Cell Differentiation/physiology; Helix-Loop-Helix Motifs/physiology; Inhibitor of Differentiation Proteins/metabolism; Blotting, Western; Cell Differentiation/genetics; Cell Line; Cell Line, Tumor; Cell Proliferation/drug effects; Cell Shape/genetics; Cell Shape/physiology; Cyclin-Dependent Kinase Inhibitor p21/metabolism; Cyclin-Dependent Kinase Inhibitor p27; Dimerization; Fluorescent Antibody Technique; Gene Expression Regulation, Neoplastic; Helix-Loop-Helix Motifs/genetics; Humans; Inhibitor of Differentiation Protein 1/genetics; Inhibitor of Differentiation Protein 1/metabolism; Inhibitor of Differentiation Protein 2/genetics; Inhibitor of Differentiation Protein 2/metabolism; Inhibitor of Differentiation Proteins/chemistry; Inhibitor of Differentiation Proteins/genetics; Intracellular Signaling Peptides and Proteins/metabolism; Mutation; Neural Cell Adhesion Molecules/metabolism; Neurofilament Proteins/metabolism; Protein Binding; Retinoblastoma Protein/metabolism; Reverse Transcriptase Polymerase Chain Reaction; Transfection; Tretinoin/pharmacology; Vascular Endothelial Growth Factor A/genetics; Vascular Endothelial Growth Factor A/metabolism