Human
melanoma cells growth arrest irreversibly, lose tumorigenic potential and terminally differentiate
after treatment with a combination of
fibroblast interferon (IFN-beta) and the
protein kinase C activator
mezerein (MEZ). Applying subtraction hybridization to this model differentiation system permitted cloning of
melanoma differentiation associated gene-7, mda-7. Expression of mda-7 inversely correlates with
melanoma development and progression, with elevated expression in normal melanocytes and
nevi and increasingly reduced expression in radial growth phase, vertical growth phase and metastatic
melanoma. When expressed by means of a replication incompetent adenovirus (Ad.mda-7) growth of
melanoma, but not normal early passage or immortal human melanocytes, is dramatically suppressed and cells undergo programmed cell death (apoptosis).
Infection of metastatic
melanoma cells with Ad.mda-7 results in an increase in cells in the G(2)/M phase of the cell cycle and changes in the ratio of pro-apoptotic (BAX, BAK) to anti-apoptotic (BCL-2, BCL-XL)
proteins. Ad.mda-7
infection results in a temporal increase in mda-7
mRNA and intracellular
MDA-7 protein in most of the melanocyte/
melanoma cell lines and secretion of
MDA-7 protein is readily detected following Ad.mda-7
infection of both melanocytes and
melanoma cells. The present studies document a differential response of melanocytes versus
melanoma cells to ectopic expression of mda-7 and support future applications of mda-7 for the gene-based
therapy of metastatic
melanoma.