N-terminal prolactin-derived fragments, vasoinhibins, are proapoptoptic and antiproliferative in the anterior pituitary

Abstract

The anterior pituitary is under a constant cell turnover modulated by gonadal steroids. In the rat, an increase in the rate of apoptosis occurs at proestrus whereas a peak of proliferation takes place at estrus. At proestrus, concomitant with the maximum rate of apoptosis, a peak in circulating levels of prolactin is observed. Prolactin can be cleaved to different N-terminal fragments, vasoinhibins, which are proapoptotic and antiproliferative factors for endothelial cells. It was reported that a 16 kDa vasoinhibin is produced in the rat anterior pituitary by cathepsin D. In the present study we investigated the anterior pituitary production of N-terminal prolactin-derived fragments along the estrous cycle and the involvement of estrogens in this process. In addition, we studied the effects of a recombinant vasoinhibin, 16 kDa prolactin, on anterior pituitary apoptosis and proliferation. We observed by Western Blot that N-terminal prolactin-derived fragments production in the anterior pituitary was higher at proestrus with respect to diestrus and that the content and release of these prolactin forms from anterior pituitary cells in culture were increased by estradiol. A recombinant preparation of 16 kDa prolactin induced apoptosis (determined by TUNEL assay and flow cytometry) of cultured anterior pituitary cells and lactotropes from ovariectomized rats only in the presence of estradiol, as previously reported for other proapoptotic factors in the anterior pituitary. In addition, 16 kDa prolactin decreased forskolin-induced proliferation (evaluated by BrdU incorporation) of rat total anterior pituitary cells and lactotropes in culture and decreased the proportion of cells in S-phase of the cell cycle (determined by flow cytometry). In conclusion, our study indicates that the anterior pituitary production of 16 kDa prolactin is variable along the estrous cycle and increased by estrogens. The antiproliferative and estradiol-dependent proapoptotic actions of this vasoinhibin may be involved in the control of anterior pituitary cell renewal.

Figure 1. N-terminal PRL fragments content in the anterior pituitary varies along the estrous cycle.

A: Recombinant 23 kDa PRL (r23-PRL), recombinant 16 kDa prolactin (r16-PRL) or pituitary protein extracts treated with β-mercaptoethanol (P+β, reducing conditions) or without β-mercaptoethanol (P, non-reducing conditions) were incubated with anti-recombinant rat PRL antibody (anti rrPRL, 1∶25000). The antibody recognizes both PRL forms. B: Anterior pituitaries from rats euthanized at diestrus I or proestrus were processed for western blot. Upper panel: Each column represents the mean ± SE of the relative increment of N-terminal PRL fragments content with respect to diestrus I. Data of each column were normalized to β-actin expression (n = 4–7 animals per group). *p<0.01 vs. diestrus I, Student's t test. Lower panel: Representative blot of pituitary proteins from rats euthanized at diestrus I or proestrus.

Figure 2. Estradiol increases the secretion and content of N-terminal PRL fragments from anterior pituitary cells in culture.

Anterior pituitary cells from OVX rats were incubated in the presence of VEH or E2 for 24 h. A: Culture media and B: cells were obtained and processed for western blot analysis. Each column represents the mean ± SE of the relative increment of N-terminal PRL fragments with respect to VEH. In B, data of each column were normalized to β-actin expression (A, n = 6 wells/group; B, n = 6 wells/group), representative of three independent experiments. *p<0.05, **p<0.01 vs. VEH without E2, Student's t test. Lower panels: Representative blots of media (A) or cells (B) cultured with VEH or E2.

Figure 3. 16 kDa PRL induces apoptosis of anterior pituitary cells.

A and B: Anterior pituitary cells from OVX rats were incubated with VEH or E2 for 24 h and with or without 16 kDa PRL (10 nM) for the last 4 h. A: Each column represents the percentage ± CI (95%) of TUNEL-positive cells (n≥1500 cells/group, left) or TUNEL-positive lactotropes (n≥300 cells/group, right), representative of four independent experiments. Data were analyzed by χ². **p<0.01 vs. respective control without 16 kDa PRL. ∧∧p<0.01, ∧p<0.05 vs. respective control without E2. B: Representative images of anterior pituitary cells showing immunoreactivity for prolactin (red) counterstained with DAPI (blue, upper panels) and DNA fragmentation determined by TUNEL method (green, lower panels). Arrowheads indicate apoptotic lactotropes. Scale bar: 50 µm. C: Anterior pituitary cells from OVX rats were incubated with E2 for 24 h, and with or without 16 kDa PRL (10 nM) for the last 4 h. The percentage of hypodiploid cells was determined by flow cytometry using PI. Left: Each column represents the mean ± SE of the percentage of sub-G1 cells (n = 3 wells/group), representative of three independent experiments. Data were analyzed by Student's t test. **p<0.01 vs. respective control without 16 kDa PRL. Right: Representative histograms of fluorescence intensity of DNA content of anterior pituitary cells incubated in the presence or absence of 16 kDa PRL.

Figure 4. 16 kDa PRL inhibits anterior pituitary cell proliferation.

A and B: Anterior pituitary cells from OVX rats were incubated for 24 h with VEH and FSK, and with or without 16 kDa PRL (10 nM) for the last 4 h. D and E: Anterior pituitary cells from OVX rats were incubated for 24 h with E2, then for 1 h with E2 and FSK, and for the last 4 h in the same media with or without 16 kDa PRL (10 nM). Proliferation was determined by BrdU incorporation (3 h) and fluorescence microscopy. A and D: Each column represents the percentage ± CI (95%) of BrdU-positive cells (n≥1000 cells/group, left) or BrdU-positive lactotropes (n≥300 cells/group, right), representative of four independent experiments. Data were analyzed by χ². **p<0.01 vs. respective control without 16 kDa PRL. B and E: Representative images of anterior pituitary cells showing immunoreactivity for prolactin (red) counterstained with DAPI (blue) and BrdU (green). Arrowheads indicate proliferating lactotropes. Scale bar: 50 µm. C and F: Anterior pituitary cells in culture were incubated with VEH (C) or E2 (F) for 24 h, and with or without 16 kDa PRL (10 nM) for the last 4 h. Cell cycle was analyzed by flow cytometry using PI. Left: Each column represents the mean ± SE of the percentage of S-phase cells (n≥4 wells/group), representative of three independent experiments. Data were analyzed by Student's t test. *p<0.05 vs. respective control without 16 kDa PRL. Right: Representative histograms of DNA content of anterior pituitary cells incubated in the presence or absence of 16 kDa PRL.