Toxoplasma gondii Activates Hypoxia-inducible Factor (HIF) by Stabilizing the HIF-1α Subunit via Type I Activin-like Receptor Kinase Receptor Signaling

Abstract

Toxoplasma gondii is an intracellular protozoan parasite that can cause devastating disease in fetuses and immune-compromised individuals. We previously reported that the ␣ subunit of the host cell transcription factor, hypoxia-inducible factor-1 (HIF-1), is up-regulated by infection and necessary for Toxoplasma growth. Under basal conditions, HIF-1␣ is constitutively expressed but rapidly targeted for proteasomal degradation after two proline residues are hydroxylated by a family of prolyl hydroxylases (PHDs). The PHDs are ␣-ketoglutarate-dependent dioxygenases that have low K m values for oxygen, making them important cellular oxygen sensors. Thus, when oxygen levels decrease, HIF-1␣ is not hydroxylated, and HIF-1 is activated. How Toxoplasma activates HIF-1 under normoxic conditions remains unknown. Here, we report that Toxoplasma infection increases HIF-1␣ stability by preventing HIF-1␣ prolyl hydroxylation. Infection significantly decreases PHD2 abundance, which is the key prolyl hydroxylase for regulating HIF-1␣. The effects of Toxoplasma on HIF-1␣ abundance and prolyl hydroxylase activity require activin-like receptor kinase signaling. Finally, parasite growth is severely diminished when signaling from this family of receptors is inhibited. Together, these data indicate that PHD2 is a key host cell factor for T. gondii growth and represent a novel mechanism by which a microbial pathogen subverts host cell signaling and transcription to establish its replicative niche.Toxoplasma gondii is an obligate intracellular protozoan parasite that can cause devastating disease in immune-compromised people and fetuses (1, 2). The ability for the parasite to cause disease is directly linked to its replication inside a parasitophorous vacuole within its host cell. From this vacuole, parasites scavenge nutrients from its host cell while causing reorganization of host organelles and cytoskeletal elements, preventing host cell apoptosis, and altering host gene expression (3-5). Therefore, Toxoplasma has developed mechanisms to manipulate host cell processes that permit it to grow. Identifying the parasite factors that cause these changes and the specific host pathways modulated by these factors is critical for developing new drugs to treat and prevent Toxoplasma infections and disease.Previously, our laboratory demonstrated that Toxoplasma activates the host cell transcription factor hypoxia-inducible factor-1 (HIF-1) 3 (6). HIF-1 is activated by a parasite-derived, secreted factor independent of direct contact between the parasite and host cell. This is in contrast to other Toxoplasma factors that control host cell signaling by virtue of their secretion into the host cell cytoplasm (7-10). Significantly, HIF-1 is important for parasite growth at both normoxic and physiological O 2 levels but dispensable for host cell replication at either O 2 levels (6).HIF-1 is a heterodimer composed of ␣ and ␤ subunits and is the master regulator of the cellular response to decreased O 2 availability (11). Although both subunits are co...

Toxoplasma gondii Development of Its Replicative Niche: in Its Host Cell and Beyond

“…Prolyl-hydroxylated HIF-1␣ is then recognized by and ubiquitylated by the Von Hippel-Lindau (VHL) ubiquitin ligase, which targets HIF-1␣ to the proteasome. Toxoplasma infection stabilizes HIF-1␣ by blocking its prolyl hydroxylation, and this correlates to a decreased abundance of PHD2, which is the PHD most critical for controlling HIF-1␣ protein levels (49). The importance of HIF-1 for parasite growth was demonstrated by the finding that parasite growth is reduced in HIF-1␣-deficient cells (48).…”

Inverse Correlation between IL-10 and HIF-1α in Macrophages Infected with Histoplasma capsulatum

“…Multiple pathogens are known to induce the transcription or stabilization of HIF-1α protein even in the absence of hypoxia (25, 18, 11, 66, 67, 19, 16). Accordingly, H. capsulatum -infected Mϕ promoted expression of HIF-1α protein as demonstrated by western blot.…”

Oxygen sensing by protozoans: how they catch their breath

“…Surprisingly, this was not simply a consequence of establishing a hypoxic microenvironment due to parasite oxygen consumption. Rather, Toxoplasma specifically activates HIF-1 by a mechanism that involves activin receptor signaling and inactivation of host PHD2 expression and activity [40]. Most significantly, parasite activation of HIF-1 is required for parasite growth under physiological O 2 tensions and does so, in part, by increasing host hexokinase-2 expression (unpublished results).…”