Differences in Apoptosis and Cell Cycle Distribution between Human Melanoma Cell Lines UACC903 and UACC903(+6), before and after UV Irradiation

Introduction of human chromosome 6 into malignant melanoma cell line UACC903 resulted in generation of the chromosome 6-mediated suppressed cell subline UACC903(+6) that displays attenuated growth rate, anchorage-dependency, and reduced tumorigenicity. We have showed that overexpression of a chromosome 6-encoded tumor suppressor gene led to partial suppression to UACC903 cell growth. We now describe the differences in apoptosis and cell cycle between UACC903 and UACC903(+6) before and after UV irradiation. MTT assay revealed 86.92±8.24% of UACC903 cells viable, significantly (p<0.01) higher than 48.76±5.31% of UACC903(+6), at 24 hr after 254-nm UV irradiation (40 J/M(2)). Before UV treatment, flow cytometry analysis revealed 6.06±0.20% apoptosis in UACC903, significantly (p=0.01) lower than 6.67±0.15% in UACC903(+6). The G0/G1, S and G2/M phase cells of UACC903 were, respectively, 54.10±0.59%, 22.31±0.50% and 16.85±0.25%, all significantly (p<0.01) different from the corresponding percentages (58.82±0.35%, 20.48±0.05%, and 13.17±0.45%) of UACC903(+6). After the UV treatment, UACC903 cells in apoptosis, G0/G1, S, and G2/M became 12.59±0.17%, 38.90±0.67%, 19.74±0.70%, and 27.01±0.66%, respectively, while UACC903(+6) cells were 24.16±0.48%, 37.97±0.62%, 19.20±0.52%, and 15.69±0.14%. TUNEL assay revealed 2.31±0.62% apoptosis in UACC903, significantly (p<0.01) lower than 9.60±1.14% of UACC903(+6), and a linear and exponential increase of apoptosis, respectively, in response to the UV treatment. These results indicate that UACC903(+6) cells have a greater tendency to undergo apoptosis and are thus much more sensitive to UV irradiation. Our findings further suggest a novel mechanism for chromosome 6-mediated suppression of tumorigenesis and metastasis, i.e., through increased cell death.