ABSTRACT
The preimplantation mammalian embryo has been shown to respond to exogenous insulin-like growth factors and insulin itself, however, the most quantitatively important source of these peptides and the receptors through which they exert their effects are unclear. Whilst the type 1 insulin-like growth factor (IGF) receptor is believed to act primarily through tyrosine phosphorylation of the substrate protein alpha IRS-1, evidence for a signalling role for the type 2 receptor is disputed, some evidence pointing to mediation through G protein-dependent calcium ion flux. We have examined the response of murine embryonic stem cells, as a model for the cells of the preimplantation embryo, to IGF-I, IGF-II, insulin and analogs of IGF-II: R6 IGF-II and des (1-6) IGF-II. In response to all of these peptides, except R6 IGF-II, elevation of intracellular cyclic AMP occurs. As R6 IGF-II binds with higher affinity to the type 2 receptor than canonical IGF-II or IGF-I, and insulin fails to interact, this suggests that the elevation of cyclic AMP in response to the other insulin related peptides (IRPs) is not through the type 2 receptor. We conclude that either the type 1 receptor has a previously uncharacterized direct or indirect effect on intracellular cyclic AMP levels, or that there is a further, as yet uncharacterized, receptor active in embryonic stem cells.
Subject(s)
Cyclic AMP/metabolism , Embryo, Mammalian/metabolism , Insulin-Like Growth Factor II/pharmacology , Insulin-Like Growth Factor I/pharmacology , Insulin/pharmacology , Stem Cells/metabolism , Animals , Calcium/metabolism , Cell Line , Cyclic AMP/analysis , Cyclic AMP/physiology , Embryo, Mammalian/chemistry , Embryo, Mammalian/cytology , Female , Insulin/metabolism , Insulin-Like Growth Factor I/metabolism , Insulin-Like Growth Factor II/analogs & derivatives , Insulin-Like Growth Factor II/metabolism , Mice , Pregnancy , Receptor, IGF Type 1/analysis , Receptor, IGF Type 1/metabolism , Receptor, IGF Type 1/physiology , Receptor, IGF Type 2/analysis , Receptor, IGF Type 2/metabolism , Receptor, IGF Type 2/physiology , Receptor, Insulin/analysis , Receptor, Insulin/metabolism , Receptor, Insulin/physiology , Stem Cells/chemistry , Stem Cells/cytologyABSTRACT
In many mammalian embryos development in vitro is arrested after the first zygotic division, a phenomenon known as the two-cell block. In the mouse several strains exhibit a two-cell block to further development and it was the purpose of this investigation to determine whether the inability of embryos to progress through the block was due to lack of insulin or insulin-like growth factors (IGFs) in the medium. Several factors have been implicated in the two-cell block, amongst which oxidative stress, glucose and missing maternal factors have been examined to date. Because of their anabolic and anti-apoptotic properties, IGFs are good candidates for such missing maternal factors. Using MF1 strain mice and M16 medium we have examined the effects of IGF-I, II and insulin on the two-cell block. No effects were discernable at concentrations known to support development of non-blocking embryos and we conclude that the IGFs and insulin may be excluded as critical factors in the two-cell block under the culture conditions used.
