Expression of the mammary gland-specific tammar wallaby early lactation protein gene is maintained in vitro in the absence of prolactin

Abstract Marsupial ELP ( early lactation protein ) and its eutherian orthologue, CTI ( colostrum trypsin inhibitor ) are expressed in the mammary gland only for the first 100 days postpartum (Phase 2A) in the tammar wallaby and during the bovine and canine colostrogenesis period 24–36 h postpartum respectively. The factors which regulate temporal ELP and CTI expression are unknown. A tammar mammary gland explant culture model was used to investigate ELP gene regulation during pregnancy and early- and mid-lactation (Phase 1, 2A and 2B respectively). Tammar ELP expression could only be manipulated in explants in vitro if the gene was already expressed in vivo . ELP expression was maximal in Phase 1 explants treated with lactogenic hormones (insulin, hydrocortisone and prolactin), but unlike LGB ( β-lactoglobulin ), ELP expression was maintained in insulin or insulin and hydrocortisone over a 12-day culture period. In contrast, ELP was down-regulated when cultured without hormones. ELP could not be induced in explants cultured from mid-lactation which suggested that transcriptional repressors may prevent ELP expression during this period.