LOSS OF SERUM RESPONSE ELEMENT-BINDING ACTIVITY AND HYPERPHOSPHORYLATION OF SERUM RESPONSE FACTOR DURING CELLULAR AGING

Human diploid fibroblasts undergo a limited number of population doubl ings in vitro and are used widely as a model of cellular aging. Despit e growing evidence that cellular aging occurs as a consequence of alte red gene expression, little is known about the activity of transcripti on factors in aging cells. Here, we report a dramatic reduction in the ability of proteins extracted from the nuclei of near-senescent fibro blasts to bind the serum response element which is necessary for serum -induced transcription of the c-fos gene. In contrast, the activities of proteins binding to the RNA polymerase core element, TATA, as well as to the cyclic AMP response element were maintained during cellular aging. While no major differences in the expression of the serum respo nse factor (SRF) that binds the serum response element were seen betwe en early-passage and late-passage cells, hyperphosphorylation of SRF w as observed in near-senescent cells. Furthermore, removal of phosphata se inhibitors during the isolation of endogenous nuclear proteins rest ored the ability of SRF isolated from old cells to bind the SRE. These data, therefore, indicate that hyperphosphorylation of SRF plays a ro le in altering the ability of this protein to bind to DNA and regulate gene expression in senescent cells.