Leptin-dependent Phosphorylation of PTEN Mediates Actin Restructuring and Activation of ATP-sensitive K+ Channels

Abstract

Leptin activates multiple signaling pathways in cells, including the phosphatidylinositol 3-kinase pathway, indicating a degree of cross-talk with insulin signaling. The exact mechanisms by which leptin alters this signaling pathway and how it relates to functional outputs are unclear at present. A previous study has established that leptin inhibits the activity of the phosphatase PTEN (phosphatase and tensin homolog deleted on chromosome 10), an important tumor suppressor and modifier of phosphoinositide signaling. In this study we demonstrate that leptin phosphorylates multiple sites on the C-terminal tail of PTEN in hypothalamic and pancreatic ␤-cells, an action not replicated by insulin. Inhibitors of the protein kinases CK2 and glycogen synthase kinase 3 (GSK3) block leptin-mediated PTEN phosphorylation. PTEN phosphorylation mutants reveal the critical role these sites play in transmission of the leptin signal to F-actin depolymerization. CK2 and GSK3 inhibitors also prevent leptinmediated F-actin depolymerization and consequent ATP-sensitive K ؉ channel opening. GSK3 kinase activity is inhibited by insulin but not leptin in hypothalamic cells. Both hormones increase N-terminal GSK3 serine phosphorylation, but in hypothalamic cells this action of leptin is transient. Leptin, not insulin, increases GSK3 tyrosine phosphorylation in both cell types. These results demonstrate a significant role for PTEN in leptin signal transmission and identify GSK3 as a potential important signaling node contributing to divergent outputs for these hormones.Efficient signaling by leptin and insulin is essential for the maintenance of body energy homeostasis, with disruptions in these processes strongly associated with diabetes and obesity (1, 2) and, at least for insulin, neurodegenerative disorders such as Alzheimer disease (3, 4). In recent years there has been a significant increase in understanding the intracellular signaling processes associated with the actions of insulin on a wide variety of cell types (5). However, our knowledge of leptin signaling is less advanced, with most studies indicating that leptin and insulin share many signaling intermediates in common, often leading to similar cellular outcomes (6, 7). In particular, signaling through the STAT (signal transducers and activators of transcription), mitogen-activated protein kinase, and PI3K 3 pathways have been reported extensively in numerous cell types for both leptin and insulin (5, 8).Nevertheless, leptin and insulin can cause differing and sometimes opposing cellular outputs, even on the same cell type. This is demonstrated in hypothalamic neurons, where electrophysiological or imaging studies show differential outcomes for leptin and insulin action (9 -11). Thus, although superficially leptin may utilize the same signaling pathways as insulin, the exact nature of the leptin-induced signaling intermediates and their interplay with one another and with individual effectors is still relatively unknown. Recently, it was demonstrated that although leptin, lik...

Leptin Elongates Hypothalamic Neuronal Cilia via Transcriptional Regulation and Actin Destabilization

“…Thus, PTEN and GSK3␤ seem to serve as downstream signals for leptin-caused F-actin rearrangement. In agreement with our results, leptin treatment also causes F-actin depolymerization via PTEN and GSK3␤ in hypothalamic neuronal cells (16,22). PTEN directly regulates actin-depolymerizing factor cofilin-1 in alveolar macrophages (42).…”

“…In our previous study, leptin treatment augmented the expression of phosphorylated PTEN and GSK3␤ in hypothalamic neuronal cells (16), which led to repressed PTEN-GSK3␤ signaling. Moreover, the study showed that leptin treatment increased ciliary length by inhibiting the PTEN-GSK3␤ pathway.…”

Leptin Regulates KATP Channel Trafficking in Pancreatic β-Cells by a Signaling Mechanism Involving AMP-activated Protein Kinase (AMPK) and cAMP-dependent Protein Kinase (PKA)

“…A number of signaling molecules, including PI3K and tyrosine kinases, were implicated in early studies (14,16), although their roles were not further substantiated. Subsequent studies show that leptin causes inhibition of the lipid/protein phosphatase PTEN, accumulation of PIP 3 , and F-actin depolymerization, events that correlate with increased K ATP conductance (15,18,19). Despite these efforts, the molecular and cellular events following leptin stimulation that lead to increased K ATP activity remain incompletely understood.…”

Leptin regulation of hippocampal synaptic function in health and disease

“…Consequently, leptin-driven inhibition of PTEN would also result in elevated PtdIns(3,4,5,)P 3 levels. In support of a possible role for PTEN, leptin activation of hypothalamic K ATP channels reportedly involves inhibition of PTEN [38]. Similarly in hippocampal neurons, expression of dominant-negative PTEN mutants (C124S or G129E) not only mirrored but also occluded the leptin-driven increase in GluA1 surface expression, suggesting involvement of PTEN inhibition in this process.…”