EVIDENCE FOR A DIAZEPAM-BINDING INHIBITOR (DBI) BENZODIAZEPINE RECEPTOR-LIKE MECHANISM IN ECDYSTEROIDOGENESIS BY THE INSECT PROTHORACIC GLAND

The diazepam-binding inhibitor (DBI) is a 10-kDa highly evolutionarily conserved multifunctional protein. In mammals, one of DBI's functions is in the activation of steroid hormone biosynthesis via binding to a specific outer mitochondrial membrane receptor (benzodiazepine recept or, BZD) and promoting cholesterol transport to the inner membrane. In this work, a multitiered approach was utilized to study the role of t his receptor-like activity in ecdysteroidogenesis by larval insect pro thoracic glands (PGs). First, both DBI protein and messenger RNA (mRNA ) levels were correlated with peak PG ecdysteroid production. In vitro ecdysteroid production was stimulated by the diazepam analogue FGIN 1 -27 and inhibited anti-DBI antibodies. The DBI protein was found distr ibuted throughout PG cells, including regions of dense mitochondria, s upposed subcellular sites of ecdysteroid synthesis. Finally, a potenti al mitochondrial BZD receptor in PG cells was demonstrated by photoaff inity labeling. These results suggest an important role for the insect DBI in the stimulation of steroidogenesis by prothoracic glands and i ndicate that a pathway for cholesterol mobilization leading to the pro duction of steroid hormones appears to be conserved between arthropods and mammals.