Insulin up-regulates vascular endothelial growth factor and stabilizes its messengers in endometrial adenocarcinoma cells.

Angiogenesis is crucial for tumor growth and dissemination. Vascular endothelial growth factor (VEGF) is a potent angiogenic factor that promotes vascular growth and therefore tumoral growth and metastasis. Overweight, frequently associated with hyperinsulinemia, constitutes the major risk factor for endometrial carcinoma. Thus, elevated insulin levels may partly explain the increased risk of endometrial cancer observed in obese postmenopausal women. The aim of the present work was to test the role of insulin in the control of VEGF expression in endometrial carcinoma cells (HEC-1A). We have shown that insulin induced a biphasic expression of VEGF messenger ribonucleic acid, with an early, but low, induction (4 h of stimulation) and a delayed, but high, induction (24 h). The delayed effect of insulin on VEGF expression involved transcriptional and posttranscriptional regulation, as evidenced by the increased rate of VEGF transcription and the prolonged half-life of VEGF messenger ribonucleic acid. Simultaneously we observed higher levels of VEGF protein in the conditioned medium of stimulated cells compared with unstimulated ones. Therefore, insulin could contribute to the increased risk of endometrial carcinoma due to its ability to induce VEGF expression and thus participate in the maintenance of an angiogenic phenotype.

Cell-associated insulin-like growth factor-binding proteins inhibit insulin-like growth factor-I-induced endometrial cancer cell proliferation.

Insulin-like growth factor I (IGF-I) is a peptidic growth factor implicated in the proliferation of a wide variety of cell types, and especially endometrial epithelial cells. Its action is modulated by the presence of IGF-binding proteins (IGFBPs) which are secreted by IGF-I target cells. The partition of IGFBPs between cell-associated and soluble form determines the potentiation or the inhibition of IGF-I action. It is commonly accepted that cell-associated IGFBPs potentiate the IGF-I action while the soluble form of IGFBPs has an inhibitory effect. In endometrial adenocarcinoma, IGF-I is involved in tumoral progression and IGFBPs may be key modulators of the IGF-I-induced cell proliferation. Here we showed that the responsiveness of human endometrial adenocarcinoma cells (HEC-IA cell line) to the mitogenic activity of IGF-I was dependent on the pre-incubation conditions. This responsiveness to IGF-I was conditioned by a differential expression of the IGF system components (IGFBPs and IGF-I receptor) and particularly of the IGFBPs. Indeed, the IGF-I-induced proliferation of the HEC-1A cells was attenuated by the presence of cell-associated IGFBPs. Moreover, the IGF-I incubation induced a release of IGFBP-3 in the culture media as the consequence of an interaction between IGF-I and the cell-associated IGFBP-3. This effect was dose-dependent and was associated with the attenuation of the IGF-I action on cellular proliferation. Thus, IGFBP-3 might be initially expressed as a cell-associated form and then released in the interstitial fluid after a direct interaction with IGF-I. Therefore, in HEC-IA endometrial adenocarcinoma cells responsive to IGF-I, the IGFBP-3 is the main binding protein expressed and both soluble and cell-associated forms act as inhibitors of IGF-I-induced cellular proliferation.

Regulation of vascular endothelial growth factor expression by insulin-like growth factor-I in endometrial adenocarcinoma cells.

Angiogenesis is crucial for tumor growth and dissemination. Vascular endothelial growth factor (VEGF) is a potent angiogenic factor that promotes endothelial cell proliferation and chemotaxis. VEGF occurs as 5 isoforms, as a result of an alternatively spliced transcript that originates from one gene, of which the 2 majors are the VEGF 121 and 165 isoforms. Our aim was firstly to determine the role of Insulin-like Growth Factor-I (IGF-I) in the regulation of VEGF expression in endometrial adenocarcinoma cells and then the mechanism by which this regulation occurs. IGF-I treatment of HEC-1A cells provoked an increase of VEGF mRNA expression that peaked at 48 hr with a 165 isoform mRNA more abundant than the 121 isoform. The IGF-I action was confirmed at the protein level, whose concentration was increased in the conditioned media. In experiments using transient transfection of VEGF promoter-luciferase constructs, the IGF-I failed to increase the activity of the VEGF promoter after a 24-hr period of IGF-I treatment, while the addition of Actinomycin D showed an increase of the VEGF mRNA half-life. Most interestingly, Northern blot analysis showed a different stability of the 2 major VEGF isoform mRNAs (VEGF 121 and 165), of which the 121 isoform was more stable than the 165 isoform. The IGF-I treatment prolonged the half-life of both of the VEGF isoform mRNAs. Our results suggest that IGF-I regulates VEGF expression in endometrial adenocarcinoma cells at the post-transcriptional level by enhancing the stabilization of the 2 major VEGF isoform mRNAs (VEGF(121) and VEGF(165)). In addition to its proliferative functions, IGF-I induces VEGF expression and participates in the maintenance of an angiogenic phenotype.