Fig. 4.

pALT^INK4a/b overexpression triggers cell-cycle arrest. Naked mole rat cells were transfected with constructs expressing HPRT (control), p15^INK4b, p16^INK4a, or pALT^INK4a/b under the CMV promoter. (A) Expression of the INK4a/b products was verified with RT-PCR. (B) Microphotograph showing cells transfected with p15^INK4b, p16^INK4a, or pALT^INK4a/b 6 d after transfection. Control cells continued to proliferate whereas cells transfected with INK4a/b products entered cell-cycle arrest. (C) Cell-cycle arrest induced by expression of p15^INK4b, p16^INK4a, or pALT^INK4a/b was quantified at indicated times after transfection by staining cells with propidium iodide and quantifying the number of cells in S-phase by FACS analysis. The experiments were repeated three times, and error bars show SD; statistical significance is indicated by asterisk. Three lines of low-passage naked mole rat skin fibroblasts were used for each experiment. *P < 0.05; **P < 0.005; ***P < 0.0005, Student’s t test.