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Differential involvement of insulin-like growth factor-I and estrogen on prolactin cells in the mouse anterior pituitary

Estrogen and insulin-like growth factor-I (IGF-I) stimulate prolactin (PRL) production, release and proliferation of PRL-producing cells (PRL cells) in the anterior pituitary. PRL cells in adult estrogen receptor α (ERα) knockout (αERKO) mice and IGF-I knockout (IGF-IKO) mice are decreased considerably in number. To investigate a correlation between 17β-estradiol (E2) and IGF-I on PRL production, IGF-I wild-type (WT) or IGF-IKO mice were ovariectomized at day 8 and the number of PRL cells was examined at days 20 and 60. Although PRL cell number at day 20 and WT or IGF-IKO mice ovariectomized at day 8 was similar to that in intact WT or IGF-IKO mice, PRL cells in adult WT or IGF-IKO mice ovariectomized at day 8 were significantly decreased as compared with those in intact WT or IGF-IKO mice. Therefore, estrogen is essential for PRL cell differentiation between days 20 and 60, regardless of IGF-I. While PRL cells in WT ovariectomized mice increased from days 20 to 60, those in IGF-IKO ovariectomized mice did not increase, suggesting that IGF-I modified PRL cell differentiation after day 20. ICI 182,780 (anti-estrogen) treatment canceled an increase of PRL cells in 30-day-old ovariectomized WT mice, indicating that the presence of ERα is important. The number of PRL cells in αERKO mice was similar to that in WT mice at day 20; however, PRL cells in αERKO mice at day 60 were not increased in number from day 20, supporting the idea that estrogen is essential for PRL cell differentiation after day 20. Finally, the percentage of PRL cells in IGF-IKO mice was decreased as compared with that in WT mice at day 20; therefore, IGF-I affects PRL cells before day 20. In conclusion, PRL cell differentiation is differently regulated by E2 and IGF-I depending on the age.