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Published on: November 28, 2013
Leptin signaling in the hypothalamus of normal rats in vivo
K C McCowen1, J C Chow, R J Smith
1Joslin Diabetes Center, Harvard Medical School, Boston, Massachusetts 02215, USA.
Abstract:
Leptin has been shown to activate multiple signaling molecules in cultured cells, including Janus kinase-2, STAT (signal transducer and activator of transcription) proteins, and mitogen-activated protein kinase, and to stimulate the DNA-binding activity of STAT3 in mouse hypothalamus. In this study, the activation of candidate leptin signaling molecules in the hypothalamus of normal rats in vivo was investigated. Fasted male Sprague-Dawley rats were injected iv with recombinant murine leptin or vehicle. Plasma leptin concentrations were determined at defined time points, and the phosphorylation of signaling proteins was assessed in hypothalamic lysates. There was a marked increase in plasma leptin concentration at 2 min and a gradual decline by 45 min after leptin injection. Immunoblotting analysis of hypothalamic lysates with a phosphospecific STAT3 antibody demonstrated a time-dependent stimulation of STAT3 tyrosine phosphorylation. STAT3 phosphorylation was first evident at 5 min and was maximal at 30 min after leptin injection. By contrast, leptin did not increase the phosphorylation of Janus kinase proteins, mitogen-activated protein kinase, or STAT1 and -5 despite abundant expression of these signaling molecules in the hypothalamus. These results differ from findings in cultured cells and in vitro systems. It remains unclear how signaling is propagated downstream from the leptin receptor to STAT3, but this may involve novel signaling intermediates.
Insights
Leptin injection in rats activates STAT3 phosphorylation in the hypothalamus, but not other signaling molecules like JAK or MAPK. This suggests unique in vivo leptin signaling pathways distinct from cell culture findings.
Area of Science:
- Neuroendocrinology
- Molecular signaling
- Physiology
Background:
- Leptin regulates appetite and metabolism.
- Leptin signaling involves Janus kinase-2 (JAK) and signal transducer and activator of transcription (STAT) proteins.
- Previous studies indicated leptin activates multiple signaling molecules in cultured cells.
Purpose of the Study:
- To investigate the in vivo activation of leptin signaling molecules in the rat hypothalamus.
- To compare in vivo findings with in vitro observations.
Main Methods:
- Male Sprague-Dawley rats were intravenously injected with leptin or vehicle.
- Plasma leptin concentrations were measured.
- Hypothalamic protein phosphorylation was assessed using immunoblotting with phosphospecific antibodies.
Main Results:
- Leptin injection rapidly increased plasma leptin levels.
- STAT3 tyrosine phosphorylation in the hypothalamus was time-dependently stimulated by leptin.
- Leptin did not affect the phosphorylation of JAK, mitogen-activated protein kinase, STAT1, or STAT5.
Conclusions:
- Leptin activates STAT3 signaling in the rat hypothalamus in vivo.
- The observed in vivo signaling differs from in vitro findings, suggesting novel signaling intermediates may be involved in leptin receptor signal propagation to STAT3.
