Figure 4. Normal (MCF12A) and cancer (HeLa MR and U251) cell populations exhibit very different senescent fractions after MNNG exposure.

Normal (MCF12A) ± siMGMT undergo ≥ 75% senescence at equitoxic concentrations of MNNG (2 µM + siMGMT; 8 µM – siMGMT). Alkylation sensitive tumor cells (HeLa MR & U251) have a low level of ≤ 5% senescence that decreases even further to < 0.01% in alkylation resistant tumor cells (HeLa MR^R & U251^R) at equitoxic concentrations of MNNG (0.2 µM for original cell lines; 2 µM for resistant subclones). Each senescence assay was performed a minimum of 4 times. Light microscopic photomicrograph (150X) of representative senescent cell of each cell line after SA-β galactosidase staining is depicted in lower part of figure, with white arrows pointing to SA-β galactosidase perinuclear stained areas in each cell [44].