Role of PLZF in melanoma progression

The promyelocytic leukemia zinc finger (PLZF) protein has been described as a transcriptional repressor of homeobox (HOX)-containing genes during embryogenesis. As we previously demonstrated a functional link between overexpression of HOXB7 and melanoma progression, we investigated the lack of PLZF as the possible cause of HOXB7 constitutive activation in these neoplastic cells. Accordingly, we found PLZF expression in melanocytes, but not in melanoma cells, a pattern inversely related to that of HOXB7. PLZF retroviral gene transduction was then performed in a panel of melanoma cell lines, and tumorigenicity was compared with that of empty vector-transduced control cell lines. Evaluation of in vitro migration, invasion and adhesion indicated that PLZF gene transduction induced a less malignant phenotype, as confirmed through in vivo studies performed in athymic nude mice. This reduced tumorigenicity was not coupled with HOXB7 repression. In order to find more about the molecular targets of PLZF, the gene expression profiles of PLZF- and empty vector-transduced A375 melanoma cells were analysed by Atlas Cancer macroarray. Among several genes modulated by PLZF enforced expression, of particular interest were integrin alphavbeta3, osteonectin/SPARC and matrix metalloprotease-9 that were downmodulated, and the tyrosinase-related protein-1 that was upregulated in all the analysed samples. This profile confirms the reduced tumorigenic phenotype with reversion to a more differentiated, melanocyte like, pattern, thus suggesting a suppressor role for PLZF in solid tumors. Moreover, these results indicate that PLZF and HOXB7 are functionally independent and that their coupled deregulation may account for most of the alterations described in melanomas.