Expression of AP-2alpha, c-kit, and cleaved caspase-6 and -3 in naevi and malignant melanomas of the skin. A possible role for caspases in melanoma progression?
Progression of melanoma is associated with loss of the transcription factor AP-2alpha and tyrosine-kinase receptor c-kit. However, the mechanisms by which these two proteins are down-regulated have not been fully elucidated. Fifty non-selected melanomas comprising ten superficial spreading melanomas (five exhibiting a radial growth phase and five a vertical growth phase), ten primary nodular melanomas, 30 melanoma metastases, and 16 naevi were investigated by direct sequencing analysis of the AP-2alpha and c-kit genes and by immunohistochemistry for the respective proteins. Because it has recently been demonstrated that AP-2alpha is preferentially cleaved by caspase-6 and to a lesser extent by caspase-3, immunohistochemistry for the cleaved (activated) forms of caspase-6 (c-casp-6) and caspase-3 (c-casp-3) was carried out. No mutations were identified in the c-kit gene, but three different point mutations were demonstrated in the activation motif of AP-2alpha in four tumours: one vertical growth phase superficial spreading melanoma, one nodular melanoma, and two metastases. Immunohistochemistry revealed progressive loss of the AP-2alpha and c-kit proteins in primary melanomas and metastases when compared with naevi. The decrease of both markers was more accentuated in the dermal component of all primary tumours, with c-kit more affected than AP-2alpha. All invasive melanomas and metastases expressed c-casp-6. c-casp-3 was expressed by 83% of the metastases and in the dermal component of one nodular melanoma. These findings suggest that the loss of AP-2alpha protein expression during the progression of melanoma could be related to mutation of the gene in only a small number of tumours, whereas the expression and activation of caspases, most prominently caspase-6, may be an important factor for the down-regulation of AP-2alpha protein. Furthermore, this study supports recent data that the activation of caspases does not inevitably result in apoptosis, but may also contribute to tumour progression in melanomas.
D053148:Caspase 3; D053178:Caspase 6; D020169:Caspases / Q000032:analysis / Q000235:genetics*; D004268:DNA-Binding Proteins / Q000032:analysis / Q000235:genetics*; D018450:Disease Progression; D004789:Enzyme Activation; D015870:Gene Expression; D006801:Humans; D007150:Immunohistochemistry / Q000379:methods; D008207:Lymphatic Metastasis; D008545:Melanoma / Q000235:genetics* / Q000473:pathology / Q000556:secondary; D009367:Neoplasm Staging; D009506:Nevus / Q000235:genetics* / Q000473:pathology; D017354:Point Mutation; D016133:Polymerase Chain Reaction; D019009:Proto-Oncogene Proteins c-kit / Q000032:analysis*; D017422:Sequence Analysis, DNA; D012878:Skin Neoplasms / Q000235:genetics* / Q000473:pathology / Q000556:secondary; D018709:Statistics, Nonparametric; D050656:Transcription Factor AP-2; D014157:Transcription Factors / Q000032:analysis / Q000235:genetics*
D004268:DNA-Binding Proteins; C493673:TFAP2A protein, human; D050656:Transcription Factor AP-2; D014157:Transcription Factors; D019009:Proto-Oncogene Proteins c-kit; C505407:CASP3 protein, human; C505514:CASP6 protein, human; D053148:Caspase 3; D053178:Caspase 6; D020169:Caspases