Both Type I and II Insulin-Like Growth Factor Receptor Binding Increase during Lactogenesis in Bovine Mammary Tissue*

Abstract

Bovine GH is a potent stimulant of lactation, and the insulin-like growth factors I and II (IGF-I and -II) are believed to mediate GH's growth-promoting actions. Since all of IGF's known actions are mediated through its receptor subtypes, we analyzed the distribution of IGF receptor subtypes in lactating and nonlactating bovine mammary tissue. Analysis of competition curves showed that IGF-I had greater potency than IGF-II in competing with [125I]IGF-I for binding to membranes prepared from both lactating and nonlactating animals. An insulin concentration of 4 micrograms/ml displaced less than 40% of the [125I]IGF-I bound to membranes prepared from both lactating and nonlactating animals, indicating that a high percentage of [125I]IGF-I was bound to the type II receptor. Lactation was associated with an increase in the total amount of [125I]IGF-I bound, and this change was due to an increase in binding to both receptor subtypes. Specifically, membranes prepared from lactating animals had a 3-fold increase in binding competed for by insulin and a 2-fold increase in binding not competed for by insulin. Affinity cross-linking of [125I]IGF-I to membranes prepared from both lactating and nonlactating animals, followed by polyacrylamide gel electrophoresis (PAGE) and autoradiography, showed that 260K and 135K bands were present. Competition experiments indicated that unlabeled IGF-I effectively competed for binding to the 260K band, whereas insulin did not. Binding to the 135K band could be inhibited by both IGF-I and insulin. The intensity of the labeled bands showed that type II receptors were relatively more abundant than type I receptors in membranes from both lactating and nonlactating animals. Membranes prepared from lactating animals showed both 135K and 127K species of the type I receptor, whereas nonlactating animals showed only the 135K band. We conclude that type I and II receptors are present in bovine mammary tissue, and type II predominate. Lactation is associated with increases in the concentration of both receptor subtypes, especially type I receptors. Lactation may be associated with structural changes in the type I receptor. These changes in receptor distribution could play a role in modulating the physiological effects of the IGFs on mammary tissue.

Differential expression of the growth hormone receptor and growth hormone-binding protein in epithelia and stroma of the mouse mammary gland at various physiological stages

“…These findings suggested that the mitogenic effect of insulin might be mediated through the high affinity type I receptors for IGFs. Indeed, IGF receptors have been found in bovine (Dehoff et al 1988), ovine (Forsyth 1996), and rat (Lavandero et al 1991) mammary tissue.…”

“…For example, IGF-I mRNA in human breast tissue localizes to stromal but not epithelial cells by in situ hybridization (Yee et al 1989). Also, bovine mammary epithelium does not express IGF-I in culture (Campbell et al 1991), although the bovine mammary gland expresses IGF receptors (Dehoff et al 1988).…”

Intestinal effects of milkborne growth factors in neonates of agricultural importance.

“…This observation would be in agreement with tracer studies in which [ 125 I]IGF-I (Prosser et al, 1987) or [ 125 I]IGF-II (Prosser and Fleet, 1992) infused into the pudic artery of the mammary gland of lactating goats was secreted into milk. The exact mechanism(s) of transfer of IGF-I into milk remains to be determined (see Baumrucker and Blum, 1993 for discussion); however, the presence of type I and type II IGF receptors in mammary tissue (Dehoff et al, 1988;Hadsell et al, 1990) may function to sequester IGF from the maternal circulation. Although both IGF-I and -II seem to be derived from maternal blood, the mechanism of transport seems to differ for the two peptides.…”

Transgenic strategies in reproductive endocrinology

“…Several workers have observed increased receptor binding to IGF-I coincident with changes in MG growth and/or secretory function (see Dehoff et al, 1988;Disenhaus et al, 1988;Collier et al, 1989;Hadsell et al, 1990;Stewart et al, 1990), in support of such a notion. Although mice transgenic for IGF-I have been produced (Mathews et al, 1988a,b: Behringer et al, 1990b, any effect(s) on reproductive function have not been reported.…”