Senescence marker protein-30 (SMP30) is an androgen-independent factor that decreases with aging. To elucidate the physiological functions of SMP30, we transfected human SMP30 cDNA into the human hepatoma cell line, Hep G2. These Hep G2/SMP30 transfectants, which stably expressed large amounts of SMP30, proliferated at a slower rate and synthesized less DNA than mock transfectants (Hep G2/pcDNA3 controls). Thus, enhanced expression of SMP30 retarded the growth of Hep G2/SMP30 cells. Ultrastructural studies by scanning electron microscopy revealed numerous microvilli covering the surfaces of Hep G2/SMP30 cells, whereas few microvilli appeared on control cells. Subsequently, transmission electron microscopy revealed that groups of Hep G2/SMP30 cells exhibited bile canaliculi and possessed specialized adhesion contacts, such as tight junctions and desmosomes, at interplasmic membranes. However, in controls, units of only two cells were seen, and these lacked specialized adhesion junctions. Moesin and ZO-1 are known to be concentrated in microvilli and at tight junctions, respectively. Double-immunostaining was performed to examine whether moesin and ZO-1 were expressed in bile canaliculi with microvilli at the apical regions of Hep G2/SMP30 cells. The intensity of moesin and ZO-1 staining in the contact regions of each cell was markedly higher in Hep G2/SMP30 than in control cells. Moreover, moesin stained more interior areas, which corresponded to the microvilli of bile canaliculi. Clearly, bile canaliculi with microvilli formed at the apical ends of Hep G2/SMP30 cells. These results indicate that SMP30 has an important physiological function as a participant in cell-to-cell interactions and imply that the down-regulation of SMP30 during the aging process contributes to the deterioration of cellular interactivity.