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The discovery of the total separation of receptor-binding and signal-generating regions of the relaxin-like factor (RLF) has provided an opportunity to investigate the mechanism of transmembrane signaling. The receptor-binding residues of RLF are in the B chain of the two-chain molecule and extend from the midregion of the central helix to the tryptophan in position B27. The signal initiation site resides in two residues before the N-terminal cysteine in each chain. For optimal signaling the RLF requires fiveL-α-amino acids preceding cysteine A10, whereas the B chain requires only three. The nature of the side chains of these amino acids is not critical for the signaling function. Heuristic arguments lead us to suggest that the peptide bond is the signal-generating feature of RLF and possibly of other peptide hormones. [ABSTRACT FROM AUTHOR]

Insulin-degrading enzyme (IDE) is a ubiquitously expressed metalloproteinase responsible for the intracellular degradation of insulin. IDE also interacts with other members of the insulin superfamily, including relaxin, but no studies have been reported regarding the interaction of other relaxin-like peptides with IDE. In this study, we determined that relaxin, relaxin-3, and InsL3 all competitively inhibited the degradation of insulin by IDE to different degrees, and all inhibited covalent cross-linking of insulin to IDE. Each of the peptides was degraded by IDE to various degrees (insulin > relaxin > InsL3 = relaxin-3). In summary, relaxin, InsL3, and relaxin-3 all bound to IDE, competed for the binding and degradation of insulin, and were all substrates for the proteolytic activity of IDE. Therefore, it is possible that in addition to insulin, IDE may be important for the cellular proteolysis of relaxin, InsL3, and relaxin-3. [ABSTRACT FROM AUTHOR]

Insulin-like peptide 3 (INSL3) is a hormone produced by fetal and adult Leydig cells of the mammalian testis. During embryonic life INSL3 is required for testicular descent, whereas in adults it is involved in bone metabolism and male germ cell survival. Despite these important roles, the molecular mechanisms regulating INSL3 expression remain poorly understood. So far, two transcription factors have been implicated in INSL3 transcription: the nuclear receptors SF1 and NUR77. Circumstantial evidence also points to a role for androgens. Using transient transfections in MA-10 Leydig cells, we found that testosterone regulates in a time- and dose-dependent manner the human INSL3 promoter. The INSL3 promoter, however, does not contain a classical androgen-responsive element. Testosterone responsiveness was found to be mediated through an element located in the proximal INSL3 promoter, which also contains a NUR77-SF1-binding site. Furthermore, we found that posttranslational modifications, such as phosphorylation and acetylation, modulate transcription factor activity and therefore also contribute to INSL3 promoter activity in Leydig cells. All together, these data provide new insights into the molecular mechanisms regulating INSL3 expression in the mammalian testis. [ABSTRACT FROM AUTHOR]