Human fibroblasts in culture invariably undergo cellular senescence, whereby they irreversible cease proliferation after a finite number of population doublings. This number is inversely related to the age of the fibroblast donor. Senescence may be a manifestation, at the cellular level, of in vivo aging and it may be important for curtailing tumorigenic transformation. Although it is well-established that senescent human fibroblasts express one or more dominant inhibitor of cell proliferation, the mechanisms responsible for the growth arrest are unknown. We have identified several changes in gene expression that occur when human fibroblasts senesce. Senescent cells: 1) underexpress a non-cell cycle regulated gene of unknown function (pHE-7); 2) express a unique, polyadenylated histone variant mRNA; 3) show normal serum-dependent induction of several mRNAs including those encoding several protooncogenes and ornithine decarboxylase (odc) but are deficient in odc enzyme activity; 4) acquire a specific, transcriptional block that prevents induction of the c-fos protooncogene. To understand the mechanisms by which these changes in gene expression occur and their impact on cell proliferation we propose to: 1) clone the pHE-7 gene and characterize it and its product, define the basis for the underexpression in senescent cells, and to determine the effect of high levels of pHE-7 expression on proliferation and senescence; 2) clone and characterize the histone variant expressed by senescent cells and identify DNA binding proteins that may be important in regulating expression in senescent cells; 3) further study the regulation of serum-inducible mRNAs and explore the possibility that some are down-regulated at the transcriptional or posttranscriptional level in senescent cells; 4) identify DNA binding proteins that associated with regulatory regions in the c-fos gene and determine the impact of high levels of fos expression on proliferation and senescence. These studies will provide a framework in which to begin to understand the molecular basis for cellular senescence and the irreversible block to cell proliferation.