Adhesion molecule expression in normal skin and melanocytic lesions. Role of UV-irradiation and architectural characteristics in nevi

The Emperor's New Clothes: A Critique of the Current WHO Classification of Malignant Melanoma

The World Health Organization's classification of skin tumors of 2018 presents melanoma as a loose assembly of independent biologic entities, each of which is characterized by a distinctive constellation of clinical, histopathologic, and molecular findings that evolve through different pathways of lesional progression from a benign to an intermediate and, ultimately, malignant tumor. The alleged pathways, however, are based on vague correlations and fail to take into account the common occurrence of lesions that cannot be assigned to either of them. Moreover, there is no such thing as a lesional progression. The evolvement of neoplasms is always a clonal and, therefore, initially focal event. In the majority of melanomas, there is no evidence of a juxtaposition of a benign, intermediate, and malignant portion. Occasionally, a melanoma may develop within the confines of a melanocytic nevus, but a nevus cannot transform into melanoma. The concept of lesional progression merely serves to handle problems of differential diagnosis because it obscures and, in fact, denies the difference between benign and malignant neoplasms. In the current classification of the World Health Organization, every lesion is said to bear some risk of malignant progression, intermediate categories are recognized for all alleged pathways, and no distinction is made between "high-grade dysplasia" and melanoma in situ. Differentiation between benign and malignant neoplasms of melanocytes may be difficult, but the concept of lesional progression does not address those problems; it merely offers evasions under the disguise of diagnoses.

Trends in Regenerative Medicine: Repigmentation in Vitiligo Through Melanocyte Stem Cell Mobilization

Vitiligo is the most frequent human pigmentary disorder, characterized by progressive autoimmune destruction of mature epidermal melanocytes. Of the current treatments offering partial and temporary relief, ultraviolet (UV) light is the most effective, coordinating an intricate network of keratinocyte and melanocyte factors that control numerous cellular and molecular signaling pathways. This UV-activated process is a classic example of regenerative medicine, inducing functional melanocyte stem cell populations in the hair follicle to divide, migrate, and differentiate into mature melanocytes that regenerate the epidermis through a complex process involving melanocytes and other cell lineages in the skin. Using an in-depth correlative analysis of multiple experimental and clinical data sets, we generated a modern molecular research platform that can be used as a working model for further research of vitiligo repigmentation. Our analysis emphasizes the active participation of defined molecular pathways that regulate the balance between stemness and differentiation states of melanocytes and keratinocytes: p53 and its downstream effectors controlling melanogenesis; Wnt/β-catenin with proliferative, migratory, and differentiation roles in different pigmentation systems; integrins, cadherins, tetraspanins, and metalloproteinases, with promigratory effects on melanocytes; TGF-β and its effector PAX3, which control differentiation. Our long-term goal is to design pharmacological compounds that can specifically activate melanocyte precursors in the hair follicle in order to obtain faster, better, and durable repigmentation.

Understanding loss of donor white blood cell immunogenicity after pathogen reduction: mechanisms of action in ultraviolet illumination and riboflavin treatment

BACKGROUND

Donor white blood cells (WBCs) present in transfusion products can lead to immune sequelae such as production of HLA antibodies or graft-versus-host disease in susceptible transfusion recipients. Eliminating the immunogenicity of blood products may prove to be of clinical benefit, particularly in patients requiring multiple transfusions in whom allosensitization is common. This study examines a method of pathogen reduction based on ultraviolet light illumination in the presence of riboflavin. In addition to pathogens, WBCs treated with this system are affected and fail to stimulate proliferation of allogeneic peripheral blood mononuclear cells (PBMNCs) in vitro.

STUDY DESIGN AND METHODS

This study sought to determine the mechanisms regulating this loss of immunogenicity. Treated cells were examined for surface expression of a number of molecules involved in activation and adhesion, viability, cell-cell conjugation, and ability to stimulate immune responses in allogeneic PBMNCs.

RESULTS

Compared with untreated controls, ultraviolet (UV)-irradiated antigen-presenting cells showed slightly reduced surface expression of HLA Class II and costimulatory molecules and had more significant reductions in surface expression of a number of adhesion molecules. Furthermore, treated cells had a severe defect in cell-cell conjugation. The observed loss of immunogenicity was nearly complete, with UV-irradiated cells stimulating barely measurable interferon-gamma production and no detectable STAT-3, STAT-5, or CD3-epsilon phosphorylation in allospecific primed T cells.

CONCLUSION

These results suggest that defective cell-cell adhesion prevents UV-irradiated cells from inducing T-cell activation.

Long-lasting molecular changes in human skin after repetitive in situ UV irradiation

It is known that UV modulates the expression of paracrine factors that regulate melanocyte function in the skin. We investigated the consequences of repetitive UV exposure of human skin in biopsies of 10 subjects with phototypes 2-3.5 taken 1-4 years later. The expression of melanogenic factors (TYR, MART1, MITF), growth factors/receptors (SCF/KIT, bFGF/FGFR1, ET1/EDNRB, HGF, GM-CSF), adhesion molecules (beta-catenin, E-cadherin, N-cadherin), cell cycle proteins (PCNA, cyclins D1, E2) as well as Bcl-2, DKK1, and DKK3, were analyzed by immunohistochemistry. Most of those markers showed no detectable changes at > or = 1 year after the repetitive UV irradiation. Although increased expression of EDNRB protein was detected in 3 of 10 UV-irradiated subjects, there was no detectable change in the expression of ET1 protein or in EDNRB mRNA levels. In summary, only the expression of TYR, MART1, and/or EDNRB, and only in some subjects, was elevated at > or = 1 year after UV irradiation. Thus the long-term effects of repetitive UV irradiation on human skin did not lead to significant changes in skin morphology and there is considerable subject-to-subject variation in responses. The possibility that changes in the expression and function of EDNRB triggers downstream activation of abnormal melanocyte proliferation and differentiation deserves further investigation.

Melanocytic Nevi With Nonsurgical Trauma: A Histopathologic Study

There is a belief among dermatopathologists that benign melanocytic nevi (BMN) may display atypical histologic characteristics when traumatized. However, to our knowledge, a systematic study of nonsurgically traumatized melanocytic nevi (TMN) has not been published. We studied a series of 92 TMN. Cases were analyzed for histologic evidence of architectural and cytologic criteria associated with atypia. Of the patients, 54 were female and 37 were male. The mean age was 38 years old (range 8-74 years old). Nevi were present, in order of frequency, on the extremities, trunk, and head/neck, but there were no acral sites. Histologic findings of trauma were as follows: parakeratosis (92%), dermal telangiectasias (61%), ulceration (51%), dermal inflammation (49%), melanin within stratum corneum (24%), and dermal fibrosis (25%). Pagetoid spread of melanocytes was limited to the site of trauma in 20% of cases and was identified away from areas of trauma in 8% of cases. Melanocytic atypia was seen in three cases. Dermal mitoses were rare (one mitotic figure in three cases). Pagetoid spread under a traumatized epidermis was relatively frequent and, in isolation, is compatible with a benign TMN. Any traumatized melanocytic lesion that displays cytologic atypia, pagetoid spread outside of the area of the traumatized epidermis, or dermal mitoses should be treated with caution because these findings were rarely seen in TMN.

[Factors influencing the microscopic appearance of melanocytic nevi]

Both clinicians and dermatopathologists must be aware of the various factors which can influence the histopathologic appearance of melanocytic nevi in order to avoid mistaken diagnoses.

Selective down-regulation of the alpha6-integrin subunit in melanocytes by UVB light

In vivo, melanocytes bind to laminin (LM) molecules of the basement membrane (BM) via the integrins alpha3beta1 and alpha6beta1, and they adhere to neighbouring keratinocytes via E-cadherin. Only few studies have addressed the impact of ultraviolet (UV) light on the interaction of melanocytes with their microenvironment. In this report, we examined the influence of UVB irradiation on the expression of the most important melanocyte-adhesion molecules (E-, N-cadherin, alpha2-, alpha3-, alpha5-, alpha6-, alphaV-, beta1-, beta3-integrins and ICAM-1) in vitro by flow cytometry. We were able to demonstrate that the alpha6-integrin subunit is selectively and reversibly down-regulated by UVB in a dwzm 150ose-dependent manner. In comparison, keratinocytes lacked UVB-inducible alterations in the expression of alpha6-integrin. In the presence of LM-1, the UVB-induced down-regulation of alpha6-integrin in melanocytes was significantly reduced. Moreover, LM-1 increased the resistance of melanocytes to UVB-induced cell death, as measured by annexinV-binding analysis. This effect was reversed by preincubation with an alpha6-integrin-blocking antibody. By immunofluorescence, we could demonstrate that UVB leads to a dose-dependent internalization of alpha6-integrin, providing an obvious explanation for the down-regulation on the outer cell surface observed by flow cytometry. We suggest that adhesion to LM-1 through alpha6-integrin represents a protective mechanism for melanocytes to withstand UVB damage. Through alpha6-integrin internalization, sunburns might alter the interaction between melanocytes and the BM, resulting in apoptosis induced by loss of anchorage (anoikis). Repeated sunburns may then lead to the selection of a population of melanocytes which are capable of anchorage-independent survival, culminating in solar nevogenesis and melanoma development.

Pagetoid melanocytosis: when is it significant?

Pagetoid melanocytosis refers to the presence of solitary and small groups of melanocytes in the superficial layers of the epidermis. Although it is generally considered to be a diagnostic hallmark of melanoma, it may also be seen in certain melanocytic naevi. Attempts to formulate reliable histological criteria for distinction between benign and malignant pagetoid melanocytosis have been only partly successful. Extensive and diffuse pagetoid spread of melanocytes, especially when it extends laterally beyond the underlying junctional component, combined with marked cytological atypia, favours melanoma (Fig. 1). In naevi, pagetoid spread tends to be more limited and there is no marked cytological atypia. Since these criteria are applicable in only a subset of cases, the correct diagnosis of melanocytic lesions must be based not only on the features of pagetoid melanocytosis but also on all histological and clinical findings. The pathogenesis of pagetoid melanocytosis is, at best, hypothetical and presumes an active infiltrative process in the setting of malignancy. However, novel data support the contention of a passive drift of melanocytes carried upwards by proliferating keratinocytes under particular circumstances, such as trauma or UV exposure. In this review, we evaluate the histological features of pagetoid melanocytosis in the light of current experimental data, reflecting on the pathogenesis of this intriguing phenomenon.

Cadherin expression pattern in melanocytic tumors more likely depends on the melanocyte environment than on tumor cell progression

BACKGROUND

Adhesion molecules have been assigned an important role in melanocytic tumor progression. By the loss of E-cadherin, melanocytes might escape the control of neighbouring keratinocytes. Although in vitro data support this hypothesis, there are yet no conclusive immunohistochemical results on cadherin expression in melanocytic tumors.

OBJECTIVE

To gain detailed insight in the expression of cadherins and their cytoplasmic binding partners, the catenins, in various types of benign and malignant melanocytic neoplasms.

METHODS

Immunohistochemical analysis of the expression of E-, P-, and N-cadherin and alpha-, beta-, and gamma-catenin in compound and dermal nevi, Spitz nevi, blue nevi, ultraviolet B (UVB)-irradiated nevi, and malignant melanomas of various tumor thickness.

RESULTS

In both nevi and melanomas, E-cadherin expression in melanocytic cells decreased, following a gradient from junctional to deeper dermal localization. The pattern of E-cadherin expression was more heterogeneous in melanomas than in nevi. In some melanomas, E-cadherin was only weakly positive in the epidermal tumor cells. P-cadherin expression was similar to that of E-cadherin. N-cadherin expression in melanocytic lesions was a rare finding, however, a small percentage of melanomas showed expression in some cell nests. Some Spitz nevi exhibited strong N-cadherin immunoreactivity. Most melanocytic cells were alpha- and beta-catenin-positive and gamma-catenin-negative. UVB irradiation did not influence the expression of cadherins and catenins in melanocytic nevi in vivo.

CONCLUSIONS

It is presumed that the gradual loss of E-cadherin expression represents a reaction of melanocytic cells to altered conditions in the dermal environment, e.g. lack of contact to keratinocytes, or new contact with dermal extracellular matrix molecules, respectively. Melanoma cells apparently are less dependent on these environmental factors and, therefore, show a more heterogeneous expression pattern. This might be of importance for the adaptation of the tumor cells to local requirements. However, in view of our results, a causative role of (loss of ) E-cadherin or (gain of ) N-cadherin for melanocytic tumor progression still remains to be proven.

Melanocyte detachment after skin friction in non lesional skin of patients with generalized vitiligo

BACKGROUND

In vitiligo, melanocytes are gradually lost in depigmented macules of the skin. The disappearance of melanocytes has, however, not been clearly observed and consequently the aetiology of the disease (autoimmune, neural, cytotoxic) is still elusive. The starting point of vitiligo macules is frequently determined by local conditions such as wounds and excoriations, but may also follow minor traumas such as pressure or repeated friction. This prominent feature is often neglected.

OBJECTIVES

To clarify the biological consequences of repeated friction on the attachment and survival of melanocytes in non lesional vitiligo skin.

METHODS

Light reproducible skin friction was performed for 4 min on the volar forearm of 18 patients with extensive vitiligo and five controls with normal healthy skin. Biopsies from the test area and control skin were taken at 1, 4, 24 and 48 h following friction. Serial sections were examined with standard light microscopy, transmission electron microscopy, histochemistry and immunohistochemistry (dihydroxyphenylalanine, HMB-45, E-cadherin and an early apoptosis marker, M30 cytoDEATH antibody).

RESULTS

The observation of sections at 1 and 48 h after friction on vitiligo skin and at all time points in controls revealed no changes. In contrast, in vitiligo skin at 4 and 24 h after friction, several melanocytes had undergone detachment and were found in various suprabasal positions, including the stratum spinosum, granular layer, and within and outside the stratum corneum.

CONCLUSIONS

Detachment and transepidermal elimination of melanocytes following minor mechanical trauma in non lesional vitiligo skin is probably the cause of depigmentation occurring in the isomorphic response (Koebner phenomenon). We propose that transepidermal elimination of melanocytes in vitiligo should be regarded as a possible mechanism of chronic loss of pigment cells, perhaps previously damaged by another process.

MMP-2, TIMP-2 and MT1-MMP are differentially expressed in lesional skin of melanocytic nevi and their expression is modulated by UVB-light

BACKGROUND

In malignant melanoma, recent studies have demonstrated an important role of matrix-metalloproteinase 2 (MMP-2), its co-activating enzyme membrane-type matrix-metalloproteinase 1 (MT1-MMP), and the endogenous inhibitor of MMP-2, tissue-inhibitor of matrix metalloproteinase 2 (TIMP-2). Melanocytic nevi are benign neoplasms of the melanocytic lineage, but may exhibit dysplastic features that can be difficult to distinguish from early stage melanoma. As shown in earlier studies, nevi show important morphological and phenotypical changes in response to ultraviolet light (UVB) irradiation.

OBJECTIVE

To clarify the role of MMP-2, TIMP-2 and MT1-MMP in UVB-irradiated vs. non-irradiated melanocytic nevi.

METHODS

Immunohistochemical comparison of the MMP-2, TIMP-2 and MT1-MMP expression pattern.

RESULTS

MMP-2 is expressed by lesional keratinocytes and its expression is up-regulated by UVB-irradiation. MMP-2 expression was not observed in melanocytic cells. TIMP-2, by contrast, is predominantly expressed by melanocytic nevus cells, and its expression is in part down-regulated by UVB-irradiation. MT1-MMP is expressed by basal keratinocytes and to a weaker extent by melanocytic nevus cells.

CONCLUSIONS

MMP-2 expression by keratinocytes in nevi probably represents the result of activation of keratinocyte turnover in lesional epidermis. MMP-2 could play a role in the downward movement of junctional nevus cells into the dermis. The reduction of TIMP-2 expression in melanocytic cells by UV-light together with the enhanced expression of MMP-2 in the adjacent epidermis may promote basement membrane degradation. The expression pattern of MT1-MMP in close proximity to epithelial-mesenchymal interfaces underlines the synergistic role of MT1-MMP in this process.

Attachment and chemotaxis of melanocytes after ultraviolet irradiation in vitro

Because ultraviolet (UV) radiation is able to influence the spatial distribution of melanocytes in melanocytic naevi in vivo, we investigated the influence of UV radiation on the ability of melanocytes to adhere to the extracellular matrix proteins fibronectin, laminin and collagen type IV in vitro. In addition, chemotaxis of melanocytes was studied using both fibronectin and the supernatants from irradiated, as well as non-irradiated, keratinocytes and fibroblasts as attractants. Melanocyte attachment to fibronectin was significantly increased 48 h after a single UV irradiation at 30 mJ/cm2 in comparison with that of non-irradiated melanocytes, whereas attachment to laminin and collagen type IV showed only minor changes after UV exposure. The UV-induced increase in attachment to fibronectin was suppressed by preincubation with antibodies against alpha5beta1 or alphavbeta3 integrin. Both immunohistochemistry and flow cytometric analysis showed an increase in alpha5beta1 integrin expression on melanocytes after UV exposure. The chemotaxis of melanocytes to fibronectin was not influenced by UV exposure. A decreasing migration rate of melanocytes towards the supernatants of UVA-irradiated fibroblasts was observed with increasing UVA doses. The chemotactic effects of conditioned medium of keratinocytes towards melanocytes was not influenced either by UVB or by UVA. The results indicate that UV radiation may alter the ability of melanocytes to adhere to certain substrates by modification of integrin expression. Because fibronectin, as the major target protein of UV-altered attachment, is located in the dermis, the UV-induced morphological changes in melanocytic lesions, with an increase in suprabasally located melanocytes within the epidermis, may be due to other changes in the adhesive properties of melanocytes.

Enhanced expression of Ki-67, topoisomerase IIalpha, PCNA, p53 and p21WAF1/Cip1 reflecting proliferation and repair activity in UV-irradiated melanocytic nevi

To investigate the effect of ultraviolet (UV) irradiation on the expression of cell cycle-associated proteins, melanocytic nevi from healthy volunteers were partially covered, irradiated with a defined UV dose, and excised 1 week thereafter. The irradiated and the protected parts were examined separately by conventional microscopy and immunohistochemistry using the antibodies Ki-S11 (Ki-67), Ki-S7 (topoisomerase IIalpha), PC10 (proliferating cell nuclear antigen [PCNA]), DO-7 (p53), 6B6 (p21WAF1/Cip1), and the melanocytic marker HMB-45. DNA nick-end labeling was used as a marker of apoptosis. Irradiation resulted in morphological changes and increased HMB-45 reactivity. Proliferation, as assessed by Ki-67 and topoisomerase IIalpha expression, was also clearly enhanced in the UV-exposed areas. This was confirmed by the appearance of occasional mitotic figures. PCNA expression levels markedly exceeded those of the proliferation markers and did not correlate with the latter in most cases. p21 immunolabeling indices were also consistently augmented after UV exposure; hence it is likely that growth-inhibitory mechanisms partly compensate for the proliferative impulse, and the disproportional rise in PCNA expression probably reflects DNA repair activity. Enhanced p53 immunostaining in four cases suggests that the induction of p21 after irradiation may be p53 mediated, whereas no concomitant apoptotic events were observed. We conclude that UV light can stimulate the proliferative activity of melanocytes in melanocytic nevi, but that simultaneously cell cycle inhibitors are activated to permit DNA repair.