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Peroxisomes in the human hepatoblastoma cell line HepG2 exhibit a high degree of plasticity. Whereas in confluent cultures they appear as small (0.1-0.3 μm) spherical particles, they undergo dramatic changes, forming elongated tubules measuring up to 5 μm on separation of cells and cultivation at low density. We recently showed that several growth factors, including nerve growth factor (NGF), induce the formation of tubular peroxisomes and that this induction is sensitive to K 252b, a specific tyrosine kinase inhibitor, suggesting the involvement of this signal transduction pathway. Because tyrosine kinase is also involved in signal transduction via the reactive oxygen species (ROS), we have analyzed in this study the effects of UV irradiation, H 2 O 2 , and oxygen on tubulation of peroxisomes. UVC irradiation induced a significant increase in formation of tubular peroxisomes (40-50% of cells) and this effect was dose-dependently inhibited by pretreatment with N-acetyl cysteine, confirming the involvement of ROS in the UV effect. Furthermore, H 2 O 2  also directly induced the tubulation of peroxisomes, although to a lesser extent. Finally, cultivation under hypoxic conditions (1.5% O 2 ) drastically reduced the inducing effect of fetal calf serum on tubulation of peroxisomes, suggesting the involvement of oxygen-mediated signaling. Taken together, our observations indicate that ROS induce the tubulation of peroxisomes in HepG2 cells. Because peroxisomes harbor most of the enzymes for catabolism of ROS, the tubulation and expansion of the peroxisome compartment could have a cell rescue function against the destructive effects of ROS.

Induction of Tubular Peroxisomes by UV Irradiation and Reactive Oxygen Species in HepG2 Cells