Analysis of the oligosaccharides on rat androgen-binding protein using serial lectin chromatography

Receptors for androgen-binding proteins: internalization and intracellular signalling

In plasma, most steroid hormones are bound and transported by the specific binding protein, testosterone-estradiol-binding globulin (TeBG). For years, it was believed that the only function of this protein was to regulate the concentration of free steroids in plasma. However, a number of reports have provided evidence for the presence of specific TeBG receptors on plasma membranes. Furthermore, the interaction of TeBG with its receptor was shown to be inhibited when steroids are bound to TeBG, suggesting that TeBG is an allosteric protein. The purpose of this manuscript is to review the evidence that androgen-binding proteins bind to membrane receptors, and, in some cells, this binding stimulates cAMP accumulation, and transfer TeBG/ABP into tissue as a consequence of receptor mediated endocytosis. Recent studies from our laboratories have demonstrated binding and uptake of TeBG by MCF-7 breast cancer cells. The interaction of unligated rabbit TeBG with membranes from MCF-7 cells resulted in a time and concentration-dependent increase in adenylate cyclase activity. The TeBG alone also had a reproducible effect on intact cells by increasing cAMP accumulation by 30-35%. The addition of DHT to cells, after TeBG has been allowed to bind, resulted in increases in cAMP of greater than 4-fold. This effect was not blocked by antiandrogens. These data support the hypothesis that extracellular SHBG is a regulator of cellular function through a membrane receptor that is coupled to adenylate cyclase.

Structure, function, and regulation of androgen-binding protein/sex hormone-binding globulin

Despite over 20 years of research, the functions of ABP and SHBG remain elusive. The major reason for this lack of knowledge has been the unavailability of natural mutants with clinical defects for study. There is strong evidence that these binding proteins do act to modulate the gene regulatory actions of nuclear sex steroid receptors by controlling the availability of androgens and estrogens. In plasma, SHBG controls the metabolic clearance rate of sex steroids. In addition there is strong evidence that they have a much broader function. The identification of plasma membrane receptors in target tissues and the finding of homologous domains in several developmental proteins support other functions. Moreover, other experiments suggest the proteins may actually be hormones or growth factors. These findings are not compatible with a model that has the proteins only regulating free steroid hormone levels. Obviously, much more experimentation will be necessary to reveal the functions of ABP and SHBG. The recent discoveries have offered several clues to their functions and open new routes for study. These experiments, coupled with newly developed techniques, such as gene knockout by homologous recombination, make one optimistic that the functions of these unique proteins will be deciphered in the near future.

Analysis of the oligosaccharides on androgen-binding proteins: implications concerning their role in structure/function relationships

Rat androgen-binding protein (rABP), human testosterone-binding globulin (hTeBG) and rabbit (rb) TeBG are heterodimeric proteins. The source of the heterogeneity arises from the differential glycosylation of a common protein core. This glycosylation results in a heavy subunit (more glycosylation) and a light subunit (less glycosylation). Glycosylation is one factor responsible for multiple charged species seen when rABP, hTeBG, and rbTeBG are analyzed by two-dimensional gel electrophoresis. Enzymatic digestion with the endoglycosidase, peptide: N-glycosidase F indicated that all three proteins have asparagine (Asn)-linked oligosaccharides as their major glycan substituent. Treatment with exoglycosidases provided evidence for terminal sialic acid, galactose and mannose and N-acetylglucosamine residues. About 16-22% of the mass of the heavy subunit and about 8-14% of the mass of the light subunit is contributed by carbohydrate. Serial lectin chromatography indicated that rABP is glycosylated differently from hTeBG and rbTeBG. About 40% of the rABP contains tri and tetraantennary complex oligosaccharides, while only about 20% of the hTeBG and TeBG from pregnant rabbits contains these types of glycans. About 9% of the TeBG from male rabbits bears these types of oligosaccharides. All of the biantennary complex oligosaccharides on rABP are fucosylated on the chitobiose core, but only 8% of those on hTeBG and none of those on rbTeBG are fucosylated in this manner. All three proteins are glycosylated at more than one site. The data indicate that the proteins may have more than one type of oligosaccharide on them. It is likely that differences in glycosylation are responsible for different physiological roles of the proteins.