DEVELOPMENTAL REGULATION OF THE OVINE BETA-LACTOGLOBULIN HUMAN SERUM-ALBUMIN TRANSGENE IS DISTINCT FROM THAT OF THE BETA-LACTOGLOBULIN AND THE ENDOGENOUS BETA-CASEIN GENES IN THE MAMMARY-GLAND OF TRANSGENIC MICE

We compared the developmental pattern of expression of the sheep beta- lactoglobulin (BLG), the chimeric BLG/human serum albumin (HSA), and t he endogenous murine beta-casein genes in the mammary gland of virgin, pregnant and lactating transgenic mice, both at the RNA (expression) and protein (synthesis and secretion) levels. The BLG and casein genes were expressed at very low levels in virgin animals and during early stages of pregnancy. The increase in the expression of these genes sta rted at the second half of pregnancy and reached a peak between the en d of pregnancy and day 10 of lactation. The accumulation of their RNA coincided with that of the corresponding proteins, indicating a transc riptional control of expression of these genes. The expression and sec retion patterns of the endogenous casein gene in transgenic and nontra nsgenic mice were indistinguishable. The hybrid BLG/HSA gene construct s displayed distinct patterns of expression in virgin animals and at e arly stage of pregnancy, from that of the BLG transgene or the endogen ous mouse milk protein gene. High levels of expression (17-60% of that on day 18 of pregnancy) were detected in the mammary gland of virgin animals. At day 5 of pregnancy there was a dramatic decrease in HSA sy nthesis and secretion in all transgenic strains tested. The down-regul ation, revealed by immunoprecipitation and immunohistochemical studies , demonstrated that at that stage of pregnancy only 10-18% of ductal s tructures contained HSA expressing cells in contrast to the majority o f ducts expressing HSA in virgin animals. These morphological studies also demonstrated that the down-regulation in HSA synthesis and secret ion was correlated with the transition from ducts comprised of a singl e layer of epithelial cells (characteristic of the virgin state) to du cts composed of multilayers of such cells. In two of the three transge nic strains tested, the down-regulation at the protein level was assoc iated with a similar decrease in HSA transcripts. In the exceptional s train no. 23, HSA transcripts continued accumulating even at this stag e. The differences in the control of expression at the RNA level betwe en these transgenic strains were also confirmed by in situ hybridizati on. Our results suggest the involvement of at least two regulatory mec hanisms effective at early stages of gestation in the control of expre ssion/secretion of the HSA transgene targeted for expression in the ma mmary gland by the BIG milk protein promoter. These putative mechanism s may play key roles in the interplay between normal mammogenesis and lactogenesis. Thus, transgenic mice expressing BLG/HSA gene constructs at early stages of gestation would be valuable in further dissecting these mechanisms. (C) 1995 Wiley-Liss, Inc.