A Human Sex Hormone-binding Globulin Isoform Accumulates in the Acrosome during Spermatogenesis

Koji Seguchi,Kevin N Hogeveen,Francis Tekpetey,Geoffrey L Hammond,David M Selva

doi:10.1074/jbc.m205903200

Abstract

Human sex hormone-binding globulin (SHBG) binds estradiol and testosterone with high affinity. Plasma SHBG is produced by hepatocytes, but the human SHBG gene is also expressed in the testis. Little is known about SHBG gene expression in the human testis, but human SHBG transcripts accumulate in a spermatogenic stage-dependent manner in the testes of mice containing an 11-kb human SHBG transgene. We have now found that human SHBG transcripts containing an alternative exon 1 sequence are located specifically in the testicular germ cells of these transgenic mice, whereas murine SHBG transcripts are confined to Sertoli cells. In addition, we have detected immunoreactive human SHBG in the acrosome during all stages of spermiogenesis in mice containing an 11-kb human SHBG transgene. Western blots of germ cell extracts from these transgenic mice and from human sperm indicate that the immunoreactive human SHBG in the acrosome composes electrophoretic variants, which are 3-5 kDa smaller than the major electrophoretic isoforms of human SHBG in the blood. This apparent size difference is due in part to differences in glycosylation of plasma and acrosomal SHBG isoforms. The function of the human SHBG isoform in the acrosome is unknown, but it binds steroid ligands with high affinity. This is the first demonstration that human SHBG transcripts encode an SHBG isoform that remains within a cellular compartment.

Highlights

Mammalian genes encoding sex hormone-binding globulin (SHBG)1 contain at least two transcription units [1, 2]
When a human SHBG cDNA that recognizes exon 6 – 8 sequences was used as probe, human SHBG transcripts were only detected in RNA extracts of testicular cells from mice containing the 11-kb human SHBG transgene
Because the relative abundance of human SHBG transcripts in the germ cell-only preparation was similar to that in the mixed population of Sertoli cells and germ cells, this suggested that the SHBG transcripts in the testis of 11-kb human SHBG transgenic mice cannot be derived from Sertoli cells

Summary

Human SHBG in the Acrosome

The alternative exon 1 sequence associated with SHBG cDNAs from a human testis library [1]. These alternatively spliced human SHBG transcripts are confined to testicular germ cells and an immunoreactive human SHBG isoform accumulates in the acrosome of developing spermatids and immature sperm in the transgenic mice. We have obtained direct evidence that this acrosomal SHBG isoform binds steroids and is present in human sperm

EXPERIMENTAL PROCEDURES

RESULTS

Relative binding affinitya

DISCUSSION