Hypoxia-Inducible Factor 1α Is Essential for Cell Cycle Arrest during Hypoxia

Abstract

A classical cellular response to hypoxia is a cessation of growth. Hypoxia-induced growth arrest differs in different cell types but is likely an essential aspect of the response to wounding and injury. An important component of the hypoxic response is the activation of the hypoxia-inducible factor 1 (HIF-1) transcription factor. Although this transcription factor is essential for adaptation to low oxygen levels, the mechanisms through which it influences cell cycle arrest, including the degree to which it cooperates with the tumor suppressor protein p53, remain poorly understood. To determine broadly relevant aspects of HIF-1 function in primary cell growth arrest, we examined two different primary differentiated cell types which contained a deletable allele of the oxygen-sensitive component of HIF-1, the HIF-1␣ gene product. The two cell types were murine embryonic fibroblasts and splenic B lymphocytes; to determine how the function of HIF-1␣ influenced p53, we also created double-knockout (HIF-1␣ null, p53 null) strains and cells. In both cell types, loss of HIF-1␣ abolished hypoxia-induced growth arrest and did this in a p53-independent fashion. Surprisingly, in all cases, cells lacking both p53 and HIF-1␣ genes have completely lost the ability to alter the cell cycle in response to hypoxia. In addition, we have found that the loss of HIF-1␣ causes an increased progression into S phase during hypoxia, rather than a growth arrest. We show that hypoxia causes a HIF-1␣-dependent increase in the expression of the cyclin-dependent kinase inhibitors p21 and p27; we also find that hypophosphorylation of retinoblastoma protein in hypoxia is HIF-1␣ dependent. These data demonstrate that the transcription factor HIF-1 is a major regulator of cell cycle arrest in primary cells during hypoxia.Mammalian cells have evolved to utilize molecular oxygen for energy production. Cells can respond differently to wide ranges of oxygen through alterations in both their metabolic states and growth rates. In recent years, several lines of evidence have indicated that hypoxia can alter cell proliferation in two distinct ways: via programmed cell death and through growth arrest. In transformed cells, hypoxia can provoke apoptosis via the p53 pathway; ultimately, this can represent a potent mechanism for the selection of p53 mutants in tumor cell populations (30,34). Nontransformed hypoxic cells, on the other hand, can undergo cell cycle arrest at the G 1 /S interface without any alteration in their long-term viability (10).It has been proposed that hypoxically induced cell cycle arrest is caused by inactivation of enzymes responsible for nucleotide synthesis, ultimately inhibiting DNA replication (19, 39). However, inhibition of nucleotide synthesis occurs only under severe hypoxia (0.01% oxygen) or anoxia, but not under moderate hypoxia (0.1 to ϳ1% oxygen) (10).In the moderately hypoxic microenvironment, various biological reactions show significant changes relative to normoxia. Numerous studies on moderate hypoxia have indicated tha...

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“…However, we excluded p53 involvement in the p21 activation in HaCaT cells due to transcriptionally inactive form of p53 in this cell line. This is consistent with and supported by previous observation that hypoxiainduced p53 also has a nonessential role in facilitating hypoxic activation of p21 promoter (Goda et al, 2003;Hammond and Giaccia, 2005). MCGA3 treatment-resultant decrease in c-Myc, which is known to MCGA3 activates p21 and enhances UVA-mediated apoptosis M-H Kweon et al repress Sp1-dependent p21 transcription and to be inhibited by iron chelation (Fan et al, 2001), raised an interesting question whether c-Myc downregulation contributes to p21 activation.…”

“…Therefore, we initially hypothesized that the ROSscavenging and iron-chelating potential of MCGA3 may play a role in the p21 induction by providing a hypoxic condition. Recent studies (Carmeliet et al, 1998;Abeysinghe et al, 2001;Goda et al, 2003) have shown that HIF-1a-involved p21 transcriptional activation without p53 transcriptional function. Our p21-promoter assays clearly reflect the p53-independent, Sp1-dependent transcriptional activation of p21 by MCGA3.…”

“…MCGA3 upregulates p21 as well as overcomes UVA-mediated early decrease of p21 Hypoxia not only induces angiogenesis and promotes cell growth and survival in cancer cells but also leads to growth arrest and apoptosis (Carmeliet et al, 1998;Goda et al, 2003;Greijer and van der Wall, 2004). Recently, hypoxia-mediated cell cycle arrest and induction of apoptosis has been shown to be regulated by p21 activation (Bossenmeyer-Pourie et al, 2002).…”

Carrot and stick: HIF-α engages c-Myc in hypoxic adaptation

“…33,34 By contrast, hypoxia upregulates CDKN1A gene in an HIF-1a-dependent way. 35,36 Furthermore, HIF-1a expression is sufficient to stimulate CDKN1A expression in normoxia, leading to cell-cycle arrest. 26,37 Of particular interest is that such gene upregulation, in contrast with the HIF-a activation cascade (Figure 1b), is independent of HIF-1a DNA-binding and transactivation domains.…”

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

“…[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.…”