A better understanding of the aging process is relevant to resolving both urgent clinical problems of America's elderly and basic biologic questions of cellular growth control. Twenty five years of gerontologic investigation in vitro utilizing various culture models has provided many tantalizing observations, including in general a loss of responsiveness to physiologic signals, but the mechanism of cellular aging remains quite obscure. This program project application proposes to employ a powerful combination of molecular biology and cellular techniques to advance our current understanding of age-associated changes in cellular proliferative capacity, drawing upon the different perspectives and expertise of the three principal investigators. Dr. Gilchrest's project will use early passage cultured human keratinocytes and fibroblasts derived from skin biopsies of healthy young adult and old adult donors to determine whether age- associated loss of growth factor responsiveness is attributable to reduced expression of genes whose protein products are required for cell division and/or to inappropriate expression of autocrine growth inhibitors. Dr. Campisi's project will use human fetal lung fibroblasts at early versus late passage to examine the regulation of specific growth factor inducible genes and possible alterations in post-receptor pathways during in vitro fibroblast senescence. Dr. Miller's project will examine the basis for age-associated loss of lymphocyte mitogenic responsiveness using T cells from young and old mice to examine intracellular pathways and induction of genes believed to participate in T cell activation. These separate projects will share molecular biology and administrative core facilities and will interact extensively with regard to both technical and conceptual issues. It is anticipated that the proposed studies will provide major new insights into the basis cellular aging generally, as well as in the skin and the immune system specifically, and will permit important cross validation of findings among the major model systems now employed in gerontologic research.