HIF has Biff – Crosstalk between HIF1a and the family of bHLH/PAS proteins
Introduction
The Hypoxia-Inducible Factor 1α (HIF1α) is an oxygen regulated member of the basic Helix-Loop-Helix/PER-ARNT-SIM (bHLH/PAS) family of transcription factors that plays key roles in embryonic and adult oxygen homeostasis. HIF1α is essential for development of embryonic vasculature and thereafter maintains roles in angiogenesis throughout life, particularly during times of oxygen deficiency, when HIF1α activity co-ordinately adjusts cellular metabolism and erythropoiesis to aid adaption and survival. To achieve these functions, HIF1α dimerises with a second bHLH/PAS protein, ARNT (also known as HIF1β), to regulate hundreds of genes across a range of tissues. HIF1α represents a Class I bHLH/PAS factor, proteins characterized by being either ubiquitously expressed and signal regulated, or alternatively, constitutively active and tissue specific. Class I factors bind DNA upon dimerision with a Class II partner factor, one of ARNT (Aryl Hydrocarbon Receptor Nuclear Translocator), ARNT2 or ARNT-Like (ARNTL, aka BMAL). Other well studied Class I factors include the Aryl hydrocarbon Receptor (AhR, aka Dioxin Receptor), which is critical for toxin metabolism and differentiation of distinct subsets of T and B immune cells [1]; Single Minded 1 (SIM1), essential for specification of neuroendocrine secreting cells of the hypothalamus and control of appetite [2], [3]; Neural PAS factors (NPAS 1, 3 & 4), which function in neuron development and synaptic plasticity [4]; and CLOCK and NPAS2, which control circadian rhythms. Members of this family all share a conserved domain organisation. The amino-terminal bHLH domain is involved in DNA binding and serves as a primary dimerization interface. This is followed by a PAS domain consisting of two PAS repeats, PAS A and PAS B, that are important for secondary dimerization and in some cases, binding small organic ligands. The carboxy-terminal halves contain either transactivation or transrepression domains for target gene regulation. Cognate DNA sequences bound by the heterodimers are variations of the canonical E-box sequence, all containing a core NNCGTG [5], [6].
Class I bHLH/PAS factors have structural similarities and commonly heterodimerise with ARNT to bind similar DNA response elements, features that invite direct crosstalk between the proteins. Additionally, an increasing number of studies are showing that there are intersections between the pathways bHLH/PAS factors control in biological functions such as immune cell regulation, the circadian rhythm and cancer progression. This review focuses on the various bHLH/PAS family members and how their functions and pathways are involved in crosstalk with the activities and functions of HIF.
Section snippets
HIF1α and SIM2 crosstalk in cancer
Single-Minded 2 (SIM2) is a Class I bHLH/PAS factor that has been identified as misregulated in a number of cancers, particularly prostate, where it has been suggested as a potential biomarker due to correlation of high levels with poor patient survival [7], [8], [9]. HIF1α can play multiple positive and negative roles in solid cancers. It primarily aids tumourigenesis and progression via adaption to hypoxia, but conversely, can also be anti-tumourigenic by activating pro-apoptotic genes. In
HIF1α crosstalk with the circadian clock
BMAL1 (Brain and Muscle ARNT-Like, or simply ARNTL) is a Class II bHLH/PAS transcription factor that, together with its Class I partner factor CLOCK, forms one of the essential core components of the circadian clock that controls central and peripheral circadian rhythms. Other core proteins include the circadian repressors, PAS domain-containing PERIOD homologues 1, 2 and 3 (PER1 PER2, PER3), and CRYPTOCHROME 1 and 2 (CRY1 and CRY2) [5]. It has long been known that HIF1α can heterodimerise with
HIF and the NPAS transcription factors
Hypoxia plays multiple roles in nervous system development and function, via both HIF-dependent and independent pathways. Through HIF, hypoxia acts as a morphogen to organize normal developmental programs [23]. This is illustrated by conditional deletion of HIF1α in the CNS leading to hydrocephalus, failure of neuronal differential and increased neural apoptosis [24]. Not surprisingly, aberrant hypoxia can lead to disrupted development and pathological processes [23], [25]. For example, genetic
HIF and AHR function in immunomodulation
The Aryl Hydrocarbon Receptor (AHR) is a ligand dependent Class I bHLH/PAS factor important for regulating development and function of immune cells. In the absence of a ligand, latent AHR is located in the cytoplasm, bound to chaperone proteins. Upon binding of aromatic hydrocarbons, typified by xenobiotics such as dioxins and PCBs (polychlorinated biphenyls), dietary indoles or tryptophan metabolites, the AHR translocates to the nucleus where it dimerises with the Class II partner factor ARNT
Conclusions and future directions
While the exploration of interplay between HIF1α and the signalling pathways of other bHLH/PAS transcription factors is in its infancy, some startling examples illustrate the potential for discoveries in this area to expand our understanding of how environmental and physiological perturbations are sensed and integrated at the molecular level. It will be interesting to see whether there are more instances of either direct competition or synergistic collaboration between HIF and other bHLH/PAS
Acknowledgements
The authors’ work has been supported by grants from the Australian Research Council, National Health and Medical Research Council, and Cancer Council SA.
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