Molecular interplay between leptin, insulin-like growth factor-1, and β-amyloid in organotypic slices from rabbit hippocampus

Background: Evidence shows that the insulin-like growth factor-1 (IGF-1) and leptin reduce beta-amyloid (A beta) production and tau phosphorylation, two major hallmarks of Alzheimer's disease (AD). IGF-1 expression involves the JAK/STAT pathway and the expression of leptin is regulated by the mammalian target of rapamycin complex 1 (mTORC1). We have previously shown that A beta reduces leptin by inhibiting the mTORC1 pathway and A beta was also suggested to inhibit the JAK/STAT pathway, potentially attenuating IGF-1 expression. As IGF-1 can activate mTORC1 and leptin can modulate JAK/STAT pathway, we determined the extent to which IGF-1 and leptin can upregulate the expression of one another and protect against A beta-induced downregulation. Results: We demonstrate that incubation of organotypic slices from adult rabbit hippocampus with A beta 42 downregulates IGF-1 expression by inhibiting JAK2/STAT5 pathway. Leptin treatment reverses these A beta 42 effects on IGF-1 and treatment with the STAT5 inhibitor completely abrogated the leptin- induced increase in IGF-1. Furthermore, EMSA and ChIP analyses revealed that leptin increases the STAT5 binding to the IGF-1 promoter. We also show that IGF-1 increases the expression of leptin and reverses the A beta 42-induced attenuation in leptin expression via the activation of mTORC1 signaling as the mTORC1 inhibitor rapamycin completely precluded the IGF-1-induced increase in leptin expression. Conclusion: Our results demonstrate for the first time that A beta 42 downregulates IGF-1 expression and that leptin and IGF-1 rescue one another from downregulation by A beta 42. Our study provides a valuable insight into the leptin/IGF-1/A beta interplay that may be relevant to the pathophysiology of AD.