Hypoxia. Cross talk between oxygen sensing and the cell cycle machinery

Abstract

A fundamental physiological property of mammalian cells is the regulation of proliferation according to O(2) availability. Progression through the cell cycle is inhibited under hypoxic conditions in many, but not all, cell types, and this G1 arrest is dependent on hypoxia-inducible factor (HIF) 1α. Components of the hexameric MCM helicase, which binds to replication origins before the onset of DNA synthesis, are present in large excess in mammalian cells relative to origins, suggesting that they may have additional functions. Screens for HIF-1α interacting proteins revealed that MCM7 binds to the amino-terminal PER-SIM-ARNT (PAS) domain of HIF-1α and stimulates prolyl hydroxylation-dependent ubiquitination and degradation of HIF-1α, whereas MCM3 binds to the carboxyl terminus of HIF-1α and enhances asparaginyl hydroxylation-dependent inhibition of HIF-1α transactivation domain function. Thus MCM proteins inhibit HIF activity via two distinct O(2)-dependent mechanisms. Under prolonged hypoxic conditions, MCM mRNA expression is inhibited in a HIF-1α-dependent manner. Thus HIF and MCM proteins act in a mutually antagonistic manner, providing a novel molecular mechanism for homeostatic regulation of cell proliferation based on the relative levels of these proteins.

Fig. 1.

Cross talk between MCM proteins and hypoxia-inducible factor (HIF)-1α. Under conditions of normoxia and the presence of proliferative signals, MCM3 and MCM7 protein levels are high and HIF-1α is subjected to hydroxylation of prolyl (Pro-OH) and asparaginyl (Asn-OH) residues. MCM7 binds to the PER-SIM-ARNT (PAS) domain of HIF-1α and also interacts with both VHL and Elongin C. MCM7 stabilizes their interaction, thereby promoting the ubiquitination and degradation of prolyl-hydroxylated HIF-1α. MCM3 binds to the inhibitory domain (ID) and inhibits HIF-1α transactivation domain (TAD) function in an asparaginyl hydroxylation-dependent manner. Under conditions of acute hypoxia, prolyl and asparaginyl hydroxylation of HIF-1α are inhibited, which inactivates the mechanisms by which MCMs block HIF-1 activity. Under conditions of chronic hypoxia, MCM3 and MCM7 mRNA and protein expression are inhibited in a HIF-1-dependent manner, further reducing inhibition of HIF-1 activity by MCMs.