Synthesis and regulation of insulin-like growth factor binding protein-5 in FRTL-5 cells.
FRTL-5 cells, a diploid nontransformed line of rat thyroid follicular cells, exhibit a marked mitogenic response to insulin-like growth factors (IGFs) when they are exposed to TSH. Because IGF binding proteins (IGFBPs) are important modulators of IGF actions, we investigated the capacity of FRTL-5 cells to synthesize IGFBPs. We found that FRTL-5 cell conditioned media contained a single band of approximately 31 kilodaltons on ligand blot analysis. This band represents IGFBP-5 because: 1) it can be immunostained with a specific antibody raised against human IGFBP-5; 2) by Northern analysis, total RNA from FRTL-5 cells contains a major 6-kilobase transcript when hybridized with a cDNA for rat IGFBP-5; and 3) no transcripts were observed when Northern blots of FRTL-5 cells were hybridized with complementary DNAs for rat IGFBP-1, -2, -3, -4 or -6. Conditioned media IGFBP-5 increased in response to IGF-I, IGF-II, and insulin in a dose-dependent fashion, compared with unstimulated FRTL-5 cells. At maximally effective concentrations IGF-I was 3.5- and 6-fold more potent than IGF-II and insulin, respectively. The addition of a monoclonal antibody (Sm 1.2) to IGF-I completely inhibited IGF-I stimulation of the IGFBP-5 6-kilobase transcript and the appearance of IGFBP-5 in FRTL-5 conditioned media. Stimulation of IGFBP-5 synthesis by the IGFs and insulin appeared to be regulated at the messenger RNA (mRNA) level, because each stimulated similar increases in both IGFBP-5 mRNA and media protein at maximally effective concentrations. TSH, on the other hand, inhibited basal levels of IGFBP-5 mRNA and attenuated the increase in IGFBP-5 mRNA stimulated by IGF-I. FRTL-5 cells provide a relatively uncomplicated model to study the regulation and action of IGFBP-5 and the mechanisms by which IGFs interact with this binding protein.
Backeljauw
P F
PF
Department of Pediatrics, University of North Carolina, Chapel Hill 27599-7220.
Dai
Z
Z
Clemmons
D R
DR
D'Ercole
A J
AJ
D000818:Animals; D002352:Carrier Proteins / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism*; D002460:Cell Line; D003470:Culture Media; D007328:Insulin / Q000494:pharmacology; D018975:Insulin-Like Growth Factor Binding Protein 5; D012333:RNA, Messenger / Q000378:metabolism; D013002:Somatomedins / Q000494:pharmacology; D013961:Thyroid Gland / Q000166:cytology / Q000378:metabolism*; D013972:Thyrotropin / Q000494:pharmacology; D013997:Time Factors
D002352:Carrier Proteins; D003470:Culture Media; D007328:Insulin; D018975:Insulin-Like Growth Factor Binding Protein 5; D012333:RNA, Messenger; D013002:Somatomedins; D013972:Thyrotropin