Regulatory networks that control hypoxia response

Hypoxia and the hypoxia-inducible factor (HIF-1)
During development, homeostasis, or disease states, cellular oxygen levels are often insufficient to meet physiological demands. This condition is termed hypoxia. Cells are poised to sense and respond to small changes in oxygen availability. Most of the critical transcriptional responses to hypoxia are controlled by hypoxia inducible factors (HIF). Transcriptional targets of human HIF-1 include genes that increase anaerobic energy production, protect cells from stress, regulate cell survival, and promote angiogenesis. HIF is essential for oxygen-dependent regulation of gene expression in mammals and in simple eukaryotes such the nematode C. elegans.

C. elegans HIF-1: a powerful genetic model system
We identified the C. elegans hif-1 gene (which encodes the HIF-1 protein). Mutants that lack a functional hif-1 gene cannot adapt to hypoxia. Jiang, Guo, and Powell-Coffman (2001) PNAS 98, 7916-21. Concurrently, the Ratcliffe lab demonstrated that the pathway for oxygen-dependent degradation of HIF-1 is evolutionarily conserved. When oxygen levels are high, HIF-1 is hydroxylated by EGL-9. HIF-1 can then bind VHL-1, which targets HIF-1 for degradation. (Epstein et al. 2001 Cell 107, 43)

One of the advantages of studying HIF-1 in C. elegans is that mutants lacking hif-1, vhl-1, or egl-9 are viable in normal culture conditions. Using microarray studies, we determined that the majority of hypoxia-induced changes in gene expression required hif-1 function in C. elegans. We were able to identify genes that were regulated by HIF-1, as well as genes that were induced by hypoxia independent via HIF-1-independent pathways. This molecular description of HIF-1 function has provided a foundation for genetic screens to identify HIF-1 regulators.Shen, Nettleton, Jiang, Kim, and Powell-Coffman (2005) J Biol Chem. 80:20580

EGL-9 regulates HIF-1 via two pathways. When oxygen levels are high, the EGL-9 enzyme hydroxylates HIF-1. This covalent modification allows VHL-1 to target HIF-1 for degradation. Our data show that EGL-9 inhibits HIF-1 via two pathways: In addition to its well-described role in regulating HIF-1 stability, EGL-9 also functions via a separate, VHL-1-independent pathway to inhibit HIF-1 activity.

Identification of novel regulators of HIF-1
One of the principal strengths of C. elegans is that it is amenable to large-scale genetic screens for mutations that disrupt a biological process of interest. We have designed genetic screens to identify HIF-1 regulators (rhy genes = regulators of the hypoxia-inducible factor). RHY-1 is an integral membrane protein that functions in a negative feedback loop to inhibit HIF-1 function.
Shen, Shao, and Powell-Coffman (2006) Genetics 174 p 1205