INCREASE IN TELOMERE SEQUENCE-BINDING ACTIVITY IN NORMAL HUMAN FIBROBLASTS IN SENESCENCE OR IN CELLS TREATED WITH PHORBOL ESTER OR N-METHYL-N'-NITRO-N-NITROSOGUANIDINE

The telomere is a specialized chromatin structure composed of unique r epetitive DNA sequences and specific nuclear proteins. Telomere sequen ce-binding activity was measured by a mobility: shift assay using nucl ear extract from normal human fibroblasts. The specific binding activi ty to the telomere sequence increased in cells that were in a senescen ce state compared to that in cells at early population doublings. Trea tment of cells with tumor promoting phorbol ester TPA induced an incre ase in the telomere sequence binding activity of nuclear extract in yo ung cells, but the increase was marginal in senescent cells. DNA-damag ing N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) also increased the tel omere sequence binding activity in young cells, but not in senescent c ells. As a reference, we measured the binding activity to NFkB sequenc e, It was activated by TPA or okadaic acid, but was not affected by MN NG or in senescence. The increase in telomere sequence-binding activit y seemed to depend on activation of tyrosine phosphorylation, since an inhibitor of Tyr-kinase abolished the increase in telomere-binding, a ctivity. The molecular weight of the major binding factor in the norma l human fibroblasts was approximately 32 kDa which is different from t hat of the telomere-associated protein, TRF-1.