Lysosome dispersion in osteoblasts accommodates enhanced collagen production during differentiation

Lysosomes are essential organelles for intracellular degradation and are generally sequestered near the cell center to receive vesicles with contents targeted for destruction. During ascorbic acid (AA)-induced differentiation of osteogenic cells ( Beck, G. R., Jr., Zerler, B., and Moran, E. (2001) Cell Growth Differ. 12, 61-83 ), we saw a marked increase in total lysosome organelles in osteoblastic cells, in addition to an enhanced endocytic rate. Interestingly, lysosomes were dispersed toward the cell periphery in differentiating osteoblasts. We determined that lysosome dispersion in differentiated osteoblasts required intact microtubules for long range transport and was dependent on kinesin motors but did not involve cytosolic acidification. Impairment of lysosome dispersion markedly reduced AA-induced osteoblast differentiation. Lysosomes were not secreted in differentiated osteoblasts, implicating them instead in intracellular degradation. We assayed the degradative capacity and saw a significant increase in DQ-ovalbumin fluorescence in differentiated osteogenic cells compared with undifferentiated control cells. Osteogenic cells are specialized for type I collagen production, and we noted enhanced secreted and intracellular collagen in AA-differentiated osteoblasts versus control cells. Importantly, osteoblasts displayed procollagen-containing vesicles that were distributed throughout the cytoplasm, a portion of which colocalized with lysosomes. Treatment of cells with 2,2'-dipyridyl to inhibit procollagen trimerization enhanced colocalization of lysosomes with procollagen-containing organelles, implicating dispersed lysosomes in collagen processing in osteogenic cells.

Vitamin C modulation of H2O2-induced damage and iron homeostasis in human cells

Vitamin C (ascorbic acid, AA) is an important antioxidant in human plasma. It is clear, however, that AA has other important, nonantioxidant roles in cells. Of particular interest is its involvement in iron metabolism, since AA enhances dietary iron absorption, increases the activity of Fe(2+)-dependent cellular enzymes, promotes Fenton reactions in vitro, and was reported to have deleterious effects in individuals with iron overload. Nevertheless, the ability of AA to modulate iron metabolism and enhance iron-dependent damage in cells, tissues, and organisms has not been fully elucidated. Here we investigated the effect of AA on iron-mediated oxidative stress in normal human fibroblasts. Incubation with physiologically relevant concentrations of AA was not harmful but sensitised cells toward H(2)O(2)-induced, iron-dependent DNA strand breakage and cell death. We also report that AA increased the levels of intracellular catalytic iron and concomitantly modulated the expression of two well-established iron-regulated genes, ferritin and transferrin receptor. In summary, we present evidence of a novel, nonantioxidant role of AA in human cells, where it increases iron availability and enhances ROS-mediated, iron-dependent damage. We suggest that AA may exacerbate the deleterious effects of metals in vivo and promote normal tissue injury in situations associated with elevated ROS production.