TIS21 (/BTG2/PC3) as a link between ageing and cancer: cell cycle regulator and endogenous cell death molecule

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.

Fig. 1

a Strong expression of TIS21 in the fetal liver of 13.5-day-old rat embyo, examined by in situ hybridization. Note the TIS21 expression in the ventricular neuronal epithelium in brain and vertebrae. b Northern blot analysis showing the expression of TIS21 until 3 days after birth, however, it is turned off 1 week after birth. CHOb, constitutive expresser in various tissues and cells, is used as a loading control

Fig. 2

Schematic representation of the mouse TIS21 gene. TIS21 gene contains two exons and one intron between the two exons. Exon 1 (1,076–1,283) contains 5′UTR (1,076–1,141) and short open reading frame, whereas exon 2 (2,491–4,865) has relatively longer open reading frame and 3′UTR (2,826–4,865) including poly A addition sites (aataaa) at 4,841–4,846

Fig. 3

Inhibition of G1/S transition by TIS21 expression. Constitutive expression of TIS21 (293 clone 9, 11) delays biosynthesis of cyclin E and CDK4 and inhibits cyclin E associated CDK activity, thereby inhibiting G1–S transition by the pRB independent manner. It has also been well established that TIS21 also regulates G1/S arrest by regulating pRB activity via inhibition of cyclin D1 expression in the pRB dependent way

Fig. 4

Induction of TIS21 expression by PKC-δ pathway and G2/M arrest. TIS21 expression is activated by PKC-δ pathway, whereas it is inhibited by cPKC isozymes. TIS21 expression is independent of p53 and p19^ARF expressions induced by DNA damage. Interestingly, constitutive expression of TIS21 induces G2/M arrest and cell death through the inhibition of cyclin B1 binding to Cdc2 and delayed degradations of cyclin A and cyclin B1, therefore, demonstrating failure of mitotic exit by TIS21 expression

Fig. 5

Mechanism of cell death by TIS21 overexpression after treatment of tumor cells, which lack of p53, with EGF. EGF induces activation of ERK_1/2, and the p-ERK then phosphorylates S¹⁴⁷ of TIS21, which in turn binds to Pin-1 protein and translocate Pin-1 from nucleus to cytoplasm, thus resulting in the cell death instead of mitosis progression. At the same time, TIS21 interrupts cyclin B1 binding to Cdc2, and inhibits mitotic progression

Fig. 6

Potential roles of TIS21^/BTG2/PC3. TIS21 can positively be expressed independent of p53 via PKC-δ pathway, whereas cPKC isozymes negatively regulate TIS21 expression. The expressed TIS21 can regulate G1 arrest and cellular senescence phenotypes by delayed biosynthesis of cyclin E and CDK4 proteins, independent of pRB. At the same time, TIS21 can regulate G1/S transition by the regulation of pRB along with inhibition of cyclin D1 expression. On the other hand, TIS21 can bind to Cdc2 kinase instead of cyclin B1, thereby inducing G2/M arrest in the p53 defective tumor cells. It is proposed that constitutive expression of TIS21 in its high expressers such as thymus, kidney, prostate and lung can be an endogenous cell death molecule through its interaction with Pin-1, when the cells are stimulated with epidermal growth factor. Moreover, TIS21 works as a developmental regulator via interaction with Smad 1 and Samd 8, and stimulates BMP signals. Consequently, TIS21 knock out mice reveal abnormal transformation of vertebral patterning as well as incomplete bone formation. TIS21 interacts also with either PRMT1 or CCR4 through CAF1/POP2, thus working as a transcriptional co-regulator