Apoptosis Regulators and Responses in Human Melanocytic and Keratinocytic Cells

Abstract

Apoptosis in keratinocytes is required for epidermal turnover, stratum corneum formation, and removal of ultraviolet-damaged premalignant cells. Its role in melanocyte homeostasis and transformation, on the other hand, has not been de¢ned, although apoptosis resistance is a commonly recognized feature of melanoma. We examined the expression of apoptosis regulators in melanocytes, keratinocytes, melanoma, and HaCat cells. Melanocytic cells expressed relatively high levels of Bcl-2, Bcl-X L , Mcl-1, C-IAP-1, C-IAP-2, XIAP, Livin, and Apaf-1. The only apoptotic regulator that was di¡erentially expressed in melanoma cells and not melanocytes was Survivin, whereas Bax was expressed in melanocytes but not in most melanoma lines. Keratinocytic cells, on the other hand, expressed high levels of FLIP and were relatively de¢cient in Bcl-2 family proteins. Levels of p53 were highest in HaCat cells and some of the melanoma lines, and barely detectable in melanocytes and keratinocytes. Next, susceptibility of these cells types to apoptosis induced by ultraviolet B, the tyrosine analog 4-tert-butylphenol, and cytotoxic drugs was examined. Melanocytes were relatively resistant to ultraviolet B, whereas keratinocytes were unresponsive to 4-tert-butylphenol. Melanocytes and keratinocytes were generally less susceptible than melanoma lines and HaCat cells to etoposide, cisplatin, and staurosporine. Induction of apoptosis in these cell types was generally associated with decreased levels of Mcl-1, XIAP, and Livin, and increased levels of p53, whereas levels of other apoptotic regulators were unaltered. These results provide insights into the potential roles of apoptosis in the function and transformation of epidermal melanocytes and keratinocytes.

Functional role of MIA in melanocytes and early development of melanoma

“…In accordance, Selzer et al (1998) could show that these genes such are equally expressed comparing melanocytes and melanoma. Recently, several groups presented Apaf-1 as the main regulator of apoptosis in malignant melanoma (Soengas et al, 2001;Bowen et al, 2003;Paggi et al, 2003). Furthermore, Apaf-1 was shown to be regulated by the transcription factor E2F-1 (Furukawa et al, 2002).…”

Role of the pro-survival molecule Bfl-1 in melanoma

“…Melanocytes were also seen to be less susceptible to cell death caused by Etoposide, Cisplatin, Paclitaxel, or the MEK inhibitor PD184352, than melanoma cells 306,444,446 .…”

“…When mouse melanocytes were stimulated with chemokines for CXCR2, they were seen to display the ability to grow colonies on soft-agar plates, representing their tumourigenic potential 445 . Indeed, the NFB pathway has been shown to be constitutively activated in melanoma cells 446 , which would result in an increase in the expression of its transcriptional targets.…”

PUMA expression is significantly reduced in human cutaneous melanomas

“…Dysregulation of several apoptotic regulators, in combination with PUMA loss, is probably responsible for melanoma chemoresistance. For example, the apoptotic inhibitors Mcl-1, Bcl-X L , XIAP, Livin, and Survivin are elevated in some melanoma cell lines, while proapoptotic Bax protein is abnormally low, compared to normal melanocytes (Bowen et al, 2003). We propose a model in which PUMA downregulation in melanoma creates an apoptotic-resistant phenotype that responds poorly to chemotherapeutic drug treatment.…”