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Pre-weaning diet and exogenous estrogen alter mammary epithelial cell proliferation and progesterone and estrogen receptor expression
Pre-weaning diet and estrogen treatment alters mammary development. Our objectives were to study the effects of diet and estrogen on mammary histology, proliferation, and expression of estrogen (ESR1) and progesterone (PR) receptors. Thirty-six Holstein heifer calves were reared on: 1) a control milk replacer (MR) fed at 454 g powder/day (R; 20% crude protein [CP], 20% fat), or 2) an enhanced MR fed at 1135 g powder/day (E; 28% CP, 25% fat). MR was fed for 8 weeks. At weaning a subset of calves were sacrificed (n = 6/diet). Remaining calves received E2 implants and were sacrificed at week 10. Treatments were: 1) R, 2) R + E2 (R-E2), 3) E, and 4) E + E2 (E-E2). One day prior to harvest calves were given bromodeoxyuridine (BrdU; 5mg/kg). At sacrifice, parenchyma (PAR) was removed and fixed. Sections from lower, middle and distal zones were stained with H and E and antibodies to measure expression of ESR1, PR, and BrdU. Comparisons with PROC GLIMMIX in SAS on a per area and per cell basis were similar. At week 8, R-fed calves had more (P < 0.01) PR expressing cells in distal PAR. But PR expression intensity was greater (P < 0.01) in E-fed calves. The proportion of cells expressing ESR1 was not affected by diet, but expression intensity was increased for E-fed calves across all zones (62 to 81%; P < 0.01). Percent BrdU positive cells was 2 and 0.5-fold greater (P < 0.01) for E-fed calves in zone 2 and 3. At week 10, calves treated with estrogen had 3.9-fold greater PR expression intensity. The intensity and percent of cells expressing ESR1 was lowest in estrogen treated calves. Overall, estrogen treated calves had the most proliferating cells (P < 0.01). However, in zone 3, E-E2 calves had a higher percentage of proliferating cells than calves on all other treatments (P < 0.01). Results indicate both diet and estrogen administration alter proliferation rates of the mammary epithelium and that changes in expression of ESR1 and PR are at least partially responsible for changes in mammary PAR development associated with enhanced pre-weaning feeding of dairy calves. However, further, more detailed analyses are needed to fully understand mechanisms at play.
Keywords: estrogen receptor, progesterone receptor, mammary gland