In situ changes in the repertoire of integrins and proteolytic enzymes have been demonstrated during melanoma metastasis. To investigate whether established human melanoma cell lines, injected into nude mice, could undergo phenotypic changes similar to those observed in in situ lesions, we studied 3 melanoma cell lines of distinct metastatic origin, adherent HT-144 and SK-MEL-2 cells, and non-adherent SK-MEL-1 cells for integrin expression, proteolytic enzyme repertoire and invasive potential after in vitro culture. Heterogeneity in integrin expression, such as elevated levels in alpha(v)beta3 in SK-MEL-1 and SK-MEL-2 cells and low expression in HT-144 cells, correlated with their in vitro invasiveness, since only the adherent HT-144 and SK-MEL-2 cells were able to invade Matrigel, and in addition, secreted a 72-kDa gelatinase. In contrast, no similar correlation could be established in nude mice, as all 3 cell lines, including the non-adherent SK-MEL-1 cells, were tumorigenic when injected s.c., while only HT-144 consistently produced experimental lung metastasis. Immunochemical analysis of the integrin profile in s.c. xenografts revealed over-expression of alpha(v), beta1 and beta3 integrins exclusively in HT-144 cells, as well as increased expression of beta3 in HT-144 cell lung metastases, as confirmed by PCR analysis using species-specific primers, while zymography and Western-blot analysis demonstrated de novo expression of the 92-kDa gelatinase MMP-9 in HT-144 xenografts. Our results highlight a positive correlation between up-regulated beta3 integrin and MMP-9 expression in human HT-144 melanoma cell tumors grown in nude mice.