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Summary: Low-oxygen conditions (hypoxia) have been associated primarily with cell-cycle arrest in dividing cells. Macrophages are typically quiescent in G0 but can proliferate in response to tissue signals. Here we show that hypoxia (1% oxygen tension) results in reversible entry into the cell cycle in macrophages. Cell cycle progression is largely limited to G0-G1/S phase transition with little progression to G2/M. This cell cycle transitioning is triggered by an HIF2α-directed transcriptional program. The response is accompanied by increased expression of cell-cycle-associated proteins, including CDK1, which is known to phosphorylate SAMHD1 at T592 and thereby regulate antiviral activity. Prolyl hydroxylase (PHD) inhibitors are able to recapitulate HIF2α-dependent cell cycle entry in macrophages. Finally, tumor-associated macrophages (TAMs) in lung cancers exhibit transcriptomic profiles representing responses to low oxygen and cell cycle progression at the single-cell level. These findings have implications for inflammation and tumor progression/metastasis where low-oxygen environments are common.

Proteasome-mediated degradation occurs with proteins principally modified with lysine-48 polyubiquitin chains. Whether the proteasome also can bind atypical ubiquitin chains, including those linked by lysine-11, has not been well established. This is critically important, as lysine-11 polyubiquitination has been implicated in both proteasome-mediated degradation and non-degradative outcomes. Here we demonstrate that pure homotypic lysine-11-linked chains do not bind strongly to the mammalian proteasome. By contrast, heterotypic polyubiquitin chains, containing lysine-11 and lysine-48 linkages, not only bind to the proteasome but also stimulate the proteasomal degradation of the cell-cycle regulator cyclin B1. Thus, while heterotypic lysine-11-linked chains facilitate proteasomal degradation, homotypic lysine-11 linkages adopt conformations that prevent association with the proteasome. Our data demonstrate the capacity of the proteasome to bind ubiquitin chains of distinct topology, with implications for the recognition and diverse biological functions of mixed ubiquitin chains.