ovarian tumor (OTU) domain-containing ubiquitin aldehyde-binding protein 1; Otub1 +/− , heterozygous Otub1 deletion; Otub1 fl /fl , mice with floxed Otub1 exons 2 and 3; pcOtub1 Wt/del , potential of conditional Otub1 deletion; PEI, polyethylenimine; scRNA, single-cell RNA (sequencing); SSC-A, side scatter-area; SSC-H, side scatter-height; TEM, transmission electron microscopy; Tx, tamoxifen; Ub, ubiquitin proteins; UBC-Cre ERT2 , Cre ERT2 transgene under the control of the ubiquitin C promoter; wbOtub1 del/del , induced whole-body Otub1 deletion; Wt, wild-type.

Significance: Limited oxygen availability (hypoxia) commonly occurs in a range of physiological and pathophysiological conditions, including embryonic development, physical exercise, inflammation, and ischemia. It is thus vital for cells and tissues to monitor their local oxygen availability to be able to adjust in case the oxygen supply is decreased. The cellular oxygen sensor factor inhibiting hypoxiainducible factor (FIH) is the only known asparagine hydroxylase with hypoxia sensitivity. FIH uniquely combines oxygen and peroxide sensitivity, serving as an oxygen and oxidant sensor. Recent Advances: FIH was first discovered in the hypoxia-inducible factor (HIF) pathway as a modulator of HIF transactivation activity. Several other FIH substrates have now been identified outside the HIF pathway. Moreover, FIH enzymatic activity is highly promiscuous and not limited to asparagine hydroxylation. This includes the FIH-mediated catalysis of an oxygen-dependent stable (likely covalent) bond formation between FIH and selected substrate proteins (called oxomers [oxygen-dependent stable protein oligomers]). Critical Issues: The (patho-)physiological function of FIH is only beginning to be understood and appears to be complex. Selective pharmacologic inhibition of FIH over other oxygen sensors is possible, opening new avenues for therapeutic targeting of hypoxia-associated diseases, increasing the interest in its (patho-)physiological relevance. Future Directions: The contribution of FIH enzymatic activity to disease development and progression should be analyzed in more detail, including the assessment of underlying molecular mechanisms and relevant FIH substrate proteins. Also, the molecular mechanism(s) involved in the physiological functions of FIH remain(s) to be determined. Furthermore, the therapeutic potential of recently developed FIH-selective pharmacologic inhibitors will need detailed assessment. Antioxid.