Expression of Bcl-2 family proteins in psoriasis

Aim

To elucidate the mechanisms involved in apoptosis of psoriatic keratinocytes by examining the expression of pro-apoptotic (Bak, Bax) and anti-apoptotic (Bcl-2, Bcl-X) Bcl-2 family of proteins, as well as the expression of p53 and Ki-67 proteins in normal skin, and uninvolved and involved psoriatic skin.

Methods

A total of 90 skin samples (30 cases of involved and uninvolved psoriatic skin and normal skin) were examined immunohistochemicaly to determine the protein expression of p53, Ki-67, Bcl-2, Bcl-X, Bax, and Bak. The results were quantified and expressed as a percentage of positive keratinocytes.

Results

There was a significant increase in Ki-67 (17.05 vs 3.65; P<0.001), Bcl-X (40.21 vs 13.97; P<0.001), Bak (89.46 vs 73.36; P<0.001), and Bax (50.00 vs 29.25; P<0.001) expression and a decrease in Bcl-2 (3.23 vs 6.25; P=0.008) expression in involved psoriatic skin, as well as an increase in Bcl-X (25.13 vs 13.97; P<0.001) expression in uninvolved psoriatic skin, when compared to normal skin. Samples with higher percentage of Ki-67 positive cells showed a higher percentage of p53 positive cells (correlation coefficient r = 0.75 in involved psoriatic samples, P<0.001; r = 0.88 in uninvolved psoriatic samples, P<0.001; and r = 0.85 in normal skin samples, P<0.001). Samples with higher percentage of p53 positive cells expressed pro-apoptotic Bak and Bax in higher percentage of cells; the correlation coefficients were r = 0.74 and r = 0.68 in involved psoriatic samples (P<0.001 for both), r = 0.75 and r = 0.69 in uninvolved psoriatic samples (P<0.001, for both), and r = 0.87 and r = 0.70 in normal skin samples (P<0.001, for both).

Conclusion

Increased expression of Bcl-X protein was associated with psoriatic epidermal hyperplasia. Strong Bax and Bak expression in involved psoriatic skin are probably inhibitory mechanisms counteracting intensive proliferation.Psoriasis is a chronic, relapsing, inflammatory, and hyperproliferative skin disorder characterized by well-circumscribed erythematosquamous lesions (1). Clinically, psoriasis is characterized by significant epidermal keratinocyte hyperplasia with proliferation, keratinocyte maturation, and turn-over rates as important mechanisms in its pathogenesis (2,3).In normal human skin, keratinocytes in the superficial layer of the epidermis undergo apoptosis and regulate proliferation of cells in the basal layer (4). As opposed to normal skin, keratinocytes derived from psoriatic plaques were shown to be resistant to apoptosis (5). Numerous TUNEL-positive keratinocytes were also positive for proliferating nuclear antigen and Ki-67, which are indicative of proliferating cells (5). Inappropriate regulation of apoptosis was proposed as a possible explanation for epidermal thickening in hyperproliferative inflammatory skin disorders (6).The list of molecular mediators influencing apoptosis is rapidly expanding with Bcl-2 and its homologous proteins, emerging as one of the most important regulators of programmed cell death and playing a crucial role in the balance between cell survival and cell death. Protein p53 is a well described tumor suppressor that plays a central role in the initiation of apoptosis and cell cycle control (2,7,8).The mechanisms involved in the psoriatic plaque formation are not completely elucidated. In the era of biological therapy, better understanding of different apoptotic cell cycle regulatory mechanisms involved in this process would enable a development of novel specific therapeutic approaches for treating psoriatic patients. In this study, we examined the expression and distribution of the pro-apoptotic (Bak, Bax) and anti-apoptotic (Bcl-2, Bcl-X) Bcl-2 family proteins, as well as the expression of p53 protein in psoriatic epidermis and normal human skin. High TUNEL-positive rate in psoriatic keratinocytes was linked to high proliferation rate, so staining of Ki-67 was also performed.