Kathleen M. Ohleth

Trophic effects of relaxin on reproductive tissue : Role of the IGF system

Although the growth promoting actions of relaxin on the reproductive tract have been well documented, the means by which relaxin stimulates reproductive tissue growth has not been identified. This report is an overview of studies from our laboratory investigating the role of the insulin-like growth factor (IGF) system in relaxin-induced growth of ovarian and uterine tissues. In the pig ovary, concentrations of relaxin that promote both theca and granulosa cell (GC) DNA synthesis in vitro also significantly (P < 0.05) increased GC IGF-I secretion. When IGF-I activity was blocked in the presence of an IGF-I antibody, the trophic effects of relaxin on GC [3H]thymidine incorporation into DNA were inhibited. However, there was no effect of relaxin on GC IGF binding proteins or IGF-I receptor. In the uterus, in vivo relaxin administration to prepubertal pigs resulted in the stimulation of growth and increases in uterine luminal IGF-I, IGF-II, and IGF binding proteins-2 and -3 secretion (P < 0.05). Thus, the trophic effects of relaxin on ovarian granulosa cells and the uterus involve tissue-specific changes in the IGF system. Additional studies are necessary to better understand the contribution of relaxin to follicular growth and uterine accommodation. These include characterization of the relaxin receptor and post-receptor binding events, as well as the potential impact of relaxin on other growth factor systems and how these systems interact to ultimately drive reproductive tissue growth.

Detection of Relaxin Secretion and Gene Expression in Individual Porcine Luteal Cells

Although large luteal cells of pregnancy contain immunoreactive relaxin (RLX), only about 50% of the cells release RLX in vitro in the reverse hemolytic plaque assay (RHPA) (1). Whether the heterogeneity in RLX secretion by subpopulations of luteal cells can be explained by differences in RLX gene expression is unknown. In situ evidence indicates the RLX gene is expressed by the corpus luteum of pregnancy, but whether a specific subset of luteal cells contains RLX transcript was not determined (2). In this study, RHPA, followed by in situ hybridization histochemistry (ISHH), was used to study the relationship between RLX secretion and gene expression, respectively, by the same luteal cell. These techniques have been used to measure peptide secretion and gene expression in individual pituitary lactotrophs (3). The objective here was to determine whether differences exist between RLX secreting and nonsecreting luteal cells in terms of expression of RLX message.

Relaxin and porcine uterine growth and remodeling

Uterine growth is the result of mitogenic activity of uterine cells, as well as remodeling of the connective tissue framework of the uterus, to facilitate uterine expansion. The stimulation of uterine growth during pregnancy is critical to accommodate growing fetuses. Among the causes of early embryo loss is competition for limited maternal resources, due to insufficient uterine accommodation, which reduces uterine capacity. Relaxin stimulates uterine growth and remodeling in prepubertal [1], pregnant [2] and ovariectomized, steroid-treated animals [3]. While the uterotropic effects of relaxin have been documented, the mechanism of relaxin action in promoting uterine growth is less clear. In our laboratory we are studying the ability of relaxin to stimulate uterine growth, independently of estradiol, using an in vivo, prepubertal gilt model in which the uterotropic actions of relaxin were first reported by Hall et al. [1]. Studies from our laboratory indicate that relaxin interacts with the IGF-I system in promoting uterine growth [4]. In addition, our working hypothesis is that relaxin acts as a growth factor in the uterus by increasing expression of elements that enhance remodeling and cell communication such as connective tissue proteases, connexin gap junction proteins and vascular endothelial growth factor (VEGF).