Fenoxycarb modulates ecdysone receptor B1 and programmed cell death of the anterior silk gland of silkworm Bombyx mori

Fenoxycarb, O-ethyl N-(2-(4-phenoxyphenoxy)-ethyl) carbamate has been shown to be one of the most potent juvenile hormone analogues against a variety of insect species. In the present study, topical application of fenoxycarb to fifth-instar larvae of the silkworm Bombyx mori (Lepidoptera: Bombycidae) was performed immediately after the fourth ecdysis (on day 0), day 3 and day 6 of the instar and then its effects on the anterior silk glands (ASG) and ecdysone receptor B1 (EcR-B1) protein were investigated during larval pupal development. Fenoxycarb application increased the instar length and prevented metamorphic events, depending on the application time. The ASGs of B. mori undergo programmed cell death during the larval-pupal metamorphosis and an insect steroid, 20-hydroxyecdysone (20E), triggers this cell death. The exact mechanism by which 20E and juvenile hormone regulates programmed cell death in insect tissues is poorly understood. To gain insights into how juvenile hormone regulates metamorphic events like programmed cell death in the anterior silk glands, we analyzed the progression of programmed cell death with morphological observations and biochemical experiments like acid phosphatase activity and DNA electrophoresis. Then we examined the EcR-B1 protein levels and their relationships with programmed cell death. Our results indicated that fenoxycarb modulates programmed cell death of the anterior silk glands and EcR-B1 protein level, depending on the application time. Fenoxycarb may exhibit its effects in at least two different ways: (i) acting on prothoracic gland secretory activity; and/or (ii) regulation of EcR-B1 expression in the anterior silk glands for programmed cell death process.