Regulation of the connexin 43 promoter in the brook trout testis: role of the thyroid hormones and cAMP.

Abstract : Gap junctions are critical for spermatogenesis. They are composed of integral proteins, the connexins. In mammals, a loss of Cx43 expression results in the inhibition of spermatogenesis. We have shown that Cx43 is expressed in the Sertoli cells of rainbow trout and that cAMP and triiodothyronine (T(3)) regulate testicular Cx43 expression in brook trout testis. The objective of this study was to determine if cAMP and T(3) act at the level of the cx43 promoter to regulate its expression. A 607 bp 5' flanking sequence of the cx43 promoter was obtained by Genome Walking. A TATA box was predicted to be located between positions -36 and -30 relative to the transcriptional initiation site. 5'-Rapid amplification of cDNA ends indicated a single transcriptional start site. Single C/EBP (-164 to -156) and tr-beta (-112 to -107) response elements were identified and electrophoretic mobility shift assays indicated the presence of competitive protein binding sites at each region. Immortalized rainbow trout gonadal cell line (RTG-2) which express cx43 and tr-beta transcripts were transfected with a vector containing the Cx43 promoter inserted into a luciferase expression vector. Transactivation of the reporter genes was stimulated by either cAMP or T(3). Sequential deletion and point mutations in either the C/EBP or tr-beta response element indicated that T(3) but not cAMP directly induced luciferase transactivation of the luciferase gene by acting on different sites of the Cx43 promoter. Together, these data indicate that T(3) stimulates cx43 expression via direct regulation of gene transcription.