Endocrine Abstracts

Objective: Insulin-like growth factor-II (IGF-II), a critical regulator of placental development, enhances proliferation and survival of human cytotrophoblasts by signalling through the IGF-1 receptor (IGF-1R). Excess IGF-II binds to IGF-2R, which mediates its transport to the lysosomes for degradation. Although considered to act solely as an IGF-II clearance receptor, IGF-2R has been implicated in mediating IGF-II-stimulated migration and invasion in trophoblast. We therefore investigated whether IGF-2R activates pathways regulating trophoblast cell turnover.

Methods: The trophoblast cell line BeWo was transfected with a siRNA sequence specific for IGF-2R or with control sequences; IGF-2R knockdown was validated by qPCR, immunohistochemistry and western blotting. Forty eight hours post-transfection, serum-starved BeWo were stimulated with IGF-I, IGF-II or the IGF-II analogue Leu²⁷IGF-II, which only binds to IGF-2R. Proliferation was assessed by immunostaining for Ki67 after 24 h. Apoptosis and necrosis were quantified by TUNEL, caspase-3/-7 activity assay and by measuring lactate dehydrogenase release after 48 h.

Results: In control cells, all IGF treatments increased the number of Ki67-positive cells (P<0.001; 2-way ANOVA); however, Leu²⁷IGF-II exhibited no mitogenic effect on cells with reduced IGF-2R expression. Similarly, all IGF treatments decreased TUNEL, caspase-3/-7 activity and lactate dehydrogenase release in control cells (P<0.001), but Leu²⁷IGF-II offered no protective effect to cells with reduced IGF-2R expression. Western blotting confirmed that Leu²⁷IGF-II did not significantly increase IGF-1R phosphorylation, indicating low affinity for this receptor. IGF-2R knockdown also significantly enhanced IGF-II-mediated rescue of apoptosis (P<0.001), and increased IGF-II-stimulated mitosis (P<0.01), confirming the role of IGF-2R as a regulator of IGF-II bioavailability.

Conclusions: The mitogenic and pro-survival effects of Leu²⁷IGF-II were only observed when BeWo expressed high levels of IGF-2R, suggesting that activation of IGF-2R can promote growth and survival of trophoblasts. Development of interventions that decrease IGF-2R expression in the placenta may provide a method for treating pregnancies complicated by growth restriction.