Critical role of STAT3 in leptin's metabolic actions

Abstract

Leptin has pleiotropic effects on glucose homeostasis and feeding behavior. Here, we validate the use of a cell-permeable phosphopeptide that blocks STAT3 activation in vivo. The combination of this biochemical approach with stereotaxic surgical techniques allowed us to pinpoint the contribution of hypothalamic STAT3 to the acute effects of leptin on food intake and glucose homeostasis. Leptin's ability to acutely reduce food intake critically depends on intact STAT3 signaling. Likewise, hypothalamic signaling of leptin through STAT3 is required for the acute effects of leptin on liver glucose fluxes. Lifelong obliteration of STAT3 signaling via the leptin receptor in mice (s/s mice) results in severe hepatic insulin resistance that is comparable to that observed in db/db mice, devoid of leptin receptor signaling. Our results demonstrate that the activation of the hypothalamic STAT3 pathway is an absolute requirement for the effects of leptin on food intake and hepatic glucose metabolism.

Figure 1

A cell-permeable SH2 domain binding phosphopeptide inhibits leptin-induced STAT3 Y705 phosphorylation in the arcuate nucleus A) Mechanism by which the STAT3 PI inhibits STAT3 activation: upon phosphorylation of Y705, STAT3 forms homodimers and translocates into the nucleus. The phosphopeptides bind to the SH2 domain of STAT3 and inhibit the recruitment of STAT3 to Jak2 and the phosphorylation of Y705. This in turn leads to inhibition of STAT3 dimerization, nuclear translocation, and transcriptional activity. Nuclear STAT3 rapidly promotes the transcription of SOCS3. B and C) In vivo validation of the STAT3 PI. Rats received first either vehicle or the Stat3 PI (75 pmol) as indicated. Thirty minutes later, rats received either 2.5 μg of recombinant leptin ± the STAT3 PI or the control peptide (75 pmol). Thirty minutes after leptin administration, rats were sacrificed and the arcuate nucleus of the hypothalamus was isolated by punch biopsy, homogenized, and analyzed by Western blot (B) and quantified by imagequant software. Leptin-induced STAT3 phosphorylation was prevented by coadministration of the STAT3 PI. Notably, the pre- and coadministration of the STAT3 PI does not suppress leptin-induced activation of Erk1/2. Administration of the STAT3 PI was able to inhibit STAT3 activation up to 180 min (C) after leptin administration. D) mRNA levels of SOCS3 in the MBH 90 min after ICV injection of vehicle, leptin, or leptin plus the STAT3 PI. Leptin induces a >3-fold increase in SOCS3 mRNA levels that is prevented once the STAT3 pathway of LRb signaling is inhibited. p = 0.11 for the vehicle-treated group compared to leptin plus STAT3 PI. *p < 0.05 versus the ICV vehicle-treated group.

Figure 2

Activation of STAT3 is required for leptin’s anorectic effect A) Experimental protocol. On day 0, rats received a single ICV injection of vehicle, 1.5 μg of leptin, 75 pmol of the STAT3 PI, or 1.5 μg of leptin plus 75 pmol of the STAT3 PI. Daily food intake and body weight were measured for 3 days. B) Food intake at 3 hr and 1–3 days after ICV administration of the indicated substances. C) Changes in food intake from baseline. D) Daily body weight during the indicated treatments. *p < 0.05 versus the ICV vehicle-treated group.

Figure 3

Activation of STAT3 is required for leptin-dependent restoration of LH surges in starved female rats A) Experimental protocol. Rats underwent ovariectomy and placement of the ICV cannula on day 1. After 7 days of recovery, vascular catheters were implanted and food was removed to begin a 3 day fast. On day 8 and 9 rats were primed with estradiol, and on day 10 they were injected with progesterone to trigger a LH surge. B) Peak LH value after progesterone injection. C) Serial measurements of LH levels from individual animals treated with ICV vehicle (upper panel), 2.5 μg of leptin (middle panel), and leptin (2.5 μg) plus the STAT3 PI (75 pmol) (lower panel).

Figure 4

Activation of STAT3 is required for leptin regulation of glucose production A) To acutely inhibit the STAT3 pathway of LRb signaling, we employed two approaches, the STAT3 PI and the adenovirally mediated expression of a dominant-negative mutant of STAT3 (STAT3DN). B) Experimental protocol for the pancreatic basal insulin clamp studies in combination with ICV leptin or vehicle infusions. This protocol is common to experiments reported here with the ICV infusion of STAT3 PI and to those in rats with MBH administration of STAT3DN adenovirus (Figure S1). C) Central administration of leptin markedly increases the rate of glucose infusion that is required to prevent hypoglycemia. This effect is negated by the central coadministration of the STAT3 PI. D) Rate of glucose production during the clamp studies. E) Glucose uptake. *p < 0.05 versus the ICV vehicle-treated group.

Figure 5

Activation of STAT3 is required for leptin regulation of liver glucose fluxes A) Flux through G6Pase. B) Glucose cycling. C) Rate of glycogenolysis. D) Rate of gluconeogenesis. E) G6Pase mRNA. F) PEPCK mRNA expression in livers at the end of the clamp procedure. *p < 0.05 versus the ICV vehicle-treated group.

Figure 6

Glucose homeostasis in s/s and db/db mice A) Experimental design for the clamp studies in wild-type (WT), db/db, and s/s mice. All mice received ~3.8 grams of chow from day 28 until clamp study. B) Glucose infusion rate during the clamp studies in WT, s/s, and db/db mice. C) Glucose production. D) Glucose uptake during the clamp in WT, s/s, and db/db mice.