Immunoexpression of Steroid Hormone Receptors and Proliferation Markers in Uterine Leiomyoma and Normal Myometrial Tissues from the Miniature Pig, Sus scrofa

Abstract

Uterine leiomyomas in miniature pet pigs occur similarly to those in women with regard to frequency, age, parity, and cycling. Clinical signs, gross, and histologic features of the porcine tumors closely resemble uterine leiomyomas (fibroids) in women. Although fibroids are hormonally responsive in women, the roles of estrogen and progesterone have not been fully elucidated. In this study, immunohistochemistry was used to assess the expression of the steroid hormone receptors, estrogen receptor alpha (ER-α), estrogen receptor beta (ER-β) and progesterone receptor (PR), and cell proliferation markers, proliferating cell nuclear antigen (PCNA) and Ki-67 in tumor and matched myometrial tissues sampled from miniature pigs. A "quickscore" method was used to determine receptor expression and labeling indices were calculated for the markers. ER-α/β and PR were localized to the nuclei of smooth muscle cells in both tissues. PR expression was intense and diffuse throughout all tissues, with correlation between tumors and matched myometria. Conversely, ER-α expression was variable between the myometrial and tumor tissues, as well as between animals. ER-β expression was low. PCNA and Ki-67 were localized to the nucleus and expression varied among tumors; however, normal tissues were overall negative. These findings support further investigation into the use of the miniature pig as a model of fibroids in women.

Keywords: fibroid; immunoexpression; miniature pig; proliferation markers; steroid hormone receptors; uterine leiomyoma.

Figure 1

Immunohistochemical stains for sex steroid hormone receptors in porcine leiomyoma and matched normal myometrium. (A, B) ER-α immunoreactivity in tumor (A) versus normal myometrium (B); (C, D) ER-β immunoreactivity in tumor (C) versus normal myometrium (D); (E, F) PR immunoreactivity in tumor (E) versus normal myometrium (F); insets = negative controls.

Figure 2

Graph shows overall average (+/− standard error of mean) quickscores for steroid receptors in uterine leiomyoma and matched myometrial tissues from miniature pigs. ER-α and PR expression levels were significantly higher in both leiomyoma and matched myometrial samples compared to expression of ER-β. Expression of PR was significantly higher than that of ER-α in both tumors and normal myometrial samples. There were no significant differences between tumor and normal tissues for each steroid receptor evaluated. a, statistically different from ER-β (p<0.05); b, statistically different from ER-α (p<0.05).

Figure 3

Proliferation marker immunohistochemical staining in a porcine leiomyoma, PCNA (A) and Ki-67 (B).

Figure 4

Minipig uterine leiomyoma: A firm, round, well-circumscribed mass (*) extends from the serosa of the uterine horn into the broad ligament. On cut surface (inset), the mass is white to tan and whorled. Arrow = uterine bifurcation; Circle = ovary.

Figure 5

Minipig uterine leiomyoma: A) Hematoxylin and eosin (H&E) stain showing well-differentiated smooth muscle cells arranged in interlacing fascicles. B) Cytoplasm of tumor cells stains positively for α-smooth muscle actin.