Hypoxia-controlled sterol and fatty acid synthesis with immune-modulatory consequences in myeloid cells

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Hypoxia evokes adaptive cell responses that are transmitted via canonical and non-canonical signals. Classical responses towards decreased oxygen availability use hypoxia-inducible factors (HIF) as transcriptional activators. There are several non-canonical transducing systems that comprise among others direct inhibition of oxygen consuming reactions. We postulate that this also applies to cholesterol metabolism. Pilot data using a SLAM-Seq (thiol-linked alkylation for the metabolic sequencing of RNA) approach in THP-1 monocytes indicates de novo transcription of several genes involved in cholesterol synthesis. We assume that this is HIF-independent and suggest direct inhibition of lanosterol demethylases, which are oxygen-dependent enzymes, belonging to the cytochrome P450 family. As a consequence we propose altered cholesterol metabolites, with cholesterol itself being decreased, while sterol intermediates such as lanosterol and dihydrolanosterol accumulate. As a molecular explanation we envision activation of SREBP (sterol regulatory element-binding protein) being responsible for global transcriptional changes. As we assume this to be a universal regulatory phenomenon we would like to understand how changes in cholesterol metabolites affect biological functions of monocytes and macrophages. We consider these cells being extremes between terminally differentiated vs. highly proliferating cells with notion that the flux through the cholesterol pathway may determine cellular sensitivity. Activation of SREBP may either alter cholesterol metabolites that in turn elicit a type I interferon response or SREBP may directly activate interferon-stimulated genes (ISG). Either way, we propose that this culminates in increased levels of interferon production. As a functional readout we like to determine whether increased interferon production enhances the anti-tumor function of macrophages. This assumption is based on the notion that our SLAM-Seq approach already indicated a hypoxia-driven type I interferon gene signature. In addition, we propose that hypoxia also alters the level of unsaturated fatty acids, a response also being SERBP driven. For these experiments we focus on SCD (sterol-CoA-desaturase) and FADS2 (fatty acid desaturase 2) as they are highly hypoxia inducible. Details concerning their expression, regulation, and function towards the viability and biology of monocytes/macrophages will be explored. We aim at understanding molecular details how hypoxia regulates cholesterol metabolism and how these alterations affect the biology of myeloid cells.