The in vivo biological function of a steroid receptor coactivator was assessed in mice in which the SRC-1 gene was inactivated by gene targeting. Although in both sexes the homozygous mutants were viable and fertile, target organs such as uterus, prostate, testis, and mammary gland exhibited decreased growth and development in response to steroid hormones. Expression of RNA, encoding TIF2, a member of the SRC-1 family, was increased in the SRC-1 null mutant, perhaps compensating partially for the loss of SRC-1 function in target tissues. The results indicate that SRC-1 mediates steroid hormone responses in vivo and that loss of its coactivator function results in partial resistance to hormone
SRC-1 is a transcriptional coactivator for nuclear receptor superfamily; it functions through its histone acetyltransferase (HAT) activity and multiple interactions with agonist-bound receptors, other coactivators such as CBP/P300, other HAT such as p/CAF, and some GTFs such as TBP and TIFIIB. SRC-1 is a member of a gene family including SRC-1, TIF-2 (GRIP-1/SRC-2), and p/CIP (RAC3/ACTR/AIB1/TRAM-1/SRC-3). Cell-free and in vitro transcription experiments have indicated that the SRC-1 family members enhance receptor-dependent transactivation of nuclear genes. It has been found that AIB1 (SRC-3) was amplified and overexpressed in many breast cancers and thus could have a role in tumorigenesis. In order to dissect the physiological role of SRC-1 in vivo, we have generated a mouse line in which the SRC-1 gene is inactivated by gene targeting.
In 8-week-old wild-type females, mammary ducts grew extensively and occupied almost the entire mammary ductal branching as well as the number of branches was substantially reduced in the mammary glands of age-matched SRC-1 null mutants. The ductal tree occupied only half the area of the mammary fat pad (panels A and B). The results suggests that SRC-1 is required for normal mammary ductal elongation.
By day 18 of pregnancy, alveolar structures in wild-type mammary glands were highly developed and appeared on all ductal sections, filling the interductal spaces. In the SRC-1 mutant mammary glands, alveoli were much less developed in terms of number and size of alveoli, and very few alveoli were observed at the ends of ducts at the same stage of pregnancy (panels C - F). In ovariectomized matured females, estrogen and progesterone stimulate a complex ductal arborization and extensive alveolar formation in mammary glands of wild-type mice. In the mammary glands of SRC-1 mutant mice, only partial ductal growth and very few alveoli were observed after hormone treatment (panels G – J). Our results indicate that the SRC-1 is required for efficient proliferation and differentiation of the mammary gland in response to estrogen and progesterone.
SRC-1 mRNA was detected in mammary tissue by Northern blot analysis. SRC-1 gene is also widely expressed in many other tissues. In the SRC-1 null mice, the mRNA of TIF-2 (SRC-2), another member in the SRC family, was slightly overexpressed.
The SRC-1 targeting vector contained 3.5-kb (5’) and 2.5-kb (3’) mouse SRC-1 genomic sequences flanking a neomycin-resistance gene (PGK-neo). Through targeted recombination, an in-frame stop codon was inserted at the Met(381) position and a 9-kb fragment of SRC-1 genomic sequence extending downstream of Met(381) was deleted in the SRC-1 mutant mice.
The SRC-1 mutant mice were established in both a C57BL6/129SvEv hybrid and 129SvEv inbreed genetic background.
Jianming Xu, Yuhong Qui, Francesco J. DeMayo, Sophia Y. Tsai, Ming-Jer Tsai and Bert O'Malley (1998) Partial Hormone Resistance in Mice with Disruption of the Steroid Receptor Coactivator-1 (SRC-1) Gene. Science, 279, 1922-1925.
Jianming Xu and Bert W. O'Malley
Department of Cell Biology
Baylor College of Medicine
Houston, TX 77030
E-mail:
jxu@bcm.tmc.edu
berto@bcm.tcm.edu