Insulin-like Growth Factor I mRNA Species Research Articles

Stability of Alternatively Spliced IGF-I mRNA in Growth Plate Chondrocytes

Insulin-like growth factor-I (IGF-I) enhances the proliferation and hypertrophy of growth plate chondrocytes leading to increased growth plate width thus promoting bone elongation. Differing quantities of the multiple IGF-I transcripts within the growth plate suggest differential regulation of IGF-I. To assess this, the relative stabilities of the 1Ea, 1Eb, and 2Ea IGF-I mRNA classes in 2-6 week old rat growth plates were evaluated. The mean estimated half-life of class lEa was 3.7 ± 0.05 h, while classes 1Eb and 2Ea decayed gradually over the course of the experiment. Estimated half-lives for each IGF-I mRNA species were unchanged at all ages examined. Incubation with Act D enhanced the transcription of class 1Ea, 1Eb, and 2Ea mRNAs to varying degrees. This implies that the differential stability of alternative IGF-I mRNA classes may be an inherent regulatory component that is not influenced by an animal's developmental state, and the differential abundance of alternative IGF-I mRNA species in the growth plate throughout development is due to transcriptional differences.

Reduction of Hepatic Insulin-Like Growth Factor I (IGF-I) Messenger Ribonucleic Acid (mRNA) during Fasting Is Associated with Diminished Splicing of IGF-I Pre-mRNA and Decreased Stability of Cytoplasmic IGF-I mRNA

The mechanisms by which fasting decreases liver insulin-like growth factor I (IGF-I) messenger RNA (mRNA) abundance have not been defined completely. In the present study, we have examined the effects of fasting in rats on hepatic IGF-I gene transcription, IGF-I pre-mRNA splicing, and cytoplasmic IGF-I mRNA stability. Using the in vitro nuclear run-on transcription technique, we observed that fasting did not change IGF-I gene transcription activity [76 ± 32 densitometric units (DU) for fasted vs. 58 ± 23 DU for control-fed rats; P = 0.1], whereas IGF-binding protein-1 (IGFBP-1) gene transcription, a positive control, was increased more than 2-fold (729 ± 157 DU for fasted vs. 261 ± 56 DU for control-fed rats; P < 0.05). This implies that fasting-induced reduction of liver IGF-I mRNA is due to events other than a decreased rate of IGF-I gene transcription. By measuring nonspliced (pre-mRNA) and spliced IGF-I transcripts in liver nuclear RNA using ribonuclease protection assays, we found that IGF-I pre-mRNA was increased in fasted rats (measured as the percentage of β-actin: 34.0 ± 5.5% for fasted vs. 8.1 ± 3.8% for control-fed rats; P < 0.01), whereas spliced IGF-I transcript remained unchanged (measured as the percentage of β-actin: 60.9± 9.2% for fasted vs. 79.0 ± 6.2% for control-fed rats; P = 0.75). We then compared this pattern of splicing to IGF-I pre-mRNA splicing in hypophysectomized rats subjected to GH stimulation and to IGFBP-1 pre-mRNA splicing in the same fasting experiment. One hour after GH injection, we observed a coordinate increase in both nonspliced and spliced IGF-I transcripts in liver nuclei of hypophysectomized rats. Fasting increased both IGFBP-1 pre-mRNA and spliced transcript. Taken together, these results indicate that the increase in IGF-I pre-mRNA in liver nuclei during fasting is caused by delayed pre-mRNA splicing, rather than increased IGF-I gene transcription. To examine the possible effect of fasting on hepatic IGF-I mRNA stability, we used an in vitro model of nutrient deprivation (fewer amino acids in culture medium) of rat hepatocyte primary culture. Each of the three major IGF-I mRNA species exhibited a shortened half-life in the amino acid-deprived media. The 7.5-kb IGF-I mRNA, however, was degraded faster than the two smaller IGF-I mRNA species. This may indicate that fasting decreases the stability of liver IGF-I mRNA in vivo. In summary, these results suggest that fasting regulates hepatic IGF-I gene expression mainly at the posttranscriptional level by delaying IGF-I pre-mRNA splicing, which attenuates mature IGF-I mRNA generation, and by accelerating the rate of degradation of IGF-I mRNA in cytoplasm.

Reduction of hepatic insulin-like growth factor I (IGF-I) messenger ribonucleic acid (mRNA) during fasting is associated with diminished splicing of IGF-I pre-mRNA and decreased stability of cytoplasmic IGF-I mRNA.

The mechanisms by which fasting decreases liver insulin-like growth factor I (IGF-I) messenger RNA (mRNA) abundance have not been defined completely. In the present study, we have examined the effects of fasting in rats on hepatic IGF-I gene transcription, IGF-I pre-mRNA splicing, and cytoplasmic IGF-I mRNA stability. Using the in vitro nuclear run-on transcription technique, we observed that fasting did not change IGF-I gene transcription activity [76 +/- 32 densitometric units (DU) for fasted vs. 58 +/- 23 DU for control-fed rats; P = 0.1], whereas IGF-binding protein-1 (IGFBP-1) gene transcription, a positive control, was increased more than 2-fold (729 +/- 157 DU for fasted vs. 261 +/- 56 DU for control-fed rats; P < 0.05). This implies that fasting-induced reduction of liver IGF-I mRNA is due to events other than a decreased rate of IGF-I gene transcription. By measuring nonspliced (pre-mRNA) and spliced IGF-I transcripts in liver nuclear RNA using ribonuclease protection assays, we found that IGF-I pre-mRNA was increased in fasted rats (measured as the percentage of beta-actin: 34.0 +/- 5.5% for fasted vs. 8.1 +/- 3.8% for control-fed rats; P < 0.01), whereas spliced IGF-I transcript remained unchanged (measured as the percentage of beta-actin: 60.9 +/- 9.2% for fasted vs. 79.0 +/- 6.2% for control-fed rats; P = 0.75). We then compared this pattern of splicing to IGF-I pre-mRNA splicing in hypophysectomized rats subjected to GH stimulation and to IGFBP-1 pre-mRNA splicing in the same fasting experiment. One hour after GH injection, we observed a coordinate increase in both nonspliced and spliced IGF-I transcripts in liver nuclei of hypophysectomized rats. Fasting increased both IGFBP-1 pre-mRNA and spliced transcript. Taken together, these results indicate that the increase in IGF-I pre-mRNA in liver nuclei during fasting is caused by delayed pre-mRNA splicing, rather than increased IGF-I gene transcription. To examine the possible effect of fasting on hepatic IGF-I mRNA stability, we used an in vitro model of nutrient deprivation (fewer amino acids in culture medium) of rat hepatocyte primary culture. Each of the three major IGF-I mRNA species exhibited a shortened half-life in the amino acid-deprived media. The 7.5-kb IGF-I mRNA, however, was degraded faster than the two smaller IGF-I mRNA species. This may indicate that fasting decreases the stability of liver IGF-I mRNA in vivo. In summary, these results suggest that fasting regulates hepatic IGF-I gene expression mainly at the posttranscriptional level by delaying IGF-I pre-mRNA splicing, which attenuates mature IGF-I mRNA generation, and by accelerating the rate of degradation of IGF-I mRNA in cytoplasm.

Effect of fasting on insulin-like growth factor (IGF)-IA and IGF-IB messenger ribonucleic acids and prehormones in rat liver.

The insulin-like growth factor I (IGF-I) gene generates by alternative splicing two IGF-I messenger RNAs (mRNAs) coding for IGF-I prehormones with different E domain sequences. In rats, these two mRNAs differ by the presence (IGF-IB) or absence (IGF-IA) of a 52-bp insert in the E domain coding region. The purpose of this study was to investigate the effect of nutritional perturbation on IGF-IA and -IB expression in rat liver. Northern blot analysis of liver mRNA revealed that the 1.5-1.9 kb and 0.9-1.2 kb IGF-I mRNA species were decreased in rats fasted for 48 h compared with either fasted-refed (48 h of each) or control-fed rats (each, P < 0.01), whereas the 7.5 kb IGF-I mRNA was decreased only when compared with the fasted-refed animals. Using semiquantitative RT-PCR, the IGF-IA transcript (114 bp amplicon) was not altered, whereas the IGF-IB transcript (166 bp amplicon) was decreased in fasted rats compared with the other two groups (both P < 0.01). We confirmed the RT-PCR results by RNase protection assay (RPA), observing that the IGF-IA (224 and 100 bases protected) was not decreased and that the IGF-IB transcript (376 bases protected), accounting for only 23% of the total IGF-I transcripts of control fed rats, was decreased by fasting. Because the results from RT-PCR and RPA do not necessarily predict full-length translatable mRNA, we subjected hepatic IGF-I transcripts to in vitro translation, and we immunoprecipitated IGF-IA and -IB prehormones. Both prehormones were translated principally from exon 1-containing mRNAs, with molecular weights of about 17K and 18K, representing 80% and 20% of the total IGF-I prehormones observed in control fed rats, respectively. Both peptides were reduced in fasted rats compared with controls (P < 0.01), and refeeding restored both. By immunoblotting of the protein extract from liver of fasted rats, IGF-IA was decreased by 77% compared with control-fed animals. Refeeding returned IGF-IA to normal. The lack of reduction of IGF-IA transcript at the alternative splice site suggests that posttranscriptional mechanisms are responsible for the reduction in steady-state IGF-I mRNAs that occurs during fasting. Additionally, we present evidence that biosynthesis of IGF-IA and -IB prehormones by liver is impaired at a posttranscriptional level.

Growth hormone (GH) modulates insulin-like growth factor I (IGF-I) and type I IGF receptor mRNA levels in the ovary of prepubertal GH-deficient rats.

In order to explore the potential role of growth hormone (GH) in modulating insulin-like growth factor I (IGF-I) gene expression in the prepubertal rat ovary, female rats were rendered GH deficient by neonatal administration of monosodium glutamate (MSG). One group of rats received vehicle and served as the control. At 21 days of age, MSG-treated rats received either GH or vehicle for 2 weeks. On days 21, 24, 28 and 31 animals were weighed and subsets were sacrificed for liver RNA extraction. The remaining animals were sacrificed at day 35 when livers and ovaries were collected, and serum was obtained for GH determinations. The IGF-I mRNA levels were estimated by Northern blots and corroborated further by slot-blot analysis. The MSG-treated rats had lower body weights (p < 0.01) and GH levels (p < 0.05) than controls. Growth hormone replacement significantly accelerated the weight gain of MSG-treated rats. At day 24 and thereafter, three RNA IGF-I species (7.5, 1.8 and 0.8-1.2 kB) were seen in the liver. In the ovary, at age 35 days, two major IGF-I mRNA species (7.5 and 0.8-1.2 kb) were seen. The MSG treatment consistently reduced the levels of both IGF-I mRNA species in the ovary. Growth hormone administration partially restored their expression, both in the liver and in the ovary. In addition, ovarian type IIGF receptor mRNA levels were increased in the MSG-treated rats when compared to controls. This trend was reversed by GH replacement.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolation of a second nonallelic insulin-like growth factor I gene from the salmon genome.

We have characterized a second nonallelic insulin-like growth factor-I (IGF-I) gene in the chum salmon (Oncorhynchus keta) genome. This gene, IGF-I.2, differs from the previously described chum salmon IGF-I gene, IGF-I.1, in the E peptide-coding portion of exon 3; specifically, the IGF-I.2 gene lacks one codon present in the IGF-I gene and contains two potential splice donor sites at the 3' end of exon 3 rather than the single, more distal site present in the IGF-I.1 gene. The expression of these two IGF-I genes could give rise to as many as six IGF-I mRNA species, each of which would encode a unique E-peptide moiety of the IGF-I prohormone. Thus, the presence of multiple, distinct IGF genes adds an additional level of complexity to IGF-I gene expression and IGF-I biosynthesis in salmon.

Functional analysis of the rat insulin-like growth factor I gene and identification of an IGF-I gene promoter.

Insulin-like growth factor I (IGF-I) mediates many of the systemic growth-promoting effects of growth hormone and also functions as a locally acting growth stimulator. In mammals, IGF-I gene expression is complicated, as the gene is transcribed and processed into multiple mRNAs (ranging in length from less than 1 to nearly 7.5 kb) that encode at least two protein precursors. As a step toward understanding the regulation of IGF-I, we report the complete organization of the rat IGF-I gene, including identification of the structural determinants for all IGF-I mRNA species, and an initial functional analysis of its promoters. The gene is composed of 6 exons distributed over nearly 80 kb of chromosomal DNA and is structurally heterogeneous. Several transcription start sites were identified within IGF-I exons 1 and 2, adjacent to presumptive promoters 1 and 2, respectively, and at least three polyadenylation sites were mapped to exon 6. To test promoter function, fusion genes were constructed linking fragments of IGF-I DNA to a reporter plasmid. Chimeric genes containing at least 395 bp of DNA from the 5'-flanking region of exon 1 enhanced luciferase activity after transfection into the IGF-I-producing SK-N-MC cell line, while fusion plasmids containing up to 1,300 bp of DNA from the 5'-flanking region of exon 2 were inactive. Relative levels of IGF-I mRNAs containing exons 1 or 2 varied among different rat tissues, although in response to acute or chronic growth hormone treatment both classes of transcripts were induced coordinately in rat liver. These observations represent the first thorough characterization of a mammalian IGF-I gene, and provide a starting point for defining the mechanisms by which growth hormone and other trophic factors regulate IGF-I gene expression.

Differential association of insulin-like growth factor I mRNA variants with polysomes in vivo.

Expression of the rat insulin-like growth factor I (IGF-I) gene results in a number of mature mRNA species that differ in size primarily at the 3' end due to differential polyadenylation site usage. Additionally, alternate splicing in both 5' and 3' regions produces RNAs which have the capacity to encode different IGF-I precursor peptides. We have analyzed total and polysomal RNAs using Northern blot analyses and solution hybridization/RNase protection assays to assess the in vivo translatability of these various IGF-I mRNA species. The results suggest that all of the known splicing variants are found on polysomes and may, therefore, be translated into a number of IGF-I precursors in vivo. One particular 5'-untranslated (UTR) variant is relatively enriched in polysomal RNA, a finding which suggests that removal of some of the 5'-UTR sequences encoded by exon 1 may enhance translatability. Of the IGF-I mRNAs with different lengths of 3'-UTR, only the shorter species were found on polysomes, suggesting that some aspect of the long 3'-UTR may prevent translation. Thus, differential processing of the primary transcript of the IGF-I gene may serve to generate IGF-I mRNA species which specify different precursors as well as to control their relative translatability.

Growth Hormone (GH) Regulation of Cytochrome P-450IIC12, Insulin-Like Growth Factor-I (IGF-I), and GH Receptor Messenger RNA Expression in Primary Rat Hepatocytes: A Hormonal Interplay with Insulin, IGF-I, and Thyroid Hormone

We have studied the GH-dependent expression of cytochrome P-450IIC12 (P-450(15)beta) mRNA and insulin-like growth factor-I (IGF-I) mRNA in primary adult rat hepatocytes. The GH receptor (GHR), being the common denominator for the GH response, was also studied. The respective mRNA levels were measured with specific solution hybridization assays. By investigating the effects of insulin, IGF-I, T3, and corticosterone, alone or in combinations, in the presence or absence of GH we concluded that GH is indeed the inducer of P-450(15)beta mRNA and IGF-I mRNA. However, insulin and IGF-I exerted a 2-fold potentiation of the GH-induced expression of the P-450(15)beta and IGF-I mRNA species. No significant effect of insulin was observed on GHR mRNA expression, but a translational or posttranslational effect on GHR was seen, in that insulin increased the binding of GH to the cells 4-fold. Furthermore, T3 caused a 9-fold increase in the GH-induced expression of IGF-I mRNA. These observations led us to postulate a possible mechanism of hormonal interplay between GH, thyroid hormone, and IGF-I in vivo, i.e. a thyroid hormone potentiation of the GH-induced IGF-I expression, which, in turn, leads to an increased GHR level and thereby a potentiation of the GH-induced expression of P-450(15)beta and, at least transiently, of IGF-I. A transcriptional mechanism of GH action on P-450(15)beta and IGF-I mRNA induction was indicated by the similar half-lives of respective mRNAs in the presence or absence of GH in cell cultures treated with actinomycin-D.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of Dietary Protein Deprivation on Insulin-Like Growth Factor (IGF)-I and -II, IGF Binding Protein-2, and Serum Albumin Gene Expression in Rat*

Circulating levels of insulin-like growth factor I (IGF-I) and serum albumin are decreased under conditions of chronic dietary protein limitation. To investigate the biochemical mechanism(s) involved in the regulation of IGF-I and serum albumin synthesis by dietary protein, we studied the effects of protein limitation on IGF-I and serum albumin gene expression in young growing rats maintained on isocaloric diets containing 20%, 12%, 8%, or 4% protein. Animals maintained on the 12%, 8%, or 4% protein diets exhibited slight, moderate, or severe growth deficiency, respectively, and a decreased abundance of hepatic IGF-I messenger RNA (mRNA). The decrease in IGF-I mRNA was most pronounced for the largest [7.7 kilobase (kb)] species, which was decreased by 87% in animals maintained on the 4% protein diet compared with animals on the 20% protein diet. The 0.9 kb species of IGF-I mRNA exhibited a smaller (46%) reduction in abundance in animals maintained on the 4% protein diet. The differential regulation of the 7.7 kb IGF-I mRNA species compared with the shorter IGF-I mRNA species suggests that a sequence or sequences within the long 3'-untranslated region of this mRNA species may play a role in regulating its abundance under conditions of protein limitation. Serum albumin mRNA was also decreased (by 62%) in the animals maintained on the 4% protein diet. The level of serum albumin gene transcription was not decreased in animals on the low protein diets, suggesting that nutrition regulates albumin mRNA at a posttranscriptional step. There was considerable animal-to-animal variability in the level of IGF-I gene transcription within each dietary group. The mean level of IGF-I gene transcription was decreased by 46% in the animals on the 4% protein diet compared with animals on the 20% protein diet, although this decrease was not statistically significant because of the animal-to-animal variability in IGF-I gene transcription within the dietary groups. Additional studies of brain RNA from animals on the four diets indicated that brain IGF-II mRNA was decreased by 57% in animals on the 4% protein diet. It has been demonstrated recently that expression of the gene for IGF binding protein-2 (IGFBP-2) is strongly induced in the liver of fasting animals. To investigate the possible regulation of the IGFBP-2 gene in the protein-limited animals, the abundance of liver and brain IGFBP-2 mRNA was analyzed in animals on the four diets.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of Fasting on Insulin-Like Growth Factor-I (IGF-I) and Growth Hormone Receptor mRNA Levels and IGF-I Gene Transcription in Rat Liver

Previous studies have indicated that the concentration of circulating insulin-like growth factor-I (IGF-I) declines in young growing rats that have been fasted or maintained on a protein-deficient diet. To investigate the molecular mechanism(s) by which IGF-I levels are regulated by nutrition, we measured the levels of IGF-I mRNA in 6-week-old male control rats fed ad libitum, rats fasted for 24, 48, or 72 h, and rats fasted for 48 or 72 h and then refed for 24 h. The abundance of several IGF-I mRNA species (8.0, 4.0, 1.7, and 1.0 kilobases) decreased in the fasting animals and rebounded after 24 h of refeeding, although not to the initial control levels. The 1 kilobase IGF-I mRNA species exhibited a 43% decrease after 24 h of fasting, a 76% decrease after 48 h of fasting, and an 82% decrease after 72 h of fasting. Hepatic GH receptor mRNA also decreased in fasting rats. This indicates that the GH receptor down-regulation that occurs in fasting is accompanied by and probably at least partly caused by a decline in GH receptor mRNA. The magnitude and kinetics of the decline in GH receptor mRNA were similar to the magnitude and kinetics of the decline in IGF-I mRNA, suggesting that the two mRNAs may be regulated by a similar mechanism. There was no significant change in the levels of liver beta-actin or serum albumin mRNA under the same conditions, indicating that the regulation of IGF-I and GH receptor mRNA was specific. In addition, the levels of brain IGF-II, beta-actin, and alpha-tubulin mRNAs were not significantly changed by fasting. To further elucidate the molecular mechanism for regulation of hepatic IGF-I mRNA, nuclear transcription elongation assays were performed using nuclei isolated from the liver of control rats, rats fasted for 72 h, and fasted-refed rats. There was considerable animal-to-animal variability in IGF-I gene transcription within each group. The mean level of IGF-I gene transcription was lower in the fasting animals than in the fed controls. However, this decrease was not statistically significant, and the magnitude of the decrease did not account for the 79% decrease in total IGF-I mRNA. These results suggest that IGF-I mRNA is regulated at least partly at the posttranscriptional level.

Dual regulation of insulin-like growth factor I expression during renal hypertrophy

We examined the regulation of insulin-like growth factor I (IGF-I) in kidney during the renal hypertrophy produced by two different experimental models: growth hormone treatment of hypophysectomized rats and compensatory hypertrophy subsequent to unilateral nephrectomy. Immunostaining for IGF-I in collecting ducts was enhanced in kidneys from growth hormone-repleted hypophysectomized rats, and the levels of IGF-I mRNAs were increased. In compensatory hypertrophy, no enhancement of the intensity of immunostaining was observed in kidneys of nephrectomized rats until 5 days postnephrectomy, at which time immunostainable IGF-I was increased markedly in medullary collecting ducts of hypertrophied kidneys compared with kidneys from sham-operated animals. No difference in steady-state levels of any IGF-I mRNA species was detected in whole kidneys or in collecting ducts from nephrectomized or sham-operated rats at any time postnephrectomy. Our findings demonstrate an increase in both IGF-I mRNA and in immunostainable IGF-I in collecting duct in the setting of growth hormone-induced renal hypertrophy but suggest that other, possibly translational, mechanisms underlie the induction of IGF-I synthesis during compensatory hypertrophy.

Expression and stability of insulin-like growth factor-I (IGF-I) mRNA splicing variants in the GH 3 rat pituitary cell line

We have employed Northern blot analyses and solution hybridization/RNase protection assays to evaluate the presence and stability of IGF-I mRNA splicing variants in the GH 3 rat pituitary cell line. All of the IGF-I mRNA size classes and IGF-I mRNAs with alternately-spliced 5′-untranslated regions and E-peptide coding regions seen in adult rat liver also were present in GH 3 cells, although the proportions of the 5′ splicing variants were significantly altered. In actinomycin D-treated cells, all IGF-I mRNA splicing variants were equally stable; thus, changes in the levels of some splicing variants were not due to differential mRNA stability. Additionally, all IGF-I mRNA size classes seen on Northern blots were equally stable; this data suggests that the large IGF-I mRNA species is not a precursor of the smaller species.

Coordinate decrease of tissue insulinlike growth factor I posttranscriptional alternative mRNA transcripts in diabetes mellitus.

In these studies, we examined the effect of excess levels of growth hormone (GH) on rat insulinlike growth factor I (IGF-I) gene expression in streptozocin-induced diabetes mellitus. A solution hybridization/RNase protection assay was used to simultaneously quantitate the relative tissue content of the variant IGF-I mRNA species arising from alternative splicing in the region encoding the COOH-terminal extension E-peptide (IGF-Ia and IGF-Ib). IGF-Ia and IGF-Ib mRNAs were markedly decreased in liver, kidney, and lung tissues of diabetic rats. Although GF stimulates IGF-I gene expression, chronic GH excess from implanted somatomammotropic tumors did not appropriately induce tissue IGF-I mRNA content in diabetic animals. Treatment of diabetic rats with insulin for 1 wk restored basal and GH-stimulated IGF-Ia and IGF-Ib mRNA content toward that present in tissues of nondiabetic rats. The ratio of IGF-Ia to IGF-Ib mRNA remained relatively constant for each tissue and was not affected by the diabetic state, chronic GH hyperstimulation, or insulin therapy, suggesting that posttranscriptional splicing is not a regulated event in these conditions. Thus, both circulating IGF-I levels and tissue IGF-I gene expression are profoundly decreased in this model of experimental diabetes. Diminished tissue availability of IGF-I from endocrine and/or paracrine sources may be responsible for the growth retardation seen in uncontrolled diabetes mellitus.

Tissue and development specific regulation of a complex family of rat insulin-like growth factor I messenger ribonucleic acids.

To obtain information about the functional significance of the structural heterogeneity that has been described for rat insulin-like growth factor I (IGF-I) cDNAs, we hybridized polyadenylated RNAs from rat tissues at different developmental stages with probes specific for two variant 5'-sequences (designated here as type 1 and type 2), with a probe specific for IB type E domain coding sequences and with a probe for E domain sequences common to IA and IB type IGF-I cDNAs. Northern blot analyses revealed that previously reported rat liver IGF-I mRNAs of estimated size 7.5-7.0, 1.9-1.5, and 1.2-0.9 kilobases each are comprised of multiple closely migrating IGF-I mRNA species containing either of two 5'-sequences and either IA or IB type E domain coding sequences. In liver, each of these detected IGF-I mRNA species showed postnatal increases in abundance. The mRNAs detected with the probe for type 2 5'-sequences were detected exclusively in postnatal liver and also showed a different pattern of postnatal increase in abundance than other IGF-I mRNA types. IGF-I mRNAs detected with the probe for IB type E domain coding sequences likewise were highly liver specific and were undetectable or barely detectable in other fetal or adult rat tissues. In contrast, IGF-I mRNAs that hybridized with probes for type 1 5'-sequences or for E domain coding sequences common to IA and IB type IGF-I mRNAs were detected in all fetal and adult rat tissues tested. These findings suggest development and tissue specific regulation of the expression of different rat IGF-I mRNA types, and also suggest a possible role of different precursor sequences encoded by the various mRNAs in targeting of IGF-I to a local site of action.