Fibroblast Cells Physiological Changes

Fibroblasts undergo striking physiological changes upon replicative senescence. Three major features distinguish a senescent fibroblast from its presenescent (early passage) counterpart: (1) an irreversible arrest of cell-cycle progression, (2) resistance to programmed cell death (apoptosis), and (3) a shift in function such that the cells no longer maintain the stromal extracellular matrix. Together, these changes are termed the senescent phenotype.

Growth arrest. Senescent cells, whether fibroblasts or other cell types, irreversibly arrest growth with an unreplicated or G1 DNA content. This growth arrest is due to two types of changes in gene expression: repression of certain genes that are essential for cell-cycle progression, and overexpression of certain genes that inhibit the cell cycle. In the case of fibroblasts, examples of genes that are repressed include those encoding the FOS proto-oncoprotein and the E2F1 transcription factor. FOS is important for the ability of fibroblasts to progress from the resting, quiescence (G0) phase through the G1 phase of the cell cycle (when cells prepare to synthesize DNA). E2F1 is important for entry into the S phase (when DNA synthesis occurs). Among the growth-inhibitory genes that are overexpressed in senescent fibroblasts are those encoding the p21 and p16 proteins—p21 and p16 inhibit cyclin-dependent protein kinases, the activities of which are essential for G1 progression; p16 is a tumor suppressor gene and appears to be critical for maintaining the senescence growth arrest in many, if not all, cell types.

Resistance to apoptosis. Fibroblasts, and several other cell types (e.g., T lymphocytes and epidermal keratinocytes), are more resistant to certain, although not all, stimuli that normally induce apoptosis. Consequently, senescent cells are quite stable. They have been shown to persist for many months or longer in culture. In addition, they very likely also persist in vivo, and hence accumulate with age.

Altered morphology and functions. One of the most visible changes that occur when cells reach replicative senescence is a change in morphology. Cell size or volume increases, often reaching double (or more) the presenescent size, and the cells accumulate intracellular vesicles, many of which are lysosomes. Fibroblasts become flatter and more irregularly shaped, showing more prominent intracellular actin fibers. For most cell types, the senescent morphology is quite distinct from that of proliferating, quiescent, or terminally differentiated cells from the same lineage.

The striking change in cell morphology very likely reflects, at least in part, the functional changes that accompany replicative senescence. In the case of fibroblasts, upon senescence the cells switch from a matrix-producing phenotype to a matrix-degrading phenotype. Senescent fibroblasts produce less collagen and elastin, which form the major structural and elastic fibers of the stroma, than presenescent cells. The decline in collagen and elastin production is not due to a general decline in matrix production, because fibronectin, a cell-associated matrix molecule, increases. In conjunction with the decline in collagen and elastin, senescent cells overexpress several matrix metalloproteinases. These enzymes degrade the stromal fibers and extracellular matrix. Senescent fibroblasts also secrete pro-inflammatory cytokines and a variety of epithelial cell growth factors. Thus, senescent fibroblasts, despite their inability to divide, adopt a phenotype that at least partially resembles a wounding response. As discussed below, senescence-associated functional changes may have a greater impact on the aging organism than the loss of cell division potential.