Insulin-like Growth factor-I Receptor Activity Research Articles

Muscle insulin-like growth factor-I modulates murine craniofacial bone growth.

Insulin-like growth factor I (IGF-I) is essential for muscle and bone development and a primary mediator of growth hormone (GH) actions. While studies have elucidated the importance of IGF-I specifically in muscle or bone development, few studies to date have evaluated the relationship between muscle and bone modulated by IGF-I in vivo, during post-natal growth. Mice with muscle-specific IGF-I overexpression (mIgf1+/+) were utilised to determine IGF-I- and muscle-mass-dependent effects on craniofacial skeleton development during post-natal growth. mIgf1+/+ mice displayed accelerated craniofacial bone growth when compared to wild-type animals. Virus-mediated expression of IGF-I targeting the masseter was performed to determine if post-natal modulation of IGF-I altered mandibular structures. Increased IGF-I in the masseter affected the mandibular base plane angle in a lateral manner, increasing the width of the mandible. At the cellular level, increased muscle IGF-I also accelerated cartilage thickness in the mandibular condyle. Importantly, mandibular length changes associated with increased IGF-I were not present in mice with genetic inhibition of muscle IGF-I receptor activity. These results demonstrated that muscle IGF-I could indirectly affect craniofacial growth through IGF-I-dependent increases in muscle hypertrophy. These findings have clinical implications when considering IGF-I as a therapeutic strategy for craniofacial disorders.

Lessons Learned from Targeting IGF-I Receptor in Thyroid-Associated Ophthalmopathy.

Complex immunological mechanisms underlie the pathogenesis of thyroid-associated ophthalmopathy (TAO). Historical models of Graves’ disease and TAO have focused almost entirely on autoimmune reactivity directed against the thyrotropin receptor (TSHR). The insulin-like growth factor-I receptor (IGF-IR) has been proposed as a second participating antigen in TAO by virtue of its interactions with IGFs and anti-IGF-IR antibodies generated in Graves’ disease. Furthermore, the IGF-IR forms with TSHR a physical and functional complex which is involved in signaling downstream from both receptors. Inhibition of IGF-IR activity results in attenuation of signaling initiated at either receptor. Based on the aggregate of findings implicating IGF-IR in TAO, the receptor has become an attractive therapeutic target. Recently, teprotumumab, a human monoclonal antibody IGF-IR inhibitor was evaluated in two clinical trials of patients with moderate to severe, active TAO. Those studies revealed that teprotumumab was safe and highly effective in reducing disease activity and severity. Targeting IGF-IR with specific biologic agents may result in a paradigm shift in the therapy of TAO.

Insulin‐like growth factor I modulates sleep through hypothalamic orexin neurons

Although sleep disturbances are common co-morbidities of metabolic diseases, the underlying processes linking both are not yet fully defined. Changes in the duration of sleep are paralleled by changes in the levels of insulin-like growth factor-I (IGF-I), an anabolic hormone that shows a circadian pattern in the circulation and activity-dependent entrance in the brain. However, the specific role, if any, of IGF-I in this universal homeostatic process remains poorly understood. We now report that the activity of orexin neurons, a discrete cell population in the lateral hypothalamus that is involved in the circadian sleep/wake cycle and arousal, is modulated by IGF-I. Furthermore, mice with blunted IGF-I receptor activity in orexin neurons have lower levels of orexin in the hypothalamus, show altered electro-corticographic patterns with predominant slow wave activity, and reduced onset-sleep latency. Collectively, these results extend the role in the brain of this pleiotropic growth factor to shaping sleep architecture through the regulation of orexin neurons. We speculate that poor sleep quality associated to diverse conditions may be related to disturbed brain IGF-I input to orexin neurons.

Insulin-like Growth Factor-I Receptor and Thyroid-Associated Ophthalmopathy.

Thyroid-associated ophthalmopathy (TAO) is a complex disease process presumed to emerge from autoimmunity occurring in the thyroid gland, most frequently in Graves disease (GD). It is disfiguring and potentially blinding, culminating in orbital tissue remodeling and disruption of function of structures adjacent to the eye. There are currently no medical therapies proven capable of altering the clinical outcome of TAO in randomized, placebo-controlled multicenter trials. The orbital fibroblast represents the central target for immune reactivity. Recent identification of fibroblasts that putatively originate in the bone marrow as monocyte progenitors provides a plausible explanation for why antigens, the expressions of which were once considered restricted to the thyroid, are detected in the TAO orbit. These cells, known as fibrocytes, express relatively high levels of functional TSH receptor (TSHR) through which they can be activated by TSH and the GD-specific pathogenic antibodies that underpin thyroid overactivity. Fibrocytes also express insulin-like growth factor I receptor (IGF-IR) with which TSHR forms a physical and functional signaling complex. Notably, inhibition of IGF-IR activity results in the attenuation of signaling initiated at either receptor. Some studies suggest that IGF-IR-activating antibodies are generated in GD, whereas others refute this concept. These observations served as the rationale for implementing a recently completed therapeutic trial of teprotumumab, a monoclonal inhibitory antibody targeting IGF-IR in TAO. Results of that trial in active, moderate to severe disease revealed dramatic and rapid reductions in disease activity and severity. The targeting of IGF-IR with specific biologic agents may represent a paradigm shift in the therapy of TAO.

Challenges in Orphan Drug Development: Identification of Effective Therapy for Thyroid-Associated Ophthalmopathy.

Thyroid-associated ophthalmopathy (TAO), the ocular manifestation of Graves' disease, is a process in which orbital connective tissues and extraocular muscles undergo inflammation and remodeling. The condition seems to result from autoimmune responses to antigens shared by the thyroid and orbit. The thyrotropin receptor (TSHR), expressed at low levels in orbital tissues, is a leading candidate antigen. Recent evidence suggests that another protein, the insulin-like growth factor-I receptor (IGF-IR), is overexpressed in TAO, and antibodies against IGF-IR have been detected in patients with the disease. Furthermore, TSHR and IGF-IR form a physical and functional complex, and signaling initiated at TSHR requires IGF-IR activity. Identification of therapy for this rare disease has proven challenging and currently relies on nonspecific and inadequate agents, thus representing an important unmet need. A recently completed therapeutic trial suggests that inhibiting IGF-IR activity with a monoclonal antibody may be an effective and safe treatment for active TAO.

BRCA1 is Expressed in Uterine Serous Carcinoma (USC) and Controls Insulin-Like Growth Factor I Receptor (IGF-IR) Gene Expression in USC Cell Lines

The insulin-like growth factor I receptor (IGF-IR) and BRCA1 affect cell growth and apoptosis. Little information is available about BRCA1 activity on the IGF signaling pathway. This study evaluated the effect of BRCA1 on IGF-IR expression. BRCA1 and IGF-IR immunohistochemistry on archival tissues (35 uterine serous carcinomas [USCs] and 17 metastases) were performed. USPC1 and USPC2 cell lines were transiently cotransfected with an IGF-IR promoter construct driving a luciferase reporter gene and a BRCA1 expression plasmid. Endogenous IGF-IR levels were evaluated by Western immunoblotting. We found high BRCA1 and IGF-IR protein expression in primary and metastatic USC tumors. All samples were immunostained for BRCA1-71% strongly stained; and 33/35 (94%) were stained positive for IGF-IR-2 (6%) strongly stained. No difference in BRCA1 and IGF-IR staining intensity was noted between BRCA1/2 mutation carriers and noncarriers. Metastatic tumors stained more intensely for BRCA1 than did the primary tumor site (P = 0.041) and with borderline significance for IGF-IR (P = 0.069). BRCA1 and IGF-IR staining did not correlate to survival. BRCA1 expression led to 35% and 54% reduction in IGF-IR promoter activity in the USPC1 and USCP2 cell lines, respectively. Western immunoblotting showed a decline in phosphorylated IGF-IR and phosphorylated AKT in both transiently and stably transfected cells. BRCA1 and IGF-IR are highly expressed in USC tumors. BRCA1 suppresses IGF-IR gene expression and activity. These findings suggest a possible biological link between the BRCA1 and the IGF-I signaling pathways in USC. The clinical implications of this association need to be explored.

Polyubiquitination of Insulin-like Growth Factor I Receptor (IGF-IR) Activation Loop Promotes Antibody-induced Receptor Internalization and Down-regulation

Ubiquitination has been implicated in negatively regulating insulin-like growth factor I receptor (IGF-IR) activity. Because of the relative stability of IGF-IR in the presence of ligand stimulation, IGF-IR ubiquitination sites have yet to be mapped and characterized, thus preventing a direct demonstration of how the receptor ubiquitination contributes to downstream molecular cascades. We took advantage of an anti-IGF-IR antibody (h10H5) that induces more efficient receptor down-regulation to show that IGF-IR is promptly and robustly ubiquitinated. The ubiquitination sites were mapped to the two lysine residues in the IGF-IR activation loop (Lys-1138 and Lys-1141) and consisted of polyubiquitin chains formed through both Lys-48 and Lys-29 linkages. Mutation of these ubiquitinated lysine residues resulted in decreased h10H5-induced IGF-IR internalization and down-regulation as well as a reduced cellular response to h10H5 treatment. We have therefore demonstrated that IGF-IR ubiquitination contributes critically to the down-regulating and antiproliferative activity of h10H5. This finding is physiologically relevant because insulin-like growth factor I appears to mediate ubiquitination of the same major sites as h10H5 (albeit to a lesser extent), and ubiquitination is facilitated by pre-existing phosphorylation of the receptor in both cases. Furthermore, identification of a breast cancer cell line with a defect in IGF-IR ubiquitination suggests that this could be an important tumor resistance mechanism to evade down-regulation-mediated negative regulation of IGF-IR activity in cancer.

Inhibition of the Insulin-Like Growth Factor I Receptor by Epigallocatechin Gallate Blocks Proliferation and Induces the Death of Ewing Tumor Cells

The insulin-like growth factor I receptor (IGFIR) has emerged as a key therapeutic target in many human malignancies, including childhood cancers such as Ewing family tumors (EFT). In this study, we show that IGFIR is constitutively activated in EFTs and that the major catechin derivative found in green tea, (-)-epigallocatechin gallate (EGCG), can inhibit cell proliferation and survival of EFT cells through the inhibition of IGFIR activity. Treatment of EFT cell lines with EGCG blocked the autophosphorylation of IGFIR tyrosine residues and inhibited its downstream pathways including phosphoinositide 3-kinase-Akt, Ras-Erk, and Jak-Stat cascades. EGCG treatment was associated with dose- and time-dependent inhibition of cellular proliferation, viability, and anchorage-independent growth, as well as with the induction of cell cycle arrest and apoptosis. Apoptosis in EFT cells by EGCG correlated with altered expression of Bcl-2 family proteins, including increased expression of proapoptotic Bax and decreased expression of prosurvival Bcl2, Bcl-XL, and Mcl-1 proteins. Our results provide further evidence that IGFIR is an attractive therapeutic target in EFTs and that EGCG is an effective inhibitor of this receptor tyrosine kinase. EGCG may be a useful agent for targeting IGFIR, either alone or in combination, with other potentially more toxic IGFIR inhibitors for the management of EFTs.

Insulin-like growth factor-I receptor blockade reduces the invasiveness of gastrointestinal cancers via blocking production of matrilysin

Insulin-like growth factor-I receptor (IGF-IR) signaling is required for carcinogenicity and proliferation of gastrointestinal (GI) cancers. We have previously shown significant therapeutic activity for recombinant adenoviruses expressing dominant-negative insulin-like growth factor-I receptor (IGF-IR/dn), including suppression of tumor invasion. In this study, we sought to evaluate the mechanism of inhibition of invasion and the relationship between IGF-IR and matrix metalloproteinase (MMP) activity in GI carcinomas. We analyzed the role of IGF-IR on invasion in three GI cancer cell lines, colorectal adenocarcinoma, HT29; pancreatic adenocarcinoma, BxPC3 and gastric adenocarcinoma, MKN45, using a modified Boyden chamber method and subcutaneous xenografts in nude mice. The impact of IGF-IR signaling on the expression of MMPs and the effects of blockade of matrilysin or IGF-IR on invasiveness were assessed using recombinant adenoviruses, a tyrosine kinase inhibitor NVP-AEW541 and antisense matrilysin. Invasive subcutaneous tumors expressed several MMPs. IGF-IR/dn reduced the expression of these MMPs but especially matrilysin (MMP-7). Insulin-like growth factor (IGF) stimulated secretion of matrilysin and IGF-IR/dn blocked IGF-mediated matrilysin induction in three GI cancers. Both IGF-IR/dn and inhibition of matrilysin reduced in vitro invasion to the same degree. NVP-AEW541 also reduced cancer cell invasion both in vitro and in murine xenograft tumors via suppression of matrilysin. Thus, blockade of IGF-IR is involved in the suppression of cancer cell invasion through downregulation of matrilysin. Strategies of targeting IGF-IR may have significant therapeutic utility to prevent invasion and progression of human GI carcinomas.

Antitumor Activity of GSK1904529A, a Small-molecule Inhibitor of the Insulin-like Growth Factor-I Receptor Tyrosine Kinase

Dysregulation of the insulin-like growth factor-I receptor (IGF-IR) signaling pathway has been implicated in the development of many types of tumors, including prostate, colon, breast, pancreatic, ovarian, and sarcomas. Agents that inhibit IGF-IR activity may be useful in treatment of patients with various cancers. Kinase assays were used to identify a selective small-molecule inhibitor of IGF-IR activity. The effects of this compound on IGF-IR signaling, cell proliferation, and the cell cycle were determined using a panel of cell lines. Antitumor activity was evaluated in human tumor xenografts growing in athymic mice. Inhibition of IGF-IR and the closely related insulin receptor (IR) was measured in vivo, and the effect on glucose metabolism was evaluated. GSK1904529A selectively inhibits IGF-IR and IR with IC(50)s of 27 and 25 nmol/L, respectively. GSK1904529A blocks receptor autophosphorylation and downstream signaling, leading to cell cycle arrest. It inhibits the proliferation of cell lines derived from solid and hematologic malignancies, with multiple myeloma and Ewing's sarcoma cell lines being most sensitive. Oral administration of GSK1904529A decreases the growth of human tumor xenografts in mice, consistent with a reduction of IGF-IR phosphorylation in tumors. Despite the potent inhibitory activity of GSK1904529A on IR in vitro and in vivo, minimal effects on blood glucose levels are observed in animals at doses that show significant antitumor activity. GSK1904529A is a promising candidate for therapeutic use in IGF-IR-dependent tumors.

A HIF‐1α‐dependent autocrine feedback loop promotes survival of serum‐deprived prostate cancer cells

We previously reported that normoxic, serum-deprived prostate cancer (PCa) cells upregulate hypoxia-inducible factor-1alpha (HIF-1alpha) protein, which promotes survival during serum deprivation via insulin-like growth factor-2 (IGF-2) upregulation. This study investigated the molecular mechanism of autocrine regulation of HIF-1alpha, IGF-2 and cell survival in serum-deprived PC-3 and LNCaP PCa cells. Cell viability was assessed by trypan blue assay. PI3K activity was inhibited with LY294002, and PTEN overexpression. mRNA was assessed by RT-PCR, and IGF-2 protein by ELISA. Activated insulin-like growth factor-I receptor (IGF-IR) was detected by probing immunoprecipitated IGF-IR for phospho-tyrosine. IGF-IR activity was inhibited with IGF-2 neutralizing antibody and IGF-IR-specific siRNA. HIF-1alpha, phospho-Akt, total-Akt and IGF-IR protein was assessed by immunoblots. HIF-1alpha was suppressed with siRNA. We detected a time-dependent increase in Akt activation during serum deprivation, and inhibition of Akt activation attenuated the serum deprivation-mediated increase in HIF-1alpha and cell survival. Importantly, IGF-2 secretion significantly increased during serum deprivation, and was accompanied by increased activation of its receptor, IGF-IR. Additionally, inhibition of IGF-2 activity markedly attenuated the serum deprivation-mediated increase in IGF-IR and Akt activation, HIF-1alpha expression, and also its own transcription, suggesting autocrine regulation of HIF-1alpha expression via IGF-2. Cross-talk between IGF-2/ IGF-IR system and PI3K-Akt pathway was further demonstrated by findings wherein IGF-IR suppression inhibited Akt activation, and IGF-IR activation was inhibited following PI3K inhibition. Furthermore, HIF-1alpha suppression attenuated the serum deprivation-mediated increase in Akt and IGF-IR activation. Collectively, our study demonstrates existence of a pro-survival HIF-1alpha-dependent autocrine feedback loop in normoxic, serum-deprived PCa cells.

Overexpression of IGF-I receptor in HeLa cells enhances in vivo radioresponse

Insulin-like growth factor I receptor (IGF-IR) is a transmembrane receptor tyrosine kinase whose activation strongly promotes cell growth and survival. We previously reported that IGF-IR activity confers intrinsic radioresistance in mouse embryo fibroblasts in vitro. However, it is still unclear whether tumor cells overexpressing IGF-IR exhibit radioresistance in vivo. For this purpose, we established HeLa cells that overexpress IGF-IR (HeLa-R), subcutaneously transplanted these cells into nude mice, and examined radioresponse in the resulting solid tumors. HeLa-R cells exhibited typical in vitro phenotypes generally observed in IGF-IR-overexpressing cells, as well as significant intrinsic radioresistance in vitro compared with parent cells. As expected, the transplanted HeLa-R tumors grew at a remarkably higher rate than parent tumors. Histological analysis revealed that HeLa-R tumors expressed more VEGF and had a higher density of tumor vessels. Unexpectedly, a marked growth delay was observed in HeLa-R tumors following 10 Gy of X-irradiation. Immunostaining of HeLa-R tumors for the hypoxia marker pimonidazole revealed a significantly lower level of hypoxic cells. Moreover, clamp hypoxia significantly increased radioresistance in HeLa-R tumors. Tumor microenvironments in vivo generated by the IGF-IR expression thus could be a major factor in determining the tumor radioresponse in vivo.

Insulin-like growth factor I receptor (IGF-IR) inhibition in trastuzumab (T) resistant HER2+ breast cancer (BrCa) cells

14049 Background: IGF-IR signaling interferes with the growth inhibitory action of T in BrCa cell lines and may play a role in clinical resistance to T. We studied the effect of IGF-IR inhibition on T resistance. Methods: HER2 and IGF-IR protein levels were determined by ELISA. T growth inhibition of HER2+ BrCa cell lines was measured by cell counting. HER2 and IGF-IR activity were determined by immunoprecipitation with HER2 and IGF-IR antibodies, followed by western blotting with phospho-tyrosine antibody. The effects of IGF- I/IGF-IR inhibition on proliferation and response to T were investigated. Results: No significant correlation was found between HER2, phospho-HER2 or IGF-IR levels and response to T in 12 HER+ BrCa lines. However, two T-conditioned cell lines show resistance to T (BT474 - 2.6 fold; SKBR3 - 1.8 fold) and have higher levels of phospho-IGF-IR compared to parental cell lines. Neither IGFBP3 nor IGF-IR antibody aIR3 significantly inhibited the growth of T-conditioned cell lines or enhanced the anti-proliferative effects of T. IGF-IR siRNA decreased IGF-IR protein levels and inhibited proliferation (29.8 ± 9.3 %) of T-conditioned SKBR3 cells. IGF-IR siRNA also enhanced T growth inhibition of T-conditioned SKBR3 cells (47.7 ± 13.7 % compared to 17.7 ± 14.8 % for T alone). The small molecule IGF-IR tyrosine kinase inhibitor (TKI), NVP-AEW541, inhibited growth of parental and T-conditioned SKBR3 cells and enhanced response to T (Table). Conclusions: Increased activation of IGF-IR may play a role in resistance to T. Inhibition of IGF-I ligand binding does not appear to alter IGF-IR mediated T-resistance. However, reducing IGF-IR protein levels or blocking IGF-IR TK activity improves response to T in resistant cells. Thus, the combination of IGF-IR TKIs, such as NVP-AEW541, with T may be beneficial for T-resistant HER2+ BrCa. [Table: see text] [Table: see text]

Down-Regulation of Insulin-Like Growth Factor I Receptor Activity by NVP-AEW541 Has an Antitumor Effect on Neuroblastoma CellsIn vitroandIn vivo

Signaling through insulin-like growth factor I receptor (IGF-IR) is important for growth and survival of many tumor types. Neuroblastoma is sensitive to IGF. We assessed the ability of NVP-AEW541, a recently developed small molecule that selectively inhibits IGF-IR activity, for neuroblastoma growth effects in vitro and in vivo. Our data showed that, in a panel of 10 neuroblastoma cell lines positive for IGF-IR expression, NVP-AEW541 inhibited in vitro proliferation in a submicromolar/micromolar (0.4-6.8) range of concentrations. As expected, NVP-AEW541 inhibited IGF-II-mediated stimulation of IGF-IR and Akt. In addition to growth inhibition, the drug also induced apoptosis in vitro. Oral administration of NVP-AEW541 (50 mg/kg twice daily) inhibited tumor growth of neuroblastoma xenografts in nude mice. Analysis of tumors from the drug-treated animals revealed a marked apoptotic pattern and a decrease in microvascularization compared with controls. Interestingly, quantitative real-time PCR detected both in vitro and in vivo a significant down-regulation of mRNA for vascular endothelial growth factor (VEGF) caused by NVP-AEW541. In addition, in Matrigel-coated chambers and in severe combined immunodeficient mice tail vein injected with neuroblastoma cells, tumor invasiveness was significantly reduced by this agent. Analysis of IGF-IR expression in a series of 43 neuroblastoma primary tumors revealed IGF-IR positivity in 86% of cases. Taken together, these data indicate that NVP-AEW541 can be considered as a novel promising candidate for treatment of neuroblastoma patients.

Insulin-like growth factor-I receptor activity is essential for Kaposi's sarcoma growth and survival

Kaposi's sarcoma (KS) is a highly vascular tumour and is the most common neoplasm associated with human immunodeficiency virus (HIV-1) infection. Growth factors, in particular vascular endothelial growth factor (VEGF), have been shown to play an important role in its development. The role of insulin-like growth factors (IGFs) in the pathophysiology of different tumours led us to evaluate the role of IGF system in KS. The IGF-I receptors (IGF-IR) were identified by immunohistochemistry in biopsies taken from patients with different AIDS/HIV-related KS stages and on KSIMM cells (an established KS-derived cell line). Insulin-like growth factor-I is a growth factor for KSIMM cells with a maximum increase of 3H-thymidine incorporation of 130±27.6% (P<0.05) similar to that induced by VEGF and with which it is additive (281±13%) (P<0.05). Moreover, specific blockade of the receptor (either by α IR3 antibody or by picropodophyllin, a recently described selective IGF-IR tyrosine phosphorylation inhibitor) induced KSIMM apoptosis, suggesting that IGF-IR agonists (IGF-I and -II) mediate antiapoptotic signals for these cells. We were able to identify an autocrine loop essential for KSIMM cell survival in which IGF-II is the IGF-IR agonist secreted by the cells. In conclusion, IGF-I pathway inhibition is a promising therapeutical approach for KS tumours.

Sequence dependence of C5-propynyl-dU,dC-phosphorothioate oligonucleotide inhibition of the human IGF-I receptor: mRNA, protein, and cell growth.

Human keratinocytes are highly responsive to mitogenic and antiapoptotic signaling by the insulin-like growth factor-I receptor (IGF-IR). IGF-IR hyperstimulation is a feature of hyperplastic skin conditions, making the IGF-IR an appealing target for antisense therapeutic intervention. In this study, we used a C5-propynyl-dU,dC-phosphorothioate oligo-2'-deoxyribonucleotide antisense 15-mer to the human IGF-IR mRNA, along with liposome transfection, to inhibit IGF-IR activity in a human keratinocyte cell line and demonstrated potent inhibition of cell growth despite the presence of serum. To investigate the sequence specificity of these effects and to establish the concentration range over which a purely antisense effect could be demonstrated, we introduced 1, 2, 4, 8, and 15 base mismatches into the oligonucleotide and analyzed changes in inhibitory efficacy. In the 10-30 nM concentration range, the introduction of 1 and 2 mismatches into the middle of the 15-mer only modestly affected inhibitory efficacy, whereas >4 mismatches profoundly reduced mRNA, protein, and growth-inhibitory effects. From these results, we conclude that (1) sequence-specific antisense inhibition of IGF-IR activity in keratinocytes is achievable, (2) potent anti-IGF-IR antisense inhibition can be achieved in vitro at concentrations as low as 10 nM, and (3) a sequence-dependent mechanism is likely to underpin the observed in vivo therapeutic effects (Wraight et al. Nat. Biotechnol. 2000;18:521) of these antisense oligonucleotides (AS-ODN) in cutaneous hyperplastic disorders, such as psoriasis.

Dephosphorylation of human insulin-like growth factor I (IGF-I) receptors by membrane-associated tyrosine phosphatases.

The insulin-like growth factor-I (IGF-I) receptor exhibits structural and functional similarities to the insulin receptor. Although the regulation of the insulin-receptor tyrosine kinase has been extensively investigated, the mechanisms involved in phosphorylation/dephosphorylation of the IGF-I receptor have received only little attention. To obtain a better understanding of the mode of IGF-I action, we have investigated the effects of protein phosphotyrosine phosphatases (PTPases) on the phosphorylation status of the IGF-I receptor. The dephosphorylation of the human IGF-I receptor by membrane-associated tyrosine phosphatases was studied by an immuno-enzymic assay based on the recognition of phosphotyrosine residues by anti-phosphotyrosine antibodies. Using intact IGF-I receptors as substrates, we show that they could be completely dephosphorylated by different cellular PTPases. Three pieces of evidence indicate that receptor dephosphorylation takes place on phosphotyrosine, i.e. the inhibition profile of phosphatase activity by zinc and vanadate, its absolute requirement for thiol compounds and the diminution of [32P]phosphotyrosine labelling of the beta subunit assessed by SDS/PAGE and phosphoamino acid analysis. Tyrosine kinase activity and autophosphorylation of the IGF-I receptor were decreased in a dose-dependent manner by PTPases, indicating that partial dephosphorylation of the receptor was associated with a decrease in its intrinsic activity. The sensitivity of the activated human IGF-I receptor to dephosphorylation on tyrosine leads to the speculation that IGF-I receptor activity might be regulated by mechanisms such as those described for the insulin receptor. Further investigation of the pathways of IGF-I receptor dephosphorylation will contribute to define the role(s) of PTPases in the overall mechanism of IGF-I signalling.