Studies on the transcriptional regulation of human sex hormone-binding globulin in testicular germ cells : mechanistic and evolutionary insights

In most mammals, the major site of sex hormone-binding globulin (SHBG) synthesis is the liver wherefrom it is secreted into the bloodstream and is the primary determinant of sex steroid access to target tissues. The minor site of SHBG synthesis is the testis and in lower mammals testicular SHBG has long been known to be synthesized and secreted by Sertoli cells. In contrast, human SHBG is expressed in testicular germ cells from an upstream alternative promoter (altP-SHBG).Transcripts arising from this region comprise an alternative first exon (1A) and the resultant protein is confined to the acrosomal compartment of the mature spermatozoa. In Chapters 3 and 4 I dissected the regulatory components of the alternative SHBG promoter and identified motifs that are required for optimal transcriptional activity from this region. Transcriptional activity is driven by two CACCC elements that appear to be functionally redundant. The transcription factor KLF4 interacts with promoter the region spanning these elements in vivo. Knockdown of Klf4 results in decreased altP-SHBG activity, while Klf4 overexpression relieves the effects of knockdown. Examination of KLF4 in testes of transgenic mice harbouring human SHBG transgenes reveals that Klf4 and SHBG have related expression patterns during the seminiferous cycle. The alternative SHBG promoter responds to dbcAMP treatment and this activation is relieved by cotreatment with the protein kinase A inhibitor, H89. Overexpression of CREM isoforms also upregulates altP-SHBG transcriptional activity. Examination testicular SHBG transcripts from members of each major primate family revealed that transcripts containing exon 1A are unique to the catarrhini phylogenetic group. In contrast SHBG transcripts in lemur testes contained exon 1, while no evidence for SHBG expression could be detected in marmoset monkey testes. In general, the exonic identity of primate testicular SHBG transcripts could be predicted based on the structure of their gene’s 5’ regulatory region. This work provides insights into how human SHBG is expressed in a stage-dependent manner throughout the seminiferous cycle and how molecular evolution of higher primate SHBG genes has resulted in distinct changes in how it is expressed in their testes.Medicine, Faculty ofObstetrics and Gynaecology, Department ofGraduat