Hypoxia Inhibits G1/S Transition through Regulation of p27 Expression

Abstract

Mammalian cellular responses to hypoxia include adaptive metabolic changes and a G 1 cell cycle arrest. Although transcriptional regulation of metabolic genes by the hypoxia-induced transcription factor (HIF-1) has been established, the mechanism for the hypoxia-induced G 1 arrest is not known. By using genetically defined primary wild-type murine embryo fibroblasts and those nullizygous for regulators of the G 1 /S checkpoint, we observed that the retinoblastoma protein is essential for the G 1 /S hypoxia-induced checkpoint, whereas p53 and p21 are not required. In addition, we found that the cyclin-dependent kinase inhibitor p27 is induced by hypoxia, thereby inhibiting CDK2 activity and forestalling S phase entry through retinoblastoma protein hypophosphorylation. Reduction or absence of p27 abrogated the hypoxia-induced G 1 checkpoint, suggesting that it is a key regulator of G 1 /S transition in hypoxic cells. Intriguingly, hypoxic induction of p27 appears to be transcriptional and through an HIF-1-independent region of its proximal promoter. This demonstration of the molecular mechanism of hypoxia-induced G 1 /S regulation provides insight into a fundamental response of mammalian cells to low oxygen tension.Cellular hypoxia is an environmental stress with important implications in developmental biology, normal physiology, and many pathological conditions, including cancer. Normal tissues display an oxygen gradient across a distance of 400 m from a blood supply; tumors often have disordered and diminished vascularization, and hypoxia occurs in tumor tissue that is Ͼ100 -200 m away from a functional blood supply (1-3). Cells may adapt to hypoxia in numerous ways, including a transition from oxidative phosphorylation to glycolysis and neovascularization. Many of these metabolic responses are mediated by the transcription factor HIF-1, 1 a heterodimer of a hypoxia-induced subunit HIF-1␣ and a constitutive subunit HIF-1␤ (ARNT), which transactivates genes encoding several glycolytic enzymes as well as the vascular endothelial growth factor gene (4 -7). Cells may also respond to hypoxia by diminishing their proliferative rates. Both invasive and noninvasive studies of a variety of normal tissues and tumors suggest that hypoxic cells may be viable but nonproliferating (8 -12). This low proliferative state may be related to the phenomenon of tumor dormancy, described in nonvascularized metastatic foci (13), and may also help explain why hypoxic tumors are relatively chemoresistant and radioresistant (14). Although some transformed cell lines undergo apoptosis in extreme hypoxia and an acidic environment, nontransformed hypoxic cells remain viable but arrest in G 1 (15)(16)(17).The best characterized molecular event necessary for the G 1 /S phase transition is phosphorylation of the retinoblastoma protein (RB) by specific cyclin-dependent kinase (CDK)-cyclin complexes (18). CDK activity can be inhibited by cyclin-dependent kinase inhibitors (CDKIs), such as p27 and p21, which then promote RB hypophosphorylation. Th...

A mathematical model of HiF-1-mediated response to hypoxia on the G1/S transition

“…As hypoxia becomes more intense, the G1 phase duration increases, in agreement with the literature [17]. For the lowest value of the pressure, we observe an asymptotic behavior, which finishes by the entrance into quiescence.…”

“…Notably, the upregulation of cyclins inhibitors, such as p21 and p27, are reported [17,22,20]. However, some authors showed that the action of HiF-1α on p27 is not so clear since the expression of p27 under hypoxia may be independent of HiF-1α [6,9].…”

Hypoxia mediates low cell-cycle activity and increases the proportion of long-term–reconstituting hematopoietic stem cells during in vitro culture

“…However, it has also been suggested that p27 Kip1 is a key hypoxic regulator of cell cycle arrest independently of HIF-1 [35]. In our study, all three CDKI genes analyzed were upregulated in LSK cells by hypoxia.…”

Hypoxia reduces adenoviral replication in cancer cells by downregulation of viral protein expression

“…5,26 Further, it has been previously demonstrated that hypoxia causes cell cycle growth arrest in G1 in nontransformed cells. [16][17][18] Our data, however, show that the reduced viral yield under hypoxic conditions was not due to cell cycle arrest of tumor cells. Although as anticipated, hypoxia was able to growth arrest normal HUVEC, hypoxia did not growth arrest the cancer cells.…”

“…[16][17][18] Although the exact mechanism is still debated, hypoxia generally reduces the activity of cyclin-dependent kinases, which leads to hypophosphorylation of the Rb protein and growth arrest in G1. [16][17][18] Moreover, the presence of cells in S-phase may be important to enable adenoviral replication. 19,20 To Figure 1 HIF-1a is expressed under hypoxic conditions.…”