Two human melanoma cell‐line variants with enhanced in vivo tumor growth and metastatic capacity do not express the β3 integrin subunit

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Abstract
The αvβ3 integrin complex is thought to play an important role in in vivo melanoma tumor growth and metastasis. However, not all human metastatic melanomas, present in lymph node biopsies, express αvβ3. In this study, we have investigated the possibility that certain melanoma cell lines can grow aggressively in vivo in the absence of αvβ3 expression. Established human melanoma cell lines (M3Da., M4Beu.) were isolated from an achromic skin metastasis of lymph nodes. Two stable variants (7GP, T1P26), derived from a poorly metastatic M4Beu. melanoma cell line, were isolated by sequential selection for spontaneous metastasis formation in an immunosuppressed newborn rat model. Flow‐cytometry analysis shows an absence of the β3 integrin subunit (less than 2% of parental levels) in the two variant melanoma cell lines (7GP, T1P26) compared to M3Da. and M4Beu. cell lines which express a relatively high number of β3 subunits. The expression levels of the integrin subunits β1, β5, β6 and αv were found to be similar for all four melanoma cell lines. Northern blot analysis confirmed the absence of β3 in 7GP or T1P26 cell lines and its presence in M3Da. and M4Beu. Moreover, similar levels of αv transcript were present in the four melanoma cell lines. The functional effect of the absence of β3 was investigated by subcutaneously implanting the variants and the melanoma cell lines in nude mice. Variant 7GP and T1P26 cell lines yielded tumors which were larger and grew at a faster rate than tumors in M3Da. or M4Beu. cell lines. The β3 integrin subunit was not detectable on the surface of cells harvested from tumors after 20 or 35 days. Similarly, subcutaneous innoculation of the two variants into immunosuppressed newborn rats gave rise to extensive spontaneous lung metastases compared to the M4Beu. cell line. These results provide evidence that a population of melanoma cells can grow aggressively in vivo and metastasize in the absence of β3 or αvβ3 integrin complex. Our results may have clinical relevance and suggest that certain types of melanomas in patients may grow and spread in the absence of the αvβ3 integrin complex.
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