Besides its proinflammatory properties, interleukin-8 (IL-8) has been suggested as an important promoter for melanoma growth. To study the role of IL-8 in melanoma biology, we determined the in vivo expression of IL-8 mRNA by in situ hybridization in primary melanoma lesions and metastases. High levels of melanoma cell-associated IL-8-specific transcripts were exclusively detected in close vicinity of necrotic/hypoxic areas of melanoma metastases, whereas both in primary melanomas and in non-necrotic metastases IL-8 expression was low or absent. To analyze further the up-regulation of IL-8 mRNA expression in necrotic/hypoxic tumor areas, human melanoma cell lines of different aggressiveness exposed to severe hypoxic stress (anoxia) were used as an in vitro model. Anoxia induced IL-8 mRNA and protein expression in the highly aggressive/metastatic cell lines MV3 and BLM but not in the low aggressive cell lines IF6 and 530. As shown by IL-8 promoter-dependent reporter gene analysis and mRNA stability assays, elevated mRNA levels in melanoma cells were due to both enhanced transcriptional activation and enhanced IL-8 mRNA stability. Interestingly, transcriptional activation was abolished by mutations in the AP-1 and the NF-kappaB-like binding motifs, indicating that both sites are critical for IL-8 induction. Concomitantly, anoxia induced an enhanced binding activity of AP-1 and NF-kappaB transcription factors only in the highly aggressive cells. From our in vitro and in vivo data we suggest that anoxia-induced regulation of IL-8 might be a characteristic feature of aggressive tumor cells, thus indicating that IL-8 might play a critical role for tumor progression in human malignant melanoma.