Effects of density and cellular aging on collagen synthesis and growth kinetics in keloid and normal skin fibroblasts

Targeting the Epigenome Reduces Keloid Fibroblast Cell Proliferation, Migration and Invasion

Keloids are pathological fibroproliferative scars resulting from abnormal collagen deposition within and beyond the margins of the initial cutaneous insult. Keloids negatively impact quality of life functionally and cosmetically, with current treatment modalities unsatisfactory. Recent studies indicate that epigenetic dysregulation is central to the development and progression of keloids. Here we evaluate the functional significance of epigenetic targeting strategies in vitro using patient-derived keloid fibroblasts treated with small molecule inhibitors of HDACs, LSD1, CoREST and p300, as potential therapies for keloids. We find that both the dual-acting CoREST inhibitor, corin, and the HDAC inhibitor, entinostat, reduce fibroblast proliferation more than the LSD1 inhibitor, GSK-LSD1; additionally, corin was the most effective inhibitor of migration and invasion across keloid fibroblasts. RNA-seq analysis of keloid fibroblasts treated with corin demonstrates coordinate upregulation of many genes including key mediators of cell adhesion such as claudins. Corin also downregulates gene sets involved in cell cycle progression, including reduced expression of cyclins A1 and B2 compared to DMSO. These results highlight a significant role for epigenetic regulation of pathologic mediators of keloidal scarring and suggest that inhibitors of the epigenetic CoREST repressor complex may prove beneficial in the prevention and/or treatment of keloidal scarring in patients.

The Keloid Disorder: Heterogeneity, Histopathology, Mechanisms and Models

Keloids constitute an abnormal fibroproliferative wound healing response in which raised scar tissue grows excessively and invasively beyond the original wound borders. This review provides a comprehensive overview of several important themes in keloid research: namely keloid histopathology, heterogeneity, pathogenesis, and model systems. Although keloidal collagen versus nodules and α-SMA-immunoreactivity have been considered pathognomonic for keloids versus hypertrophic scars, conflicting results have been reported which will be discussed together with other histopathological keloid characteristics. Importantly, histopathological keloid abnormalities are also present in the keloid epidermis. Heterogeneity between and within keloids exists which is often not considered when interpreting results and may explain discrepancies between studies. At least two distinct keloid phenotypes exist, the superficial-spreading/flat keloids and the bulging/raised keloids. Within keloids, the periphery is often seen as the actively growing margin compared to the more quiescent center, although the opposite has also been reported. Interestingly, the normal skin directly surrounding keloids also shows partial keloid characteristics. Keloids are most likely to occur after an inciting stimulus such as (minor and disproportionate) dermal injury or an inflammatory process (environmental factors) at a keloid-prone anatomical site (topological factors) in a genetically predisposed individual (patient-related factors). The specific cellular abnormalities these various patient, topological and environmental factors generate to ultimately result in keloid scar formation are discussed. Existing keloid models can largely be divided into in vivo and in vitro systems including a number of subdivisions: human/animal, explant/culture, homotypic/heterotypic culture, direct/indirect co-culture, and 3D/monolayer culture. As skin physiology, immunology and wound healing is markedly different in animals and since keloids are exclusive to humans, there is a need for relevant human in vitro models. Of these, the direct co-culture systems that generate full thickness keloid equivalents appear the most promising and will be key to further advance keloid research on its pathogenesis and thereby ultimately advance keloid treatment. Finally, the recent change in keloid nomenclature will be discussed, which has moved away from identifying keloids solely as abnormal scars with a purely cosmetic association toward understanding keloids for the fibroproliferative disorder that they are.

Cellular senescence as a possible mechanism for halting progression of keloid lesions

Keloid scarring is a consequence of aberrant wound healing that leads to expansion of the scar beyond the confines of the skin injury. Keloid scars are characterized by excessive extracellular matrix disposition, prolonged proliferation of fibroblasts, increased angiogenesis, and inflammatory cell infiltration. There is no single satisfactory treatment for keloid, and it can lead to severe disfigurements and bodily dysfunction. Thus, clarification of the mechanisms underlying keloid formation, as well as those that prevent it from behaving as a malignant tumor, has significant consequences not only for treatment of keloid but also for the prevention of malignant tumor formation. Senescence is an irreversible form of growth arrest that has been shown to play a role, both in vitro and in vivo, in preventing malignant tumorigenesis upon oncogenic stress. In this study it is shown that fibroblasts embedded inside keloid scars proliferate at a slower rate compared with either those residing at the proliferative edges of the scar or normal fibroblasts. Likewise it is demonstrated that keloid fibroblasts exhibit a cell-cycle arrest with a G2/M DNA content and a higher rate of senescence. The results also indicate that levels of the tumor suppressor protein PML are higher in the active regions of keloid. The study therefore suggests that senescence is one possible mechanism by which keloid is maintained in a benign state. On this basis, "pro-senescence therapy" should be taken into consideration when designing treatment strategies for keloid.

‘Young’ Oral Fibroblasts Are Geno/Phenotypically Distinct

Wound healing within the oral mucosa results in minimal scar formation compared with wounds within the skin. We have recently demonstrated distinct differences in the aging profiles of cells (oral mucosal and patient-matched skin fibroblasts) isolated from these tissues. We hypothesized that the increased replicative potential of oral mucosal fibroblasts may confer upon them preferential wound-healing capacities. Passage-matched early cultures of oral mucosal fibroblasts and skin fibroblasts demonstrated distinct gene expression profiles, with several genes linked to wound healing/tissue repair. This was related to an increased ability of the ‘replicatively younger’ oral mucosal fibroblasts to repopulate a wound space and reorganize their surrounding extracellular matrix environment, key activities during the wound-healing process. We conclude that oral mucosal fibroblasts exhibit a preferential healing response in vivo, due to their ‘replicatively younger’ phenotype when compared with that of patient-matched skin fibroblasts.

Keloid Pathogenesis and Treatment

BACKGROUND

Keloid management can be difficult and frustrating, and the mechanisms underlying keloid formation are only partially understood.

METHODS

Using original and current literature in this field, this comprehensive review presents the major concepts of keloid pathogenesis and the treatment options stemming from them.

RESULTS

Mechanisms for keloid formation include alterations in growth factors, collagen turnover, tension alignment, and genetic and immunologic contributions. Treatment strategies for keloids include established (e.g., surgery, steroid, radiation) and experimental (e.g., interferon, 5-fluorouracil, retinoid) regimens.

CONCLUSION

The scientific basis and empiric evidence supporting the use of various agents is presented. Combination therapy, using surgical excision followed by intradermal steroid or other adjuvant therapy, currently appears to be the most efficacious and safe current regimen for keloid management.

A new surgical treatment of keloid: keloid core excision

Keloids and hypertrophic scars result from excessive collagen deposition, the cause of which is not yet known. Unlike hypertrophic scars, keloids frequently persist at the site of injury, often recur after excision and always overgrow the boundaries of the original wound. There have been many trials to control keloids, but most of them have been unsuccessful. The authors propose a new surgical technique to treat keloids and name it keloid core extirpation. They excise the inner fibrous core from the keloid and cover the defect with a keloid rind flap, which is arterialized by the subcapsular vascular plexus. The authors treated 24 keloids of the ear, trunk, face, and genitalia with keloid core excision. Four cases of partial rind flap congestion or necrosis occurred. Those patients who healed primarily after surgery showed no evidence of keloid recurrence as long as they were followed. The authors have found the keloid core extirpation technique to be excellent in preventing keloid recurrence, with no adjuvant therapy after surgery.

Keloids and hypertrophic scars: review and treatment strategies

Keloids and hypertrophic scars represent exuberant forms of scar formation that frequently are pruritic, painful, and occasionally form strictures. As well, they may result in significant cosmetic disfigurement. Recent years have seen an increased understanding in the molecular and biological mechanisms of keloidal scar formation, allowing for the development of more specific therapeutic options for these lesions. Despite these developments, keloids and hypertrophic scars remain difficult to manage. Clinical, histopathological, and biochemical features of keloids and hypertrophic scars, as well as treatment guidelines, are discussed.

An investigation of preferential fibroblast wound repopulation using a novel in vitro wound model

To overcome the difficulties of studying wounding and wound repopulation in monolayer systems, a 3-dimensional model of wound repopulation has been developed which allows the in vitro investigation of fibroblast migration in response to experimental wounding. This model was utilized to determine whether fibroblasts derived from sites which demonstrate preferential healing (child and oral mucosal fibroblasts) possessed an increased ability to repopulate experimental wounds when compared to adult dermal fibroblasts. Fibroblasts were established from specimens derived from healthy donors undergoing minor elective surgery. Standard wounds were created in fibroblast populated collagen lattices (FPCLs) which were then overlaid upon an extracellular wound matrix. Fibroblast repopulation of the wounds was studied over 12 days using light- and scanning electron microscopy and quantified using computerized image analysis. Wound repopulation by fibroblasts derived from child donors (n = 3) was significantly (P < 0.001) more rapid than their adult tissue-matched counterparts (n = 3). Wound repopulation by oral mucosal fibroblasts (n = 3) was significantly greater than that exhibited by age-matched dermal fibroblasts (n = 3; P < 0.05). These differences were not reflected in differences in DNA synthesis (P > 0.5) or cell number (P > 0.5) within similar attached FPCL systems. These findings further support the concept of a gradual transition from the fetal to adult phenotype in wound healing. The potential applications of the model are discussed.

Characterization of a new in vitro burn wound model

Previous studies of the healing process in burn wounds have largely been dependent on different animal models. These models are not only ethically questionable but are also combined with several systemic variables that are difficult to control and standardize. The aim of this study was to develop a standardized and repeatable in vitro model of the burn wound in human skin. Burn wounds with a standardized area and depth were created in human skin from routine breast reduction operations and incubated in vitro. The re-epithelialization of the wounds was followed throughout the time of incubation (14 days) by fixing and staining wounds every second day. After 14 days of incubation, the viability of the cells in the epidermis and dermis was confirmed by isolation and culture in vitro. The wounds incubated in 10 per cent fetal calf serum were shown to heal after 7 days, whereas wounds incubated in 2 per cent serum did not show any sign of re-epithelization. However, both epidermal and dermal cells from wounds incubated in 2 per cent serum were shown to be viable after 2 weeks of incubation. These findings indicate that the in vitro model can be of great value for studies of the different phases of the healing process in burn wounds as well as stimulatory effects on the wound healing process by different pharmacological agents and treatments.

The effect of TGF-beta on keloid fibroblast proliferation and collagen synthesis

Keloids are characterized by an overabundant deposition of collagen, and they recur frequently following excision. Fibroblasts isolated from keloid tissue and maintained in cell culture continue to express an increased capacity to produce collagen. In an effort to define the mechanisms responsible for keloid formation, the potential of exogenous transforming growth factor beta 1 (TGF-beta 1) to differentially affect DNA synthesis and collagen expression in cultured human fibroblasts derived from keloid or normal dermis was investigated. In this study, TGF-beta 1 at a concentration of 5.0 ng/ml was found to stimulate DNA synthesis of keloid-derived fibroblasts to a greater extent than fibroblasts derived from normal dermis. With a microassay to measure levels of collagenase-digestible radiolabeled proteins, TGF-beta 1 was found to elicit a greater increase in absolute collagen synthesis in keloid-derived fibroblasts compared with fibroblasts derived from normal dermis. Examination of tRNA(pro) pool-specific activities indicated that these observed differences in rates of collagen synthesis were not the result of unequal rates of proline transport or pool size. Likewise, TGF-beta 1 did not alter the uptake of vitamin C, an essential cofactor and mediator needed for maximal collagen expression. The increase in collagen synthesis by keloid-derived fibroblasts treated with TGF-beta 1 was accompanied by a corresponding increase in procollagen type I mRNA levels, indicating that the differential response of keloid and normal dermal fibroblasts to this growth factor is occurring primarily at a pretranslational level. These results suggest a unique sensitivity of keloid fibroblasts to TGF-beta 1 and thus a possible role for this mediator in keloid pathogenesis.

Differential extracellular matrix gene expression by fibroblasts during their proliferative life span in vitro and at senescence

Increasing evidence supports the idea that the finite proliferative life span of normal fibroblasts is a differentiation-like phenomenon. If this were correct, an ordered sequence of differential gene expression should be associated with the in vitro progression of cells from low passage to high passage (senescence). To define the pattern of expression of fibroblast differentiation-associated genes during this in vitro progression, we have determined the temporal pattern of expression of extracellular matrix (ECM) genes in Syrian hamster dermal fibroblasts as a function of passage level and percentage of proliferative life span in vitro. Steady-state mRNA levels were determined by Northern and dot blot analyses of total cellular RNA hybridized with cDNA probes specific for fibronectin, procollagen alpha 1III, and procollagen alpha 1I. Cells were analyzed at 24 hr postconfluence to minimize the presence of actively proliferating cells, and because maximal levels of fibronectin, alpha 1III, and alpha 1I mRNAs were observed 24 hr postconfluence. Unique, multiphasic patterns of expression of each of these ECM components were observed as the cells progressed from low passage to high passage. As the cells reached midhigh passage, fibronectin mRNA levels increased. This midpassage increase in fibronectin was followed by an increase in the level of alpha 1III mRNA as the cells reached the end of their in vitro proliferative life span, and then alpha 1I when the cells entered the postmitotic senescent phase, at which time the level of fibronectin mRNA also declined. A similar overlapping cascade pattern of up-regulation of these genes is seen during development and wound repair. This suggests that as cultured fibroblasts reach the end of their proliferative life span, they reinitiate a gene expression program used in tissue development and repair.

Ultrastructure of dermal fibroblasts during development and aging: relationship to in vitro senescence of dermal fibroblasts

One approach to understanding the relationship between in vitro cellular senescence and in vivo aging is to define the development and aging of cells in vivo and then to compare these in vivo properties with the in vitro behavior of the same cells. The Syrian hamster is being used as an experimental aging model to investigate the intrinsic developmental program of dermal fibroblasts in vivo (proliferation, extracellular matrix (ECM) production, quiescence, and reactivation during wound repair) in order to determine whether the in vivo differentiation program and mature function of these cells is related to their in vitro proliferation and senescence pattern. The ultrastructure of dermal fibroblasts from midfetal development through old age is described, and a working hypothesis of the development and aging of dermal fibroblasts is proposed as a framework for further evaluation of the relationship between in vitro proliferative capacity of dermal fibroblasts and in vivo developmental and age-related changes in the dermis.

Alteration of collagen composition and cross-linking in keloid tissues

Collagen composition and cross-linking in human keloid and normal skin tissues were analyzed biochemically. CNBr peptides were separated by 2-dimensional (2-D) mapping and high performance liquid chromatography (HPLC). The amounts of type I and type III collagen was quantified by 2-D scanning densitometry of fluorographs of 2-D maps derived from samples radioactively labelled in vitro by [3H]-NaBH4 in dimethylformamide. Keloid tissues contained 31.6 +/- 2.2 percent type III collagen as compared to 21.4 +/- 2.7 percent type III present in normal human skin dermis. HPLC profiles of CNBr peptides showed that approximately 5 percent of the high molecular weight material in keloids is mercaptoethanol reducible, compared to insignificant amounts in normal skin. 2-D maps derived from CNBr peptides of keloid collagen demonstrated thiol reduction sensitive alpha 1(III)-CB9 dimer as well as 24,000- and 32,000-dalton CNBr peptides, which were not mercaptoethanol reduction sensitive in normal skin due to cross-linking via the lysyl oxidase pathway. Also, a group of 20,000- to 25,000-dalton CNBr peptides, in the alpha 1(I)-CB6 cross-linking region were prominent in keloid tissues.

Cellular senescence revisited: a review

The field of cellular senescence (cytogerontology) is reviewed. The historical precedence for investigation in this field is summarized, and placed in the context of more recent studies of the regulation of cellular proliferation and differentiation. The now-classical embryonic lung fibroblast model is compared to models utilizing other cell types as well as cells from donors of different ages and phenotypes. Modulation of cellular senescence by growth factors, hormones, and genetic manipulation is contrasted, but newer studies in oncogene involvement are omitted. A current consensus would include the view that the life span of normal diploid cells in culture is limited, is under genetic control, and is capable of being modified. Finally, embryonic cells aging in vitro share certain characteristics with early passage cells derived from donors of increasing age.

Cell culture studies on neurofibromatosis (von Recklinghausen's disease). IV. Collagen synthesis

Comparative measurements of collagen synthesis as a portion of total protein synthesis were performed with fragments of peripheral neurofibromas, of skin adjacent to the tumors, and of unaffected skin from patients with neurofibromatosis (NF). No significant difference was found among these various samples. Collagen synthesis was also measured in cell cultures derived from peripheral neurofibromas of eight NF patients and from skin of ten healthy donors. No differences observed in the following respective parameters: dependence on the concentration of fetal calf serum; dependence on cell population density; portion of synthesized collagens in the culture medium and the cell layer. The ratios of synthesized collagen types III to type I, determined in five pairs of NF and control strains, vary within the same range.

Inhibitory effect of NH4Cl on secretion of collagen in human gingival fibroblasts

The general protein synthesis in human gingival fibroblasts as measured by 14C-proline incorporation was only moderately inhibited by 10 mM NH4C1 during incubation for 36 h. The proportion secreted as noncollagen protein and recovered from the cellular fraction as collagen was not significantly affected, whereas a pronounced inhibitory effect on the secretion of collagen was evident after 24 h. This effect was dose dependent, with a significant inhibition of collagen secretion even at 2 mM ammonia. The applied concentrations of NH4C1 had no significant effect on the hydroxylation of prolyl residues in collagen. Ammonia had no inhibitory effect on the secretion of fibronectin, another major secretory protein from fibroblasts. When comparing different lysosomotropic agents; NH4C1, chloroquine and methylamine, the most prominent effect was consistently found to be an inhibition of the secretion of collagen.

Altered rates of collagen synthesis in in vitro aged human lung fibroblasts

Absolute rates of protein and collagen synthesis based on prolyl-tRNA as the precursor were determined in two age groups of IMR-90 human lung fibroblasts. Compared with midrange fibroblasts [population doubling level (PDL) = 20 to 30] aged fibroblasts (PDL greater than 40] were larger in size in terms of protein and RNA per cell, generally proliferated more slowly, exhibited different steady state [3H] proline transfer RNA (tRNA) precursor pool specific radioactivities, synthesized collagen at a substantially lower rate, and exhibited a reduction in the percent commitment to collagen synthesis. Total protein synthetic rates were reduced slightly in aged versus midrange fibroblasts but the difference was not statistically significant. Proliferative capacity (PDL/wk) correlated better with these changes than cumulative PDL. Cell size (protein/cell) was the variable that had the highest correlation with the reduction in collagen synthesis observed in human lung fibroblasts. Thus, an important differentiated function of human lung fibroblasts, collagen synthesis, is greatly diminished in vitro in large, slowly dividing fibroblasts.