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The hypoxia-inducible transcription factor (HIF) controls (in an oxygen-dependent manner) the expression of a large number of genes whose products are involved in the response of cells to hypoxia. HIF is an αβ dimer that binds to hypoxia response elements (HREs) in its target genes. Human HIF-α has three isoforms, HIF-1α, HIF-2α and HIF-3α, of which the roles of HIF-3α are largely unknown, although it is usually regarded as a negative regulator of HIF-1α and HIF-2α. The human HIF-3α locus is subject to extensive alternative splicing, leading to at least seven variants. We analyzed here the effects of the long variants and the short variant HIF-3α4 on the hypoxia response. All these variants were found to interact with HIF-β, HIF-1α and HIF-2α. The long HIF-3α variants were localized in the nucleus in hypoxia, while HIF-3α4 was cytoplasmic. Interaction of the HIF-3α variants with HIF-1α inhibited the nuclear translocation of both. None of the long HIF-3α variants was capable of efficient induction of an HRE reporter in overexpression experiments, but instead inhibited the transcriptional activation of the reporter by HIF-1 and HIF-2. Unexpectedly, siRNA knock-down of the endogenous HIF-3α variants led to downregulation of certain HIF target genes, while overexpression of individual long HIF-3α variants upregulated certain HIF target genes in a variant and target gene-specific manner under conditions in which HIF-β was not a limiting factor. These data indicate that the HIF-3α variants may have more versatile and specific roles in the regulation of the hypoxia response than previously anticipated.

CORTISONE given to adult animals decreases the amount of alkali-soluble1 and neutral salt-soluble2,3 collagen in the skin. In the urinary excretion of hydroxyproline, however, no effect has been found4,5. In the chick embryo, a marked increase in the content of free hydroxyproline occurs in the tissues 6–10 days after a single injection of 1 mg of cortisone acetate6,7. However, when cortisone is given to the pregnant rat, the contents of free and bound hydroxyproline in the tissues of the new-born young are decreased8. This discrepancy has been attributed to the fact that 1 mg of cortisone greatly reduces the weight of the treated chick embryos and hence the absolute amounts of free and bound hydroxyproline in the cortisone-treated embryos are lower than in the controls, although their content per unit of dry tissue is higher7. The present work indicates that cortisone also decreases the content of free hydroxyproline per unit of dry tissue, but when cortisone is given as a single injection, a marked increase later occurs in the content of free hydroxyproline.

DURING recent years increasing attention has been paid to free and peptide-bound hydroxyproline in order to explain their role in the metabolism of collagen1–10. Since chick embryos contain relatively large amounts of free hydroxyproline at least1,11–13, it seemed worth while to follow the contents of various hydroxyproline containing fractions during the development of the chick embryo.