A Dominant-Negative Isoform of Hypoxia-Inducible Factor-1(alpha) Specifically Expressed in Human Testis

Spermatogenesis in the seminiferous tubuli of the testis occurs under a high proliferation rate, suggesting considerable oxygen consumption. Because of the lack of blood vessels, the oxygen partial pressure in the lumen of these tubuli is very low. We previously identified a testis isoform of the hypoxia-inducible factor (HIF)-1(alpha) in the mouse, termed mHIF-1(alpha)I.1. Here, we demonstrate that expression of mHIF-1(alpha) I.1 increases during puberty, further demonstrating its gene induction in postmeiotic germ cells. Using 5ʹ-rapid amplification of cDNA ends, we identified a novel HIF-1(alpha) isoform in the human testis, called hHIF-1(alpha) Te. Like mHIF-1(alpha)I.1, hHIF-1(alpha)Te mRNA is derived from an alternative promoter-first exon combination, but with a different genomic organization and a different nucleotide sequence. Reverse transcription-polymerase chain reaction analysis confirmed that hHIF-1(alpha)Te is exclusively expressed in the testis. As determined by immunofluorescence of ejaculated sperm cells, HIF-1(alpha)protein is mainly localized in the postacrosomal head and in the midpiece of spermatozoa. Though overlapping with mitochondrial localization in human and mouse spermatozoa, neither hHIF-1(alpha)Te nor hHIF-(alpha)associated with mitochondria. In contrast with the ubiquitously expressed HIF-1(alpha)protein and the mouse testis-specific mHIF-1(alpha)I.1 isoform, the hHIF-1 (alpha) Te mRNA sequence predicts a protein with an N-terminal truncation of the DNA-binding domain. As shown by yeast two-hybrid assays, hHIF-1(alpha)Te still formed heterodimeric complexes with HIF-1(beta). However, hHIF1(alpha) Te was incapable of forming a DNA-binding HIF-1 complex. Overexpression of exogenous hHIF-1(alpha) Te resulted in the inhibition of the endogenous HIF-1 transcriptional activity, demonstrating that the testis-specific hHIF-1(alpha)Te isoform is a dominant-negative regulator of normal HIF-1 activity.