UCSF

Understanding cell biology of three-dimensional (3D) biological structures is important for more complete appreciation of in vivo tissue function and advancing ex vivo organ engineering efforts. To elucidate how 3D structure may affect hepatocyte cellular responses, we compared global gene expression of human liver hepatocellular carcinoma cell line (HepG2) cells cultured as monolayers on tissue culture dishes (TCDs) or as spheroids within rotating wall vessel (RWV) bioreactors. HepG2 cells grown in RWVs form spheroids up to 100 mum in diameter within 72 h and up to 1 mm with long-term culture. The actin cytoskeleton in monolayer cells show stress fiber formation while spheroids have cortical actin organization. Global gene expression analysis demonstrates upregulation of structural genes such as extracellular matrix, cytoskeletal, and adhesion molecules in monolayers, whereas RWV spheroids show upregulation of metabolic and synthetic genes, suggesting functional differences. Indeed, liver-specific functions of cytochrome P450 activity and albumin production are higher in the spheroids. Enhanced liver functions require maintenance of 3D structure and environment, because transfer of spheroids to a TCD results in spheroid disintegration and subsequent loss of function. These findings illustrate the importance of physical environment on cellular organization and its effects on hepatocyte processes.