the conclusion is the importance of preventing contaminating bacteria becoming established and forming biofilms.

Inhibition of keratinocyte migration by lipopolysaccharide

A critical process in cutaneous wound healing is reepithelialization by keratinocytes that closes the breach in the epidermis. Chronic wounds fail to reepithelialize despite the presence of activated and proliferative keratinocytes around the wound perimeter. This type of wound is generally colonized to a greater or lesser extent by bacteria. This study examines the possibility that bacterial products might directly inhibit keratinocyte migration. Using conventional scratch assays, we observed a dose-dependent inhibition of keratinocyte migration by lipopolysaccharide (LPS) derived from either Pseudomonas aeruginosa or Escherichia coli. Although the P. aeruginosa preparation appeared to be slightly more inhibitory, both gave half-maximal inhibition at 0.5-0.6 ng/mL. Migration of fibroblasts was not inhibited. The result could not be attributed to a cytotoxic effect of the LPS. LPS inhibition of migration was relieved by neutralizing antibodies to toll-like receptors (TLR), 40% by anti-TLR2 and 75% by anti-TLR4. We conclude that keratinocyte migration is inhibited by bacterial products, detected through TLR4 and also through TLR2. Because chronic wounds always show some presence of bacteria, these findings provide a possible explanation for the lack of healing found in ulcers.

Commercial considerations in tissue engineering

Tissue engineering is a field with immense promise. Using the example of an early tissue-engineered skin implant, Dermagraft, factors involved in the successful commercial development of devices of this type are explored. Tissue engineering has to strike a balance between tissue culture, which is a resource-intensive activity, and business considerations that are concerned with minimizing cost and maximizing customer convenience. Bioreactor design takes place in a highly regulated environment, so factors to be incorporated into the concept include not only tissue culture considerations but also matters related to asepsis, scaleup, automation and ease of use by the final customer. Dermagraft is an allogeneic tissue. Stasis preservation, in this case cryopreservation, is essential in allogeneic tissue engineering, allowing sterility testing, inventory control and, in the case of Dermagraft, a cellular stress that may be important for hormesis following implantation. Although the use of allogeneic cells provides advantages in manufacturing under suitable conditions, it raises the spectre of immunological rejection. Such rejection has not been experienced with Dermagraft. Possible reasons for this and the vision of further application of allogeneic tissues are important considerations in future tissue-engineered cellular devices. This review illustrates approaches that indicate some of the criteria that may provide a basis for further developments. Marketing is a further requirement for success, which entails understanding of the mechanism of action of the procedure, and is illustrated for Dermagraft. The success of a tissue-engineered product is dependent on many interacting operations, some discussed here, each of which must be performed simultaneously and well.

Skin substitutes to enhance wound healing

Biologically-based skin substitutes have developed as commercial products over the last 5 years. The first generation includes the collagen-based synthetic device, Integra, and Alloderm, which is based on devitalised and cross-linked human dermis. These are used as dermal replacements for third degree burns. Within the last year, the tissue-engineered product, Dermagraft-TC, has become available. While originally intended as a temporary covering for severe burns, Dermagraft-TC has proved to markedly improve the healing of deep second degree burns. The earliest living skin substitutes used autologous keratinocytes expanded in vitro. Two new products containing living cells, Dermagraft and Apligraf, are expected to be approved shortly for diabetic foot ulcers and venous stasis ulcers, respectively. Dermagraft is produced by growing human fibroblasts on a three-dimensional scaffold. The cells actively proliferate and lay down extracellular matrix to generate a papillary dermis-like device that shows a combination of angiogenic, growth factor and cell adhesion properties that enhance healing in diabetic foot ulcers. The production of Apligraf includes casting human fibroblasts in collagen, in order to generate a dermal equivalent on which is grown an epidermis. The structure is akin to a skin graft and is so applied. Despite Dermagraft and Apligraf being of allogeneic origin, rejection has not been an issue in clinical trials and possible contamination by pathogens has been eliminated as a concern through extensive testing. These developments represent a new concept and are expected to revolutionise wound care. They may also provide a platform for gene therapy applications.

Tissue-engineered skin substitutes

The development of tissue engineered skin substitutes has reached a mature stage; major remaining questions relate to areas of greatest potential utility. Research is mainly concerned with extending indications of existing products, including treatment of the full range of acute, chronic, surgical and cosmetic wounds, and in other applications such as soft tissue augmentation and gene delivery. Among the few advances in the composition of skin substitutes is the inclusion of endothelial cells to provide a pathway for vascularisation by the host system.

SB203580 promotes EGF-stimulated early morphological differentiation in PC12 cell through activating ERK pathway

MAP kinases have important role in PC12 cell differentiation, since the activities of both extracellular regulated protein kinase (ERK) and p38 have been indicated as necessary signal for PC12 cell differentiation. Epidermal growth factor (EGF) and NGF both activate ERK and p38 in PC12 cells, but only NGF trigger differentiation. It has been proposed that the duration of ERK activation determines the switch from proliferation to differentiation, since EGF causes more transient activation of ERK than NGF in PC12 cells. Here we report that treatment of PC12 cells with EGF in the presence of SB203580, a widely used p38 inhibitor, caused differentiation. The pro-differentiation effect of SB203580 in EGF-treated PC12 cells was found to be independent of its function of p38 inhibition but was through an effect on the ERK pathway that has been recently reported (Kalmes et al. [1999] FEBS Lett. 444: 71-74; Hall-Jackson et al. [1999] Onc. 18: 2047-2054). We found that SB203580 by itself did not affect the activity of ERK1/2 but significantly extended EGF-induced ERK activation in PC12 cells, which resulted in early morphological differentiation. Our data indicated that although both ERK and p38 are required for PC12 cell differentiation, activation of p38 is not required when ERK is superactivated. Our data provided further evidence for the threshold theory that differentiation is determined by the duration of ERK activation.

Tissue-engineered skin substitutes

The last two years have seen new tissue-engineered skin substitutes come onto the market and begin to resolve the various roles to which each is best suited. It is becoming evident that some of the very expensive cell-based products have cost-benefit advantage despite their high price and are valuable within the restricted applications for which they are intended. The use of skin substitutes for testing purposes has extended from epidermal keratinocytes to other integumentary epithelia and into preparations containing multiple cell types in which reactions resulting from paracrine interactions can be examined. Challenges remain in the application of gene therapy techniques to skin substitutes, both the control of transgene expression and in the selection of suitable genes to transfect. A coming challenge is the production of tissue-engineered products without the use of animal products other than human cells. A challenge that may be diminishing is the importance of acute rejection of allogeneic tissue-engineered skin substitutes.

Overexpression of trehalose synthase and accumulation of intracellular trehalose in 293H and 293FTetR:Hyg cells

A humanized clone containing the trehalose-6-phosphate synthase and trehalose-6-phosphate phosphatase (otsA/B) has been constructed. Using the Gateway Cloning System (Invitrogen, Inc.), the otsA/B genes have been placed under the control of the CMV promoter (pEXPcmv-otsA/B) or the CMV promoter and the tet operator (pEXP cmv TetO-otsA/B). The pEXPcmv-otsA/B clone has been introduced into 293H cells using LIPOFECTAMINE 2000 and the intracellular concentration of trehalose has been evaluated. The 293H cells accumulate 4-5 microg trehalose/mg dry weight and this concentration increases to 7-10 microg trehalose/mg dry weight if trehalose is included in the growth medium. The pEXPcmv TetO-otsA/B clone has been transfected into 293FTetR:Hyg cells which contain the tet repressor integrated into the genome. When these transfected cells are grown in the absence of tetracycline, no intracellular trehalose is detected. Inclusion of 0.3 microg/ml tetracycline in the growth medium results in the accumulation of 11-14 microg trehalose/mg dry weight, a value which increases to 19-20 microg trehalose/mg dry weight if trehalose is included in the growth medium. The data for the 293FTetR:Hyg cells indicate that intracellular trehalose accumulates in response to the addition of tetracycline. This system will allow us to manipulate the intracellular concentration of trehalose and to evaluate the desiccation tolerance of these cells as a function of intracellular trehalose concentration.

Modification of fibroblast gamma-interferon responses by extracellular matrix

Fibroblasts from scaffold-based three-dimensional human cultures have been demonstrated to colonize ulcer wound beds and persist for at least 6 mo without rejection. This study examines the expression in these cultures of molecules associated with activation of the immune system in acute rejection. Studies in monolayer cultures showed that fibroblasts expressed CD40 at about 10% of the surface density seen in umbilical vein endothelial cells, whereas HLA-DR was undetectable. In these cultures, both molecules were induced by gamma-interferon. In scaffold-based three-dimensional cultures, however, a majority of the fibroblasts showed little induction of CD40 and HLA-DR in response to gamma-interferon, although HLA class I expression was increased. Fibroblasts re- isolated from the three-dimensional cultures and cultured in monolayers recovered HLA-DR induction in response to gamma-interferon. Fibroblasts cultured in an alternative three-dimensional system using collagen gels showed CD40 and HLA-DR induction by gamma-interferon in the same manner as monolayer cultures. Comparison of phosphorylation of signal transducer and activator of transcription 1 on tyrosine-701 showed it to be similar in monolayer and three-dimensional culture, and phospho-signal transducer and activator of transcription 1 moved into the nucleus. Induction of the class II transcription activator was greatly reduced, however. We propose that interaction of fibroblasts with the fibroblast-derived extracellular matrix is an important modulator of gamma-interferon responsiveness and that this interaction may play a role in the low immunogenicity of allogeneic fibroblasts grown on scaffolds.

Comparison of the stress response to cryopreservation in monolayer and three-dimensional human fibroblast cultures: stress proteins, MAP kinases, and growth factor gene expression

Stress responses induced in fibroblasts by cryopreservation were compared in suspension or three-dimensional cultures at various times up to 5 days of recovery. Cryopreservation caused an 86% inhibition in [(35)S]methionine incorporation, with recovery over 2 days to 45% &plusmn: 14% of its original value. Stress proteins, including heat shock protein (hsp) and glucose-regulated proteins (GRP), detected by immunoblotting, responded with transient increases in cellular content (hsp27 and hsp90 in suspension and three-dimensional culture, and hsp70 only in three-dimensional culture), decreases at 24 h (hsp56, hsp70, hsp90, and GRP78 in three-dimensional culture and hsp90 in suspension), or little change (hsp70 in suspension). Polyacrylamide gel electrophoresis of [(35)S]methionine-labeled proteins showed transient induction of hsp47 within 4 h, and increased synthesis of hsp90 and GRP78 and other unidentified proteins at 24 h, but no change in hsp70. The mitogen-activated protein (MAP) kinase, p38, showed a transient increase after thawing, followed by a peak in extracellular signal-regulated kinase at 24 h. The stress-activated protein kinase (JNK) was not activated. In both stress protein and MAP kinase responses, the three-dimensional cultures showed a more intense response than fibroblasts in suspension. Although some responses were related to osmotic and cold stress during freezing, others were unique. Cryopreservation induced mRNA for selected growth factors, including vascular endothelial growth factor (VEGF) and platelet-derived growth factor (PDGF) A chain, which increased 5- to 20- fold at 48 h returning to basal levels by 120 h. Our results indicate the novel finding that cryopreservation of fibroblasts grown in three-dimensional culture induced a specific cellular stress response including growth factors.

Human three-dimensional fibroblast cultures express angiogenic activity

Human neonatal fibroblasts were cultured on a lactate-glycollate copolymer scaffold for 12-16 days to form a three-dimensional dermal equivalent tissue. The cellular content of vascular endothelial growth factor (VEGF) mRNA in these three-dimensional cultures was 22-fold greater than that observed in the same fibroblasts grown as monolayers. No induction was shown by hepatocyte growth factor (HGF) or angiopoietin 1 indicating that the effect was specific to VEGF. The predominant VEGF splice variant, detected by RT-PCR corresponded to the 121 amino acid form, with less of the 165 amino acid form. The cell-associated forms (189 and 206 amino acids) comprised less than 1% of the total VEGF mRNA. VEGF and HGF proteins, determined by ELISA, were secreted in physiologically significant amounts, 0.5-4 ng per 24 h/10(6) cells. Conditioned medium from the three-dimensional cultures stimulated proliferation of endothelial cells in a dose-dependent manner and induced cellular expression of integrin alpha(v)beta(3). Conditioned medium from the same dermal fibroblasts cultured in monolayer showed little angiogenic activity in any of these assays. Using the chorioallantoic membrane (CAM) angiogenesis assay, the cultures stimulated blood vessel production 2.8-fold over scaffold alone. VEGF-neutralizing antibody reduced the vessel development in the CAM to the level in the scaffold control. Anti-HGF antibody had no significant effect. In conclusion, three-dimensional cultures of dermal equivalent tissue express angiogenic activity to a greater extent than monolayer cultures, some of which can be assigned to VEGF.

Trehalose expression confers desiccation tolerance on human cells

Many organisms that withstand desiccation express the disaccharide trehalose. We have now expressed the otsA and otsB genes of Escherichia coli, which encode trehalose biosynthetic enzymes, in human primary fibroblasts using a recombinant adenovirus vector. Infected cells produced increased amounts of trehalose with increasing multiplicity of infection (MOI). Human primary fibroblasts expressing trehalose could be maintained in the dry state for up to five days. Fourier transform infrared spectroscopy indicated that dry, but viable, human cells contained no detectable water. This study shows that mammalian cells can be engineered to retain viability in the absence of water.

Three-dimensional fibroblast culture implant for the treatment of diabetic foot ulcers: metabolic activity and therapeutic range

Dermagraft is three-dimensional, allogeneic, human neonatal dermal fibroblast culture grown on a degradable scaffold and cryopreserved. Clinical trials for treatment of diabetic foot ulcers showed optimal healing within a therapeutic range of metabolic activity, determined by 3[4,5-dimethylthiazol-2-yl]2,5-diphenyltetrazolium bromide (MTT) reduction. Actions of Dermagraft in wound repair include colonization by cells and provision of growth factors and cytokines, both activities dependent on living cells. Cells in the cryopreserved culture showed 60% viability by dye exclusion and, when isolated, were able to proliferate in monolayer culture. Protein synthesis by Dermagraft was inhibited 70-98% by cryopreservation, but, if within the therapeutic range, recovered to 45-85% of the prefreeze value over 48 h. Subtherapeutic Dermagraft showed variable, low recovery. Expression of mRNA for vascular endothelial growth factor (VEGF), platelet-derived growth factor A chain, and insulin-like growth factor-1 was reduced >83% in subtherapeutic compared with therapeutic Dermagraft. Granulocyte colony-stimulating factor and VEGF protein secretion, determined by enzyme-linked immunosorbent assay (ELISA), and angiogenic activity also depended on therapeutic range. VEGF secretion dropped sharply with MTT reductase in subtherapeutic tissue. The data demonstrate the critical dependence of the therapeutic properties of this living dermal implant on recovery of protein synthesis, growth factor expression, and angiogenesis, determined by metabolic activity.

A metabolically active human dermal replacement for the treatment of diabetic foot ulcers

Tissue engineering, the science of growing living human tissues for transplantation, promises to revolutionize aspects of medical care. Ulcers of the skin of the feet of diabetic patients are a serious health problem and a major cause of amputations. Dermagraft, a tissue-engineered, living human dermal tissue, which provides normal growth factors and matrix proteins, has been implanted to replace a patients' destroyed dermises and heal these ulcers. Large-scale clinical studies and in vitro experiments have demonstrated the importance of controlling specific product parameters, especially the metabolic activity of the tissue, to provide, upon implantation into the wound bed, a living tissue that facilitates healing. Implanting tissue within a defined therapeutic range of metabolic activity dramatically improves healing of diabetic foot ulcers, with significantly more ulcers healed completely in a shorter time. In this new, rapidly moving science, such elucidation of the mechanism of action is vital to ensure that tissues will provide their intended benefit.

The response of human dermal microvascular endothelial cells to hypoxia

Dermal microvascular endothelial cells (DMEC) exposed to hypoxic conditions show a rapid induction of several proteins that do not increase in other cell types placed in a similar environment. These DMEC proteins differ from the well-characterized stress proteins that have been observed in a wide variety of cultured cell types. The DMEC proteins are induced rapidly, within 2-4 h, and are expressed transiently. They include a group of acidic proteins (pI approximately 5-5.2) with molecular weights in the range 100,000-120,000 and at least one glycoprotein (pI 5.1, M(r) 57,000) that is probably expressed on the cell surface. In some primary DMEC cell strains, this response is accompanied by a transient overall increase in protein synthesis. The oxygen-regulated proteins (ORP) that are induced in most other cell types under hypoxic conditions show little variation in their rate of synthesis in DMEC within the first 24 h. The response of DMEC differs from that of umbilical vein endothelial cells (UVEC) and from spindle-shaped cells derived from DMEC, that show a response to hypoxia that is similar to most other cell types. The changes seen in DMEC proteins take place in the same time scale as ischemia-reperfusion injury and may reflect the specialized change of functions of the microvasculature observed under conditions of hypoxic stress in vivo.

Miniature laser light scattering instrumentation for particle size analysis

We describe the design, construction, and testing of a miniature, all-solid state laser light scattering instrument for determination of particle sizes and distributions using photon correlation spectroscopy techniques (i.e., quasielastic or dynamic light scattering). Detailed comparative tests with standard photon correlation spectroscopy equipment are presented.

Miniature, solid state photon correlation laser Doppler velocimetry

We describe the design, construction and testing of a miniature, solid state laser Doppler velocimeter (LDV) for measurement of the velocities of gases, liquids and solid surfaces, especially using photon correlation techniques. Detailed comparative tests with standard LDV equipment are presented.

Effects of filaggrin breakdown products on the growth and maturation of keratinocytes

Filaggrin is degraded to amino acids in the stratum corneum. We tested the hypothesis that the resulting high concentrations of amino acids might be involved in the control of keratinocyte maturation. An amino acid mixture, with the composition of the filaggrin breakdown products, inhibited the growth of cultured human keratinocytes at 0.1 M. Inhibition of protein synthesis was progressive with time. After 24-h exposure, 50% inhibition occurred at 0.3-0.4 M amino acids. When the cultures were incubated with the amino acids for 7 days, the concentration giving half-inhibition was reduced to 0.2 M. No change was observed in the pattern of keratin synthesis, although synthesis of protein with Mr of 36,000 was disproportionately inhibited. It is concluded that filaggrin breakdown products are not involved in the control of keratinocyte differentiation.

Comparative effects of growth inhibitors on DNA replication, DNA repair, and protein synthesis in human epidermal keratinocytes

Cultured human epidermal keratinocytes were used as a model system for testing compounds with potential therapeutic effect against hyperproliferative skin disorders. We have investigated whether each test compound caused direct damage to the DNA or inhibited DNA repair and/or seminconservative replication of DNA, as well as its effect on the overall rate of protein synthesis and on expression of specific keratin genes. The following compounds were studied: (a) inhibitors of DNA polymerase alpha [aphidicolin and its derivative aphidicolin glycine], (b) inhibitors of topoisomerases [novobiocin, nalidixic acid, teniposide, etoposide, and 4'-(9-acridylamine) methanesulfon-m-anisidide], (c) modifiers of chromatin structure [sodium butyrate, 3-aminobenzamide, and nicotinamide], (d) inhibitors of calmodulin activation and protein kinase C [chlorpromazine and trifluoperazine]; and (e) drugs used in clinical dermatology [anthralin, fluocinolone acetonide, ketoconazole, and hydroxyurea]. The compounds were tested at concentrations at which they were known from the literature to be effective in their respective actions. Among the groups of compounds studied, the topoisomerase inhibitors were particularly interesting since they caused no detectable damage to DNA but exhibited maximal inhibitory effect on replication combined with minimal inhibition of DNA repair. In addition most of the topoisomerase inhibitors, particularly novobiocin, changed the pattern of gene expression by inhibiting the synthesis of certain keratins and inducing a Mr 67,000 protein in the prekeratin fraction. These properties combined with minimal systemic side effects may encourage the clinical exploration of some topoisomerase inhibitors for antiproliferative therapy of skin disorders.

A rapid assay for DNA repair synthesis using neutral and alkaline sodium iodide gradients

A modification to the method for estimation of repair DNA synthesis in cultured mammalian cells using density labeling which reduces the time and labor involved is described. Centrifugation times are reduced by using high-molecular-weight DNA which is loaded onto the top of a preformed sodium iodide gradient. Cerenkov counting is used to locate the region of the gradient containing light DNA for rebanding and likewise to identify appropriate fractions from alkaline gradients for acid precipitation for scintillation counting. The procedures give results indistinguishable from cesium chloride equilibrium gradients in a shorter time, with less effort, and at a lower cost.

Epidermal characteristics related to skin cancer susceptibility

We have compared the basal cell labeling index and cellular architecture in samples of epidermis removed by vacuum blistering from people with or without a personal history of skin cancer. Donors with no family history of skin cancer showed a basal cell labeling index of 5.5% with a standard deviation of 1.3%. Those not personally affected but with a family history gave 4.1% +/- 0.4% but among cancer patients the value was approximately doubled to 11.5% +/- 2.7%. The proportion of cells replicating was reduced after ultraviolet irradiation, with a D0 of 40 J/m2 for UVB but no difference could be demonstrated between individuals with or without a history of skin cancer.

Ultraviolet radiation sensitivity of proliferating and differentiated human neuroblastoma cells

The effects of ultraviolet (U.V.) radiation were studied on a cloned line of human neuroblastoma cells in proliferative and differentiated growth modes, the latter being induced by serum deprivation. The neuroblastoma cells were found to be unusually sensitive in comparison with HeLa cells when survival was measured by colony formation in soft agar, the differentiated mode being the most sensitive. Ultraviolet radiation sensitivity was associated with very low DNA repair capacity as measured by DNA repair synthesis and by removal of M. luteus endonuclease-sensitive sites from irradiated DNA. The greater sensitivity of the differentiated cells appeared to be related to a greater degree of DNA damage at a given U.V. dose, resulting from altered cell geometry in the growth mode. The neuroblastoma cells showed little or no post-irradiation inhibition of DNA replication at low U.V. doses, suggesting that it is the repair process rather than the DNA damage which is responsible for inhibiting replication.

Frequency of U.V. induced protein-DNA crosslinks in cell lines of different sensitivities

We have measured U.V.-induced protein-DNA crosslinking in two cultured human cell lines of different sensitivities. Using a previously published method, involving SDS-protein precipitation, we obtained a biphasic response with an initial slope of 0.6 per cent DNA J-1 m2 up to 50 J m-2 and a second-phase slope of 0.12 per cent DNA J-1 m2 with a background of 22 +/- 13 per cent. Rigorous washing of SDS-protein precipitates reduced background binding to about 5 per cent with a linear U.V. effect up to 100 J m-2 of 0.038 per cent DNA J-1 m2. Binding was judged to be covalent on the grounds of stability to boiling and represented 4.1 crosslinks pg DNA-1 J-1 m2 or 60 crosslinks cell-1 J-1 m2. Similar results were obtained for both cell lines. We conclude that the differences in U.V. survival between cell lines is not related to the extent of protein-DNA crosslinking. We have been unable to detect repair of these lesions in either cell line.

Resistance of human melanoma cells to ultraviolet radiation

A series of five human melanoma cell lines has been demonstrated to be highly resistant to ultraviolet (UV) radiation, with a D0 of 400 ergs/sq mm. Melanotic melanoma cells were found to increase their production of melanin following UV radiation, whereas some amelanotic cells did not. Melanotic and amelanotic melanoma cell lines exhibited the same UV resistance; melanoma and nonmelanoma cells formed the same numbers of thymine dimers at a given UV dose. These data imply that melanin does not play a major role in protecting DNA of melanoma cells against UV damage in culture. The rates of removal of thymine dimers from DNA of melanoma cells were comparable to those in UV-sensitive, nonmelanoma cell lines, so that rapid excision repair does not explain UV resistnace in the melanoma cells. No DNA strand breakage was detected in a melanoma cell line at moderate UV doses.

The fractionation of adenosine-rich oligoribonucleotides on polyethyleneimine-cellulose

After combined pancreatic and T(1) ribonuclease treatment of RNA, a characteristic series of products of the type A(n)N are obtained, where N can be any of the four ribonucleosides. Depending on whether phosphatase treatment is used before the addition of labelled phosphate at the 5'-terminus with bacteriophage phosphokinase, the labelled oligonucleotides obtained may or may not possess a phosphate group at the 3'- as well as the 5'-end. The behaviour of these characteristic products after electrophoresis on cellulose acetate strips followed by chromatography on polyethyleneimine-cellulose thin-layer plates was examined.