Pituitary tumor transforming gene (PTTG) regulates placental JEG-3 cell division and survival: Evidence from live cell imaging

The pituitary transforming gene, PTTG, is abundantly expressed in endocrine neoplasms. PTTG has recently been recognized as a mammalian securin based on its biochemical homology to Pds1 p. PTTG expression and intracellular localization were therefore studied during the cell cycle in human placental JEG-3 cells. PTTG mRNA and protein expressions were low at the G(1)/S border, gradually increased during S phase, and peaked at G(2)/M, but PTTG level were attenuated as cells entered G(1). In interphase cells, wild-type PTTG, an epitope-tagged PTTG, and a PTTG-EGFP conjugate all localized to both the nucleus and cytoplasm, but in mitotic cells, PTTG was not observed in the chromosome region. PTTG-EGFP colocalized with mitotic spindles in early mitosis and was degraded in anaphase. Intracellular fates of PTTG-EGFP and a conjugate of EGFP and a mutant inactivated PTTG devoid of an SH3-binding domain were observed by real-time visualization of the EGFP conjugates in live cells. The same cells were continuously observed as they progressed from G(1)/S border to S, G(2)/M, and G(1). Most cells (67%) expressing PTTG-EGFP died by apoptosis, and few cells (4%) expressing PTTG-EGFP divided, whereas those expressing mutant PTTG-EGFP divided. PTTG-EGFP, as well as the mutant PTTG-EGFP, disappeared after cells divided. The results show that PTTG expression and localization are cell cycle-dependent and demonstrate that PTTG regulates endocrine tumor cell division and survival.