In this study, we evaluated whether proinsulin may differentially bind to and activate the two insulin receptor (IR) isoforms (IR-A and IR-B), because IR-A is a relatively low-specificity receptor that is prevalent in fetal and cancer cells and is able to mediate the growth effects of IGF-II. Mouse R(-) fibroblasts devoid of IGF-I receptor (IGF-IR) and stably transfected with cDNA encoding either human IR-A or IR-B (R(-) /IR-A and R(-) /IR-B cells) were used.
Three human cancer cell lines were also studied.
We found that proinsulin stimulated phosphorylation of IR-A with an EC(50) of 4.5 ± 0.6 nm and displaced [(125)I]insulin from IR-A with a similar EC(50). In contrast, proinsulin EC(50) values for stimulation of IR-B phosphorylation and for [(125)I]insulin displacement from IR-B were approximately 7-fold higher.
Proinsulin did not bind or activate IGF-IR or IR/IGF-IR hybrids.
Via IR-A, proinsulin activated the ERK/p70S6K pathway to a similar degree as insulin but elicited a weaker Akt response.
Despite its low metabolic activity, proinsulin was almost equipotent as insulin in inducing cell proliferation and migration in cells expressing various IR-A levels.
In conclusion, proinsulin is a selective IR-A ligand and may induce biological effects through this IR isoform.