Insulin-like growth factor I in cultured rat astrocytes: expression of the gene, and receptor tyrosine kinase.

Abstract

Gene expression, receptor binding and growth-promoting activity of insulin-like growth factor I (IGF I) was studied in cultured astrocytes from developing rat brain. Northern blot analysis of poly(A)+ RNAs from astrocytes revealed an IGF I mRNA of 1.9 kb. Competitive binding and receptor labelling techniques revealed two types of IGF receptor in astroglial cells. Type I IGF receptors consist of a-subunits (Mr 130 000) which bind IGF I with significantly higher affinity than IGF 11, and fl-subunits (Mr 94 000) which show IGF Isensitive tyrosine kinase activity. Type II IGF receptors are monomers (Mr 250 000) which bind IGF H with three times higher affinity than IGF I. Both types of IGF receptor recognize insulin weakly. DNA synthesis measured by cellular thymidine incorporation was stimulated 2-fold by IGF I and IGF H. IGF I was more potent than IGF H, and both were significantly more potent than insulin. Our findings suggest that IGF I is synthesized in fetal rat astrocytes and acts as a growth promoter for the same cells by activation of the type I IGF receptor tyrosine kinase. We propose that IGF I acts through autocrine or paracrine mechanisms to stimulate astroglial cell growth during normal brain development.

Insulin Promotes the Hydrolysis of a Glycosyl Phosphatidylinositol in Cultured Rat Astroglial Cells

“…From previous reports it is known that insulin, lOF-T, and TGF-IT receptors are present in astroglial cells (Lowe et al, 1986;Ballotti et al, 1987;Han et al, 1987;Ocrant et al, 1988). Our results indicate that (a) insulin receptors were present in our astroglial cultures and (b) our observed effect of insulin on OPT and DAG turnover was mediated through its own receptors and not through TOF-T and/or TGF-IT receptors.…”

“…Our results indicate that (a) insulin receptors were present in our astroglial cultures and (b) our observed effect of insulin on OPT and DAG turnover was mediated through its own receptors and not through TOF-T and/or TGF-IT receptors. Relative to the first of these observations we identified a '251-labeled band that proved to be the astroglial insulin receptor a-subunit, according to its molecular mass (130 kDa) and relative binding affinities by insulin, TOF-IT, and IGF-I (Lowe et al, 1986;Ballotti et al, 1987;Han et al, 1987). The small differences in the concentrations of the TGFs needed to displace labeled insulin suggest the existence of hybrid receptors and might explain the effect of TGF-IT reported here.…”

“…In this sense, insulin is known to modify the metabolic activities of astroglial cells through the stimulation of glucose uptake and protein and lipid synthesis (Kum et al, 1987(Kum et al, , 1988Van der Pal et al, 1988). Because TOF-I does not produce OPT hydrolysis in astrocytes, other mechanisms of action may be related to the effects of this polypeptide, i.e., mitogenesis of astrocytes and promotion of glial cell development (Ballotti et al, 1987;Van der Pal et al, 1988). It is interesting that TGF-I receptors, IRS-1, and P1-3' kinase have been identified in rat brain and are components of another signal transduction pathway involved in the mechanism of both TGF-I and insulin action (Folli et al, 1994).…”

“…Accordingly, primary astroglial cell cultures have been used to identify and study the properties of insulin receptors (Lowe et al, 1986) and the effects of insulin on glucose uptake, glycogen synthesis, and pyrimidine nucleoside uptake and incorporation into nucleic acid (Kum et a!., 1987(Kum et a!., , 1988. Also, insulin-like growth factor (IGF)-I and IGF-II receptors and certain biological effects of these growth factors have been reported in astroglial cells (Ballotti et al, 1987;Han et al, 1987;Ocrant et al, 1988;Master et al, 1991).In the present study, we used cultured rat astroglial cells as a model to investigate whether GPI might be involved in the mechanism of insulin signal transduction in neural cells. Also, the effects of IGF-I and IGF-II on GPI turnover of astrocytes pretreated or not with platelet-derived growth factor (PDGF) and/or epidermal growth factor (EGF) were compared with those of insulin in an attempt to know whether these three homologous peptides might act through common or different signal transduction pathways.…”

“…Accordingly, primary astroglial cell cultures have been used to identify and study the properties of insulin receptors (Lowe et al, 1986) and the effects of insulin on glucose uptake, glycogen synthesis, and pyrimidine nucleoside uptake and incorporation into nucleic acid (Kum et a!., 1987(Kum et a!., , 1988. Also, insulin-like growth factor (IGF)-I and IGF-II receptors and certain biological effects of these growth factors have been reported in astroglial cells (Ballotti et al, 1987;Han et al, 1987;Ocrant et al, 1988;Master et al, 1991).…”