Demonstration of androgen-binding protein gene expression in primary neuronal and astrocyte cultures

Testicular androgen-binding protein (ABP) and liver (plasma) sex hormone-binding globulin (SHBG) are extracellular carrier proteins. They are encoded by the same gene and have the identical primary amino acid sequence. The ABP/SHBG gene is also expressed in rat brain; immunoreactive neurons are located throughout the male and female brain and immunoreactive fibers are present in the ventral hypothalamus. ABP/SHBG RNA transcripts are synthesized from two promoters, P(1) and P(A). P(1) regulates synthesis of the mRNA that encodes secreted ABP, whereas, a P(A) mRNA encodes a nonsecreted ABP-like protein without steroid binding properties. P(A) and P(1) transcripts are expressed in the brain. In this study, we demonstrated by Northern hybridization analysis that primary neuronal and astrocyte cultures express ABP mRNA. PCR analysis of ABP cDNA revealed that astroglia express transcripts originating from promoters P(1) and P(A), whereas, neuronal cDNAs contain primarily PA transcripts. These ABP cDNA-hybridizing products represent mRNAs that encode secreted ABP and the nonsecreted ABP-like protein. Furthermore, it was shown by immunohistochemistry that cultured astrocytes contain immunoreactive ABP. Western blots of astrocyte proteins yielded immunoreactive ABP migrating as 38,000-, 34,000-, and 24,060-M(r) proteins. The sizes of these proteins do not correspond to the protein-encoding properties of P(1) or P(A) ABP mRNA or any known alternatively processed ABP mRNAs, suggesting that they are derived by proteolytic processing of ABP or the ABP-like protein (46,000 Da). The function of the ABP gene products in brain is not obvious. We speculate that ABP may act as a carrier protein for an unknown ligand in axonal transport.