Apoptosis is induced by anti-Fas antibody alone in cultured human keratinocytes

Morphometry and epidermal fas expression of unexposed aged versus young skin

Background:

Identifying the molecular mechanisms of intrinsic aging is critical in developing modalities for reversal of cutaneous aging.

Objective:

The objective was to evaluate the expression of epidermal Fas, epidermal thickness, collagen, and elastic fibers degeneration in unexposed skin of aged individuals compared with young ones.

Materials and Methods:

Skin biopsies were taken from normal skin of the back of 22 old subjects (age range: 48-75 years) and 15 young subjects (age range: 18-28 years). Skin sections were stained with hematoxylin and eosin, Masson trichrome, orcein. Epidermal thickness was measured with image analyzer and scoring was done for collagen and elastic fiber degeneration. Fas immunostaining was done. Quantitative and qualitative data were compared statistically between the old and young subjects.

Results:

A statistically significant decreased epidermal thickness was found in old compared with young skin (P<0.05). A statistically significant number of patients showed decreased epidermal thickness, density, and fragmentation of both collagen and elastic fibers in old compared with young skin (P<0.001). Epidermal Fas expression was detected in 19 of 22 old subjects (86.4%) compared with 2 of 15 young subjects (13.3%) (P<0.001). There was no statistically significant correlation between age of old subjects and each of epidermal thickness, collagen, and elastic fiber degeneration.

Conclusion:

The decreased epidermal thickness and morphological alteration of collagen and elastic fibers are not correlated with aging and Fas-mediated apoptosis could be involved in thinning of the epidermis in unexposed aged skin.

FasL and TRAIL induce epidermal apoptosis and skin ulceration upon exposure to Leishmania major

Receptor-mediated apoptosis is proposed as an important regulator of keratinocyte homeostasis in human epidermis. We have previously reported that Fas/FasL interactions in epidermis are altered during cutaneous leishmaniasis (CL) and that keratinocyte death through apoptosis may play a pathogenic role for skin ulceration. To further investigate the alterations of apoptosis during CL, a keratinocyte cell line (HaCaT) and primary human epidermal keratinocytes were incubated with supernatants from Leishmania major-infected peripheral blood mononuclear cells. An apoptosis-specific microarray was used to assess mRNA expression in HaCaT cells exposed to supernatants derived from L. major-infected cultures. Fas and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) mRNA and protein expression were significantly up-regulated, and apoptosis was detected in both HaCaT and human epidermal keratinocyte cells. The keratinocyte apoptosis was partly inhibited through blocking of Fas or FasL and even more efficiently through TRAIL neutralization. Up-regulation of Fas on keratinocytes in epidermis and the presence of FasL-expressing macrophages and T cells in dermis were previously reported by us. In this study, keratinocytes expressing TRAIL, as well as the proapoptotic receptor TRAIL-R2, were detected in skin biopsies from CL cases. We propose that activation of Fas and TRAIL apoptosis pathways, in the presence of inflammatory mediators at the site of infection, leads to tissue destruction and ulceration during CL.

Knock down of p53 levels in human keratinocytes increases susceptibility to type I and type II interferon-induced apoptosis mediated by a TRAIL dependent pathway

BACKGROUND

Keratinocytes (KCs) in healthy skin only undergo death following differentiation to produce stratum corneum. By contrast, in inflammatory pathological conditions featuring type I (IFN-alpha) and type II (IFN-gamma) interferons KCs undergo premature apoptosis.

OBJECTIVE

To define apoptotic susceptibility of KCs, response to interferons was examined. Since molecular cross-talk occurs between interferons and p53, potential mechanistic roles for p53 in KC apoptosis were investigated.

METHODS

Knock down of p53 was performed, and apoptotic response to addition of interferons was assessed using FACS and by staining for activated caspase 3 and TUNEL. Elucidation of death pathway was accomplished by using a dominant negative death receptor construct and a neutralizing TRAIL antibody.

RESULTS

Reduction in p53 levels in KCs by siRNA treatment enhanced, rather than reduced, apoptotic responses to IFN-alpha plus IFN-gamma. In an immortalized human KC cell line (HaCaT cells with both p53 alleles mutated) enhanced apoptotic susceptibility to interferon exposure was also observed. The mechanism for this enhanced apoptosis involved induction of TRAIL and its interaction with death receptors, as blocking the death receptor pathway using dominant negative FADD, or by addition of neutralizing antibody against TRAIL, reduced the apoptotic response to IFN-alpha and IFN-gamma.

CONCLUSION

These results indicate IFN-alpha plus IFN-gamma triggers apoptosis independent of p53 in HaCaT cells, and also demonstrate an unexpected survival role for p53 in human KCs as regards apoptotic responsiveness to cytokines such as IFN-alpha and IFN-gamma involving activation of TRAIL-related death receptors. Strategies enhancing p53 regulated survival proteins in KCs may be of therapeutic benefit in skin disorders characterized by activated immunocytes triggering premature KC apoptosis.

Interferon (IFN)-gamma is a main mediator of keratinocyte (HaCaT) apoptosis and contributes to autocrine IFN-gamma and tumour necrosis factor-alpha production

BACKGROUND

Apoptosis of keratinocytes or intestinal epithelial cells is an important pathophysiological mechanism of organ damage during acute graft-versus-host disease.

OBJECTIVES

To analyse in detail the mediators and their mutual interaction leading to keratinocyte apoptosis.

METHODS

Experiments were performed using a keratinocyte cell line (HaCaT) and human skin explant cultures.

RESULTS

Supernatants (SN) of major histocompatibility complex nonmatched mixed lymphocyte cultures (MLCs) induced apoptosis in HaCaT cells and also in keratinocytes from skin biopsies. Although both interferon (IFN)-gamma and Fas ligand (FasL) were detected in MLC-SN by enzyme-linked immunosorbent assay, the apoptosis-inducing capacity could be fully abrogated by neutralization of IFN-gamma, but not by neutralization of FasL. Recombinant (r) IFN-gamma induced HaCaT keratinocyte apoptosis in a dose- and time-dependent manner. Induction of HaCaT apoptosis by rFasL alone was induced only at higher doses than present in MLC-SN, but apoptosis was dramatically enhanced in the presence of rIFN-gamma. Further synergistic effects with IFN-gamma in the induction of apoptosis were also observed with agonistic antitumour necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) receptor 2 antibody, soluble TRAIL and TNF-alpha. However, in contrast to FasL and TRAIL, TNF-alpha alone did not induce HaCaT apoptosis. Interleukin-1beta and lipopolysaccharide did not enhance the apoptosis-inducing effect of IFN-gamma. Beside its apoptosis-inducing capacity in HaCaT cells, rIFN-gamma also induced autocrine IFN-gamma production, and combined treatment with IFN-gamma and TNF-alpha induced autocrine TNF-alpha production. Neutralization of autocrine IFN-gamma protected HaCaT cells from apoptosis.

CONCLUSIONS

Taken together, our data suggest a central role for IFN-gamma in HaCaT keratinocyte apoptosis but also show the importance of co-acting mediators such as TNF-alpha, TRAIL and FasL, which potentiate the effect of paracrine and autocrine IFN-gamma and TNF-alpha release.

The contribution of the Fas/FasL apoptotic pathway in ulcer formation during Leishmania major-induced cutaneous Leishmaniasis

Cutaneous leishmaniasis (CL), caused by the intracellular protozoan Leishmania major, is characterized by lesion formation and ulceration at the site of infection. The mechanism of ulcer formation during CL is not fully understood. The expression of Fas and FasL and the levels of apoptosis in skin biopsies and in restimulated blood mononuclear cells from patients with 1 to 7 months of L. major-induced CL were analyzed using immunohistochemistry and fluorescence-activated cell sorting analysis. The levels of soluble Fas and FasL were also analyzed by enzyme-linked immunosorbent assay. A substantial number of apoptotic keratinocytes were observed mainly in the superficial epidermis of morphologically active and healing CL skin samples. Fas expression was increased on epidermis in active CL, whereas Fas expression was similar in healing and healthy epidermis. FasL-expressing macrophages and T cells were found in subepidermal infiltrate, mainly in active disease. When CL peripheral blood mononuclear cells were restimulated with L. major, Fas was up-regulated on effector T cells, and high levels of sFasL were secreted. Supernatants from restimulated cultures induced apoptosis in human keratinocytes (HaCaT), possibly through Fas/FasL interactions. Our results indicate that FasL-expressing effector T cells and macrophages may act to induce apoptosis and ulcer formation in Fas-expressing keratinocytes during L. major infection.

Intravenous immunoglobulin does not improve outcome in toxic epidermal necrolysis

Intravenous Immunoglobulin (IVIG) has been proposed as a beneficial therapy for toxic epidermal necrolysis (TEN). However, this has been based on a limited amount of Class 5 evidence. To compare outcomes in TEN patients treated in our burn unit since 1999, when we began to use IVIG (IG group), with TEN patients treated between 1995 and 1999 who did not receive IVIG (control group). Retrospective cohort review of the records of all TEN patients admitted between April 5, 1995 and December 4, 2002. There were 16 patients in the IG group (age 53 +/- 21 years, with initial rash involving 65 +/- 29% TBSA) and 16 patients in the control group (age 52 +/- 20 years, with initial rash involving 65 +/- 27% TBSA). The IG group received 0.7 +/- 0.2 g/kg/day of IVIG for 4 +/- 1 days. There were no significant differences between the groups with respect to the length of stay, duration of mechanical ventilation, severity of systemic inflammatory response syndrome and multiple organ dysfunction syndrome, or the incidence of sepsis. Significant progression of the wound occurred in 13% of the IG patients and in 27% of control patients, whereas no wound progression was observed in 47% of the IG patients and in 18% of the control patients (P =.299). The time to healing did not differ between IG and control groups (11.2 +/- 3.6 vs 11.4 +/- 2.6 days, respectively). There was no significant difference in the mortality rate between the IG group (25%) and the control group (38%). There were no complications from IVIG aside from one case of hyponatremia from the hypotonic IVIG solution. Although there may have been a trend towards less severe wound progression in patients who received IVIG, this was not associated with any substantial improvement in outcome in our TEN patients. A prospective randomized study with a larger sample size is needed to confirm our findings.

Intravenous immunoglobulin prophylaxis for recurrent Stevens-Johnson syndrome

Human intravenous immunoglobulin has been described as a treatment for patients with Stevens-Johnson syndrome and toxic epidermal necrolysis. We describe the prophylactic use of intravenous immunoglobulin to prevent Stevens-Johnson syndrome in a woman undergoing cardiac catheterization with a previous history of four episodes of Stevens-Johnson after receiving intravenous contrast dye.

Enhanced expression of p16 in seborrhoeic keratosis; a lesion of accumulated senescent epidermal cells in G1 arrest

BACKGROUND

Seborrhoeic keratosis (SK) is a common skin disease associated with skin ageing and photoageing, but only limited studies have been performed on SK and the senescence of keratinocytes.

OBJECTIVES

We sought to clarify the genetic basis of SK and the senescence of keratinocytes.

METHODS

Expression of p16, cyclins A, D and E, p21, p53, retinoblastoma (Rb) gene product and telomerase-associated protein 1 (TP1) in SK was examined by immunohistochemistry. DNA fragmentation in SK was detected by the terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick end labelling method. We cultured keratinocytes from SK lesions and non-lesional epidermis and examined expression of p16, observed morphology of the cultured cells by light and electron microscopy and measured survival time.

RESULTS

p16, a cyclin-dependent kinase inhibitor, was expressed in all cells from SK lesions, whereas normal keratinocytes expressed p16 only in the granular cells. Other factors such as cyclins A, D and E, p21, p53, Rb gene product, and TP1, were not expressed in SK cells. These results suggest that p16 expression is a marker of SK and that p16 has a role in the pathogenesis of SK. DNA fragmentation was not detected in four of five SK tissue samples; one of the SK tissue samples showed DNA fragmentation only in the superficial upper layer of an SK lesion, suggesting that apoptosis was inhibited in SK cells. In contrast, normal epidermis showed DNA fragmentation in the granular and squamous layers. Immunohistochemical examination of cultured SK cells also revealed the presence of p16. A greater number of SK cells survived after 3 weeks of culture in comparison with normal keratinocytes. Features of senescence, such as a balloon-like appearance after lengthy culture and increased amounts of tonofilaments in cytoplasm, were observed in SK cells in culture.

CONCLUSIONS

These results suggest that SK is a benign neoplasm where keratinocytes in a senescent condition and G1 arrest are accumulated.

Differentiation and apoptosis in human immortalized sebocytes

Increased cell volume, accumulation of lipid droplets in the cytoplasm, and nuclear degeneration are phenomena indicating terminal differentiation of human sebocytes followed by holocrine secretion and cell death. The molecular pathways of natural and induced sebocyte elimination are still unknown, however. In this study, SZ95 sebocytes were found to exhibit DNA fragmentation after a 6 h culture followed by increased lactate dehydrogenase release after 24 h, indicating cell damage. With the help of morphologic studies and using Oil Red detection of cellular lipids, cell enlargement, accumulation of lipid droplets in the cytoplasm, and nuclear fragmentation could be observed under treatment with arachidonic acid. Staurosporine, a potent inhibitor of phospholipid Ca2+-dependent protein kinase, increased externalized phosphatidylserine levels on SZ95 sebocytes, detected by annexin V/propidium iodide flow cytometry, as early as after 1 h, whereas dose-dependent reduction of bcl-2 mRNA and protein expression, enhanced DNA fragmentation, and increased caspase 3 levels, detected by caspase 3 inhibitor/propidium iodide flow cytometry, were found after 6 h of treatment. SZ95 sebocyte death was detected as early as after 6 h of SZ95 sebocyte treatment with high staurosporine concentrations (10(-6)-10(-5) M). 5Alpha-dihydrotestosterone (10(-8)-10(-5) M) did not affect externalized phosphatidylserine levels and DNA fragmentation in SZ95 sebocytes but slightly decreased lactate dehydrogenase cell release. Neither acitretin nor 13-cis retinoic acid (10(-8)-10(-5) M) affected externalized phosphatidylserine levels, DNA fragmentation, and lactate dehydrogenase cell release, despite the increased caspase 3 levels under treatment with 13-cis retinoic acid. The combined staurosporine and 13-cis retinoic acid treatment enhanced DNA fragmentation in SZ95 sebocytes to the same magnitude as in cells only treated with staurosporine. In conclusion, SZ95 sebocytes in vitro undergo apoptosis, which can be enhanced by the terminal differentiation inductor arachidonic acid or by staurosporine and leads to cell death. 5Alpha-dihydrotestosterone inhibits SZ95 sebocyte death without involving apoptotic pathways, and retinoids did not affect the programmed death of human sebocytes. The latter result fits well with the currently reported inability of normal skin cells to undergo apoptosis after treatment with retinoids, in contrast to their malignant counterparts.

Fas counterattack in cholangiocarcinoma: A mechanism for immune evasion in human hilar cholangiocarcinomas

AIM: To investigate FasL expression in hilar cholangiocarcinoma tissues and cultured cholangiocarcinoma cells, and to assess its ability to induce apoptosis.

METHODS: We studied the expression of FasL by human hilar cholangiocaroinomas tissues by immunohistochemistry, and the QBC939 cholangiocarcinoma cell line by RT-PCR, immunohistochemistry, and Western Blot. TUNEL and flow cytometry were used to detect apoptotic cells.

RESULTS: Prevalent expression of FasL was detected in 39 resected hilar cholangiocarcinoma tissues. TUNEL staining disclosed a high level of cell death among lymphocytes infiltrating FasL positive areas of tumor. FasL mRNA and protein expressions in cholangiocarcinoma cells could induce Jurkat cells.

CONCLUSION: Hilar cholangiocarcinomas may elude immunological surveillance by inducing, via Fas/FasL system, the apoptosis of activated lymphocytes.

Dendritic cells induce the death of human papillomavirus-transformed keratinocytes

Although human papillomavirus (HPV) antigens are expressed in a majority of (pre)neoplastic lesions (squamous intraepithelial lesions; SILs) of the uterine cervix, progression to invasive cancer may occur, which suggests that the presentation of viral antigens to the immune system is deficient in some SILs. To determine whether professional antigen-presenting cells die in SILs, we assayed for the apoptosis of immature dendritic cells (DC) in organotypic cultures of HPV-transformed keratinocytes, which reproduce many features of in vivo observed SILs. Unexpectedly, the infiltration of organotypic cultures by DC specifically induced the apoptosis of HPV+ tumor cells, whereas DC were not affected. In the same conditions and in coculture experiments, apoptosis was not observed in normal keratinocytes. The induction of apoptosis required membrane contacts between DC and HPV-transformed keratinocytes. Although the HPV+cell lines were sensitive to the effects of TRAIL, soluble TRAILR2-Fc did not block the DC-induced apoptosis. Furthermore, although FasL and Fas were detected on DC and HPV+ cell lines, respectively, functional analysis revealed that this pathway is not responsible for the apoptosis induced by the DC. All together these results suggest that DC may be at the interface between innate and adaptive immunity by inducing the apoptosis of (pre)neoplastic cells.

Treatment of drug-induced toxic epidermal necrolysis (Lyell's syndrome) with intravenous human immunoglobulins

Toxic epidermal necrolysis (TEN) is a rare drug-induced life-threatening disease. Currently, the disease is only treated by supportive and antiseptic measures. Quite recently intravenous immunoglobulins (IG) were shown to be a promising TEN treatment. The rationale for their use is based on the fact that keratinocyte apoptosis in TEN involves the CD95 (APO-1/Fas) cell surface receptor-ligand system. We successfully treated a TEN patient with high dose of intravenous IG. The clinical recovery appeared exceptionally rapid. Immunohistochemistry showed that the IG action probably developed on the CD95 receptor-ligand system at the keratinocytes surface.

High-dose proinflammatory cytokines induce apoptosis of hair bulb keratinocytes in vivo

BACKGROUND

Hair loss following skin inflammation may in part be mediated by keratinocyte (KC) apoptosis. While the effects of different cytokines or other apoptosis stimulating agents such as interferon (IFN)-gamma or tumour necrosis factor (TNF)-alpha on KC apoptosis in vitro have been addressed in several studies, little is known about the effects of proinflammatory cytokines on KC apoptosis in vivo.

OBJECTIVES

To study the effects of intradermally injected TNF-alpha, interleukin (IL)-1beta and IFN-gamma on KC apoptosis in the back skin of C57BL/6 mice.

METHODS

Apoptosis in epidermal and hair bulb KCs was analysed by immunohistology using TUNEL staining.

RESULTS

Injection of TNF-alpha induced a significantly higher number of apoptotic cells within the epidermis than vehicle; all three proinflammatory cytokines together further increased their number. Intrafollicular hair bulb KCs were much more susceptible to apoptosis induction by TNF-alpha or IL-1beta; their injection significantly upregulated apoptosis after 6 h, which was further increased after 24 h. The combination of all cytokines together accelerated intrafollicular apoptosis after 6 h by doubling the number of apoptotic cells per hair bulb, compared with the effects of TNF-alpha or IL-1beta alone.

CONCLUSIONS

These data suggest that programmed cell death of proliferating KCs in vivo can be induced by proinflammatory cytokines.

Inhibition of keratinocyte apoptosis by IL-15: a new parameter in the pathogenesis of psoriasis?

Keratinocytes (KC) are important source of and targets for several cytokines. Although KC express IL-15 mRNA, the functional effects of IL-15 on these epithelial cells remain to be dissected. Investigating primary human foreskin KC and HaCaT cells, we show here by semiquantitative RT-PCR and flow cytometric analysis that both translate IL-15 and IL-15R mRNA and express IL-15 and IL-15Ralpha protein on the cell surface, suggesting that human KC can employ IL-15 for juxtacrine signaling. While IL-15 exerted no significant effect on KC proliferation and IL-6 or IL-8 secretion, IL-15 inhibited both anti-Fas and methylcellulose-induced KC apoptosis in vitro. This is in line with the recognized potent anti-apoptotic effects of IL-15. IL-2, whose receptor shares two components with the IL-15R, failed to inhibit KC apoptosis. Together with the role of IL-15 in sustaining chronic immune reactions, this invited the question of whether a reduction of KC apoptosis by IL-15 may be involved in the pathogenesis of psoriasis, a chronic hyperproliferative inflammatory skin disease characterized by abnormally low KC apoptosis in the epidermis. Remarkably, compared with nonlesional psoriatic skin and skin of healthy volunteers, lesional psoriatic epidermis showed high IL-15 protein expression in the epidermis and enhanced binding activity for IL-15. Therefore, antagonizing the inhibitory effects of IL-15 on KC apoptosis deserves exploration as a novel therapeutic strategy in psoriasis management.

Fas ligand is highly expressed in acute leukemia and during the transformation of chronic myeloid leukemia to blast crisis

Fas ligand (FasL) induces apoptosis in susceptible Fas-bearing cells and is critically involved in regulating T-cell immune responses. It is highly expressed in several human malignancies, and a role in the suppression of antitumor immune responses has been suggested. We evaluated FasL expression in leukemia and normal hematopoietic cells. By Western blotting, all acute leukemic cell lines (n = 9) and primary samples of acute leukemic marrow (n = 4) revealed high levels of FasL. In contrast, much weaker signals were observed in samples of normal marrow (n = 5), and either weak or intermediate expression was seen in chronic myeloid leukemia (CML) in chronic phase (n = 7). Additional leukemic samples were examined by immunohistochemistry. Staining for FasL was negative in 7 of 9 cases of chronic-phase CML, whereas all cases of CML in blast crisis (n = 6), acute lymphoblastic leukemia (n = 6), and acute myeloid leukemia (n = 11) stained strongly in 60 to 100% of nucleated cells. FasL+ leukemic cell lines did not trigger Fas-mediated apoptosis in either Jurkat cells or activated human T lymphocytes, possibly related to the intracellular location of the ligand. Western analysis of normal marrow subpopulations revealed that most FasL in marrow mononuclear cells was expressed by CD7+ lymphocytes. FasL also was strongly expressed in CD34+ hematopoietic progenitor cells from both normal and chronic-phase CML marrow, suggesting a correlation with primitive maturation stage. In summary, high levels of FasL expression were associated with aggressive biologic behavior in leukemia, including transformation of CML to blast crisis. This could potentially represent a response to loss of proapoptotic Fas signaling, which is known to occur in acute leukemic blasts.

Contact-dependent growth inhibition and apoptosis of epidermal growth factor (EGF) receptor-expressing cells by the membrane-anchored form of heparin-binding EGF-like growth factor

Heparin-binding epidermal growth factor-like growth factor (HB-EGF) transduces mitogenic signals through the EGF receptor (EGFR). There are two forms of HB-EGF, the membrane-anchored form (pro-HB-EGF) and the soluble form (sHB-EGF). We studied the biological activity of pro-HB-EGF by using a model in which pro-HB-EGF-expressing effector cells was co-cultured with EGFR-expressing target cells. The DER cell, an EGFR-expressing derivative of the interleukin-3-dependent hematopoietic 32D cell line, grows well in the presence of EGF or sHB-EGF without IL-3. When DER cells were co-cultured on a monolayer of Vero-H cells overexpressing pro-HB-EGF, growth inhibition and subsequent apoptosis were induced in the DER cells even in the presence of excess amounts of EGF or sHB-EGF. Such growth inhibition of DER cells was abrogated when specific antagonists for pro-HB-EGF were added in the culture medium or when direct contact of DER cells with Vero-H cells was prevented, indicating that pro-HB-EGF is involved in this inhibitory effect. Pro-HB-EGF-induced apoptosis of DER cells was also observed even in the presence of IL-3. This rules out the possibility of simple competition between soluble EGFR ligands and pro-HB-EGF. Moreover, 32D cells expressing EGFR mutant composed of the extracellular and the transmembrane domain of EGFR and the cytoplasmic domain of erythropoietin receptor did not undergo apoptosis by co-culture with Vero-H cells, indicating that the inhibitory signal induced by pro-HB-EGF-expressing Vero-H cells is mediated to DER cells via EGFR and that the cytoplasmic domain of EGFR is essential for pro-HB-EGF-induced apoptosis. From these results, we concluded that pro-HB-EGF has unique biological activity through cell-cell contact that is distinct from the activity of sHB-EGF.

Bowen's disease showing spontaneous complete regression associated with apoptosis

Spontaneous regression is sometimes seen in malignant skin tumours. We report a 68-year-old woman whose Bowen's disease showed spontaneous complete regression. Prominent infiltration of T cells and increased vascularity were found in the upper dermis of the regressed lesion. Strong expression of Fas (APO-1/CD95) antigen, an apoptosis-related tumour necrosis factor receptor family protein, in the primary lesion and faint expression following regression suggest the involvement of Fas-mediated apoptosis in the spontaneous complete regression of our patient's Bowen's disease.

The role of apoptosis in wound healing

Wound healing involves a series of rapid increases in specific cell populations that prepare the wound for repair, deposit new matrices and finally, mature the wound. Upon completing their tasks, these specific cell types must be eliminated from the wound prior to the progression to the next phase of healing. The most logical method of cellular down-regulation is through apoptosis. Apoptosis allows for the eliminations of entire populations without tissue damage or an inflammatory response. This review discusses which cells dominate the various phases of tissue repair and how the cellular pattern may vary after differing types of injury. The potential mechanisms involved in the down-regulation of inflammation and fibrosis are also covered. The studies that support the hypothesis that apoptosis is involved in the regulation of wound healing are discussed. The evidence supporting potential cell signals involved in the induction of apoptosis in tissue repair are examined. Finally, the review ends with a presentation of how dysregulation of apoptosis can lead to pathologic forms of healing such as excessive scarring and fibrosis. By understanding the mechanisms controlling apoptosis and tissue repair, one may eventually develop therapeutic modalities to minimize scarring, a final pathway for many disease processes.