Mutations of ovine and bovine placental lactogens change, in different ways, the biological activity mediated through homologous and heterologous lactogenic receptors

Abstract

The biological activities of ovine (o) and bovine (b) placental lactogens (PLs) and their mutated analogues were compared using several binding and in vitro bioassays. In almost all cases, the biological activities of these analogues mediated through rat (r) prolactin receptor (PRLR) showed little or no change, despite a remarkable decrease in their capacity to bind to the extracellular domain of rPRLR and despite compromised stability of the 2:1 complexes. These results indicate that mutations impairing the ability of oPL or bPL to form stable complexes with lactogenic receptors do not necessarily lead to a decrease in the biological activity, because the transient existence of the homodimeric complex is still sufficient to initiate the signal transduction. In contrast, oPL and bPL analogues completely, or almost completely, lost their ability to activate homologous PRLRs, and some of them even acted as site-2 antagonists. To explain the difference between the activity transduced through homologous and that transduced through heterologous PRLRs, we propose the novel term 'minimal time of homodimer persistence'.This concept assumes that in order to initiate the signal transduction, the associated kinase JAK2 has to be transphosphorylated and this requires a 'minimal time' of homodimer existence. In the case of homologous interaction between ruminant PLs and homologous PRLRs, this 'minimal time' is met, though the interaction with homologous PRLRs has a shorter half-life than that with heterologous PRLRs. Therefore oPL or bPL are active in cells possessing both homologous and heterologous PRLRs. Mutations of oPL or bPL lead to reduced affinity and, consequently, the 'time of homodimer persistence' is shortened. Although in the case of heterologous interaction the 'minimal time' is still sufficient to initiate the biological activity, in homologous interactions, which are already weaker than heterologous interactions, further destabilization of the complex shortens its persistence to below the 'minimal time', leading to full or partial loss of biological activity.

Ovine Placental Lactogen Specifically Binds to Endometrial Glands of the Ovine Uterus1

“…The amount of SEAP-oPL was estimated using the biological activity of the SEAP along with the calculated mass of the fusion protein, a method that is not entirely quantitative. Mutations impairing the ability of oPL or bovine PL to form stable complexes with lactogenic receptors do not necessarily lead to a decrease in the biological activity because the transient existence of the homodimeric complex is still sufficient to initiate signal transduction [30]. Collectively, available results can be interpreted to indicate that the majority of oPL binding sites in the endometrial GE consist of a homodimer of two oPRLRs and the minority of oPL binding sites are a heterodimer of the oPRLR and oGHR.…”

“…Comparative binding studies and in vitro bioassays using heterologous receptors prompted several research groups to suggest that oPL induces somatogenic effect through oGHRs (see [17] for review). Recent evidence indicates that conclusions drawn from heterologous interactions may be misleading [17,29,30]. Ovine PL appears to be an oGHR antagonist [29].…”

“…Ovine PL appears to be an oGHR antagonist [29]. Moreover, homologous in vitro assays indicated that oPL can bind to a homodimer of the long form of the oPRLR and to a heterodimer of oPRLR and oGHR [28,30]. However, the existence and role of these receptors for oPL in vivo has not been determined.…”

The Role of Prolactin in Fish Osmoregulation: A Review

“…Furthermore, in a mammalian study examining the effects of GH analogues with an increased affinity for the second GH receptor of the dimeric complex on biological activity, there was no increase in either cell proliferation or the activation of signal transduction (Pearce et al, 1999). Helman et al (2001) propose that there is a "minimal time of homodimer existence" required for the activation of signal transduction pathways. Perhaps this "minimal time" is greater for tilapia than for other vertebrates, thus the 2:1 complex must persist longer in order to elicit a biological response.…”

Differences in affinities between the homologous and the heterologous rabbit prolactin–receptor interaction with respect to proliferation and differentiation activities

“…PL activates the heterologous but not the homologous GHR (Herman et al, 1999). Mutations change in different ways the activity of PL mediated through homologous (drastical reduction) versus heterologous (no effect) lactogenic receptors (Helman et al, 2001).…”