Cell interactions in initiation of mammary epithelial proliferation by oestradiol and progesterone in prepubertal heifers

In prepubertal cattle, mammary development is characterized by the growth of an epithelial-rich parenchyma (PAR) into the mammary fat pad (MFP). This proliferation and accumulation of mammary epithelial cells require estrogen. Paradoxically, both epithelial cell proliferation and PAR accumulation rate decline with rising plasma estrogen as puberty approaches. The possibility that variation in abundance of estrogen receptors (ERs) in PAR or MFP accounts for a portion of these effects has not been examined in cattle. Additionally, we recently demonstrated that MFP is highly responsive to exogenous estrogen, suggesting that this tissue may play a role in coordinating estrogen's effects on PAR; however, the developing bovine MFP has yet to be studied in detail. To address these hypotheses, Holstein heifers were assigned to planes of nutrition supporting body growth rates of 950 (E) or 650 (R) g/day and harvested every 50 kg from 100 to 350 kg body weight (BW). Post-harvest, their mammary glands were dissected into PAR and MFP compartments. Transcript abundance of genes encoding members of the ER family (ERa, ERb, and estrogen-related receptor a-1 (ERRa)) and estrogen-responsive genes (IGF-I and progesterone receptor (PR)) were measured in both mammary compartments by quantitative real-time RT-PCR. Significant expression was detected for all genes in both compartments, with the exception of the ERb gene. Transcript abundance of both ERa and IGF-I decreased linearly with increasing BW within both compartments. ERRa and PR expressions decreased with increasing BW in PAR but not in MFP. Nutrition stimulated ERa and ERRa expression in the PAR but had no effect on IGF-I or PR in either PAR or MFP. Overall, ERa and IGF-I transcript abundance are consistent with the drop in mammary epithelial cell proliferation and PAR accretion observed over development, but do not support a negative effect of nutrition on PAR growth.

Section: Discussionmentioning

confidence: 99%

Ontogenic and nutritional regulation of steroid receptor and IGF-I transcript abundance in the prepubertal heifer mammary gland

Meyer

Rhoads

Capuco³

et al. 2007

“…The second factor was applied after a surgery recovery period of 30 days and consisted of daily s.c. injection of 17b-estradiol (Cestrogen) or excipient (corn oil, Kestrogen). Estrogen was administrated at a dose of 0$1 mg/kg body weight as previous work has demonstrated that this dose elicits an increase in mammary epithelial cell proliferation in the prepubertal bovine (Woodward et al 1993, Berry et al 2001. Injections were administered on 3 consecutive days.…”

Section: Animalsmentioning

confidence: 99%

Estrogen-dependent responses of the mammary fat pad in prepubertal dairy heifers

Meyer¹,

Capuco²,

Boisclair³

et al. 2006

Ovaries are absolutely required for development of the mammary parenchyma (PAR) in cattle, reflecting estrogendependent epithelial cell proliferation. However, the estrogen receptor (ER) that mediates the mammary estrogen effects, ERa, is absent in proliferating epithelial cells. In the mouse, this discrepancy is explained in part by the ability of the mammary fat pad (MFP) to synthesize epithelial cell mitogens such as IGF-I in response to estrogen. Consistent with a similar role for the bovine MFP, 30% of its fibroblasts and adipocytes were immunoreactive for ERa in prepubertal dairy heifers. To assess estrogen-dependent gene expression in the MFP, 16 prepubertal dairy heifers were randomly assigned to a 2!2 factorial. The first factor was ovarian status, with heifers undergoing bilateral ovariectomy or left intact at 4$6 months of age. The second factor was applied 30 days after surgery and consisted of injection of estrogen or excipient. After 3 days of injection, heifers were administered an intrajugular bolus of bromodeoxyuridine (BrdU) and slaughtered 2 h later. The estrogen injection, but not ovarian status, caused significant increases in the fraction of epithelial cells labeled with BrdU and produced tissue-specific effects on gene expression. In the PAR, estrogen injection increased IGF-I gene expression by twofold despite reductions of 50% or more in ERa mRNA abundance and the fraction of epithelial cells immunoreactive for ERa. The estrogendependent increase in IGF-I mRNA was greater in the MFP, presumably because estrogen failed to downregulate ERa expression in this mammary compartment. Finally, estrogenresponsiveness of the MFP appears unique among the bovine fat depots as estrogen injection did not induce IGF-I expression in its s.c. counterpart. Our data demonstrate that the bovine MFP is highly responsive to exogenous estrogen, consistent with a role for this tissue compartment in communicating its effects on epithelial cell proliferation.

“…Although growth hormone (GH) and estrogen are stimulators of mammary development (Topper & Freeman 1980, Tucker 1981, Sejrsen et al 1986) and mammary epithelial cell proliferation in heifers (Woodward et al 1993, Berry et al 2001, the mechanisms by which these hormones act are unclear. Accumulated evidence suggests that locally produced growth factors and their binding proteins, especially insulin-like growth factor (IGF)-I and IGF-binding proteins (IGFBP), may mediate the effects of systemic hormones in the ruminant mammary gland.…”

Section: Introductionmentioning

confidence: 99%

Interactions between the ovary and the local IGF-I axis modulate mammary development in prepubertal heifers

Berry

Howard²,

Jobst³

et al. 2003

The objective was to determine the effects of ovariectomy and epithelial-stromal interactions on mammary development and local expression of IGF-I and IGF-binding protein (IGFBP) mRNA in prepubertal heifers. An epithelium-free ('cleared') fat pad (CFP) was prepared in two glands in each of 14 Holstein heifers, aged 1-3 months. Eight of the calves were also ovariectomized. Serum concentrations of GH, IGF-I and prolactin were not affected by ovariectomy. At 6 months of age, calves were killed to provide mammary samples of parenchyma, CFP and intact fat pad (MFP). Total mammary mass was reduced in ovariectomized calves (130 21 g vs 304 25 g; P<0·001), and in several cases parenchymal tissue was essentially absent. Uterus weight was also reduced by ovariectomy (14·5 3·8 g vs 30·4 4·5 g; P<0·05). In support of our hypothesis that local IGF-I mediates prepubertal mammary development, mRNA expression of IGF-I was lower in ovariectomized than in control calves (62·1 7·8 vs 91·6 7·8 arbitrary units; P<0·05). Specific binding of IGF-I to mammary parenchymal microsomes was also reduced by ovariectomy (377 142 vs 868 82 c.p.m.; P<0·01), suggesting decreased sensitivity to IGF-I. Expression of IGFBP-3 and IGFBP-5 mRNA were not influenced by ovariectomy. Expression of IGF-I, IGFBP-3 and IGFBP-5 mRNA did not differ between CFP and MFP, suggesting that expression of these factors was not influenced by interactions between stroma and developing epithelium. Overall, the data suggested that interactions between the ovary and the local IGF-I axis act to optimize the availability and effectiveness of IGF-I within the gland to stimulate mammary growth.