Deregulation of collagen phagocytosis in aging human fibroblasts: effects of integrin expression and cell cycle.
Intracellular degradation of collagen by phagocytosis in fibroblasts is essential for physiological remodeling of the extracellular matrix in a wide variety of connective tissues but imbalances between degradation and synthesis can lead to loss of tissue collagen. As aging is associated with loss of dermal and periodontal collagen and with increased lysomomal enzyme content in fibroblasts, we examined the regulation of collagen phagocytosis by integrin expression and the cell cycle in an in vitro fibroblast aging model. Two different fibroblast lines (CL1; CL2) at the fourth subculture were passaged up to replicative senescence to model aging processes in vitro. Cells were incubated with collagen-coated or BSA-coated green fluorescent beads for 3 h to assess alpha 2 beta 1-integrin-mediated or nonspecific phagocytosis, respectively. Single-cell suspensions were stained with DAPI and sulforhodamine 101 to separate cycling G1 and noncycling G0 cells. Staining for alpha 2-integrin, bead internalization, and bivariate analyses of DNA/protein content were measured by three-color flow cytometry. Serum deprivation was used to induce increases in the proportion of G0 cells. For G1 cells, the proportion of collagen phagocytic cells was > 50% for all passages and collagen beads were internalized > 5-fold more frequently than BSA beads. In contrast, G0 cells with diploid DNA content but low protein content exhibited greatly reduced phagocytic capacity (< 10% of cells internalized collagen or BSA beads), the number of beads per cells was 4-fold less, and alpha 2 integrin expression was very low compared to G1 cells. The proportion of collagen phagocytic cells and the proportion of alpha 2-integrin-positive cells increased with transit through the cell cycle. At higher passage numbers mean cell volume and cytoplasmic granularity were reduced approximately 30% but at replicative senescence cells with large surface area and subdiploid DNA predominated. The proportion of collagen and BSA phagocytic G1 cells increased 1.5- and 5-fold, respectively, and the number of beads per cell increased < 3-fold. However, surface alpha 2-integrin staining remained unchanged. These data indicate that the collagen and nonspecific internalization pathways were greatly unregulated, independent of cell cycle phase, and that cellular aging in vitro strongly influences the specificity and rate of phagocytic processes in fibroblasts. We suggest that age-related loss of collagen in connective tissues undergoing turnover may be a manifestation of a deregulated increase of collagen phagocytosis in which the net loss of degraded collagen exceeds new synthesis.