Neuroblastoma is the most common extracranial solid tumor of childhood. The heterogeneous microenvironment of solid tumors contains hypoxic regions associated with poor prognosis and chemoresistance. Hypoxia implicates the actin cytoskeleton through its essential roles in motility, invasion and proliferation. However, hypoxia-induced changes in the actin cytoskeleton have only recently been observed in human cells. Tropomyosins are key regulators of the actin cytoskeleton and we hypothesized that tropomyosins may mediate hypoxic phenotypes.
Neuroblastoma (SH-EP) cells were incubated ± hypoxia (1 % O2, 5 % CO2) for up to 144 h, before examining the cytoskeleton by confocal microscopy and Western blotting.
Hypoxic cells were characterized by a more organized actin cytoskeleton and a reduced ability to degrade gelatin substrates. Hypoxia significantly increased mean actin filament bundle width (72 h) and actin filament length (72-96 h). This correlated with increased hypoxic expression and filamentous organization of stabilizing tropomyosins Tm1 and Tm2. However, isoform specific changes in tropomyosin expression were more evident at 96 h.
This study demonstrates hypoxia-induced changes in the recruitment of high molecular weight tropomyosins into the actin stress fibres of a human cancer. While hypoxia induced clear changes in actin organization compared with parallel normoxic cultures of neuroblastoma, the precise role of tropomyosins in this hypoxic actin reorganization remains to be determined.