Abstract
TIS21(/BTG2/PC3), orthologs of mouse, human and rat, respectively, is initially identified as one of the early growth response genes and induced by various stimulations. TIS21 belongs to antiproliferative (APRO) gene family containing the BTG-Box A (Y(50)-N(71)) and BTG-Box B (L(97)-E(115)), which are highly conserved among various species. On the other hand, it has lately been found that the expression of TIS21 is constitutive and high in thymus, lung alveolar epithelium, proximal tubule of kidney and basal cell layer of prostate acini. Potential roles of TIS21 have been suggested as transcriptional co-regulator, differentiation and antiapoptotic factor in neurogenesis, key mediator of the stage-specific expansion of thymocyte and negative regulator of hematopoietic progenitor expansion, and tumor suppressor gene in both mouse and human. In addition, as pan-cell cycle regulator TIS21 induces G1/S arrest by pRB dependently and pRB independently and G2/M arrest and cell death in the p53 null tumor cells, and regulates the development of vertebrate patterning in mouse, paraxial mesoderm development in zebrafish, and notochord development in Xenopus. It has been known that the expression of TIS21 depends on the induction of wt p53 when cells are damaged, however, it can also be upregulated p53 independently by the activation of PKC-delta pathway in tumor cells. The characteristic roles of TIS21 are discussed in the present review: (1) TIS21 inhibits early phase of carcinogenesis in its high expressers such as kidney, prostate, breast and thymus: Loss of constitutive and high expression of TIS21 was observed in the precancerous lesions as well as tumor tissues. As an endogenous cell death molecule, TIS21 may be involved in translocation of Pin-1 to cytoplasm. Pin-1 subsequently interacts with Serine(147) residue in TIS21 protein, resulting in mitochondrial depolarization. (2) TIS21 regulates transition of cell cycle at G1/S and G2/M phases in cancer cells with inactive pRB and/or p53, as well as in normal cells by regulating pRB/p16(INK4a) pathway. The latter has already been well elucidated; TIS21 inhibits the expression of cyclin D1, thus resulting in the arrest of cells at G1/S phase by pRB and p53 dependent manner. On the other hand, TIS21 inhibits degradations of cyclin A and cyclin B1 at G2/M phase, and directly binds to Cdc2, resulting in the failure of mitotic exit and then increasing the tumor cell death, when stimulated by high concentration of EGF. Therefore, TIS21 can be suggested as a pan-cell cycle modulator. (3) TIS21 regulates embryo development by activating BMP signal through interaction with Smad 1 and Smad 8, thereby regulating vertebral patterning in mice. It is also involved in notochord development in Xenopus and paraxial mesoderm development in zebrafish. Based on the previous report that the expression of TIS21 is involved in the induction of senescence after chemotherapy of cancer cells, which can be a mechanism to resist carcinogenesis, TIS21(/BTG2/PC3), the endogenous cell death molecule and pan-cell cycle regulator, might be a link between cellular senescence and carcinogenesis.