A role of phosphodiesterase-3B pathway in mediating leptin action on proopiomelanocortin and neurotensin neurons in the hypothalamus

Abstract

Leptin signaling in the hypothalamus is required for normal food intake and body weight homeostasis. Recent evidence suggests that besides the signal transducer and activator of transcription-3 (STAT3) pathway, several non-STAT3 pathways mediate leptin signaling in the hypothalamus. We have previously demonstrated that leptin stimulates phosphodiesterase-3B (PDE3B) activity in the hypothalamus, and PDE3 inhibitor cilostamide reverses anorectic and body weight reducing effects of leptin. To establish the physiological role of PDE3B signaling in the hypothalamus, we examined if leptin signaling through the PDE3B pathway is responsible for the activation of proopiomelanocortin (POMC) and neurotensin (NT) neurons, which are known to play a critical role in energy homeostasis. To this end, we assessed the effect of cilostamide on leptin-induced POMC and NT gene expression in the rat hypothalamus. Results showed that while central injection of leptin significantly increased both POMC and NT mRNA levels in the medial basal hypothalamus, cilostamide completely reversed this effect of leptin suggesting a PDE3B-activation dependent induction of POMC and NT gene expression by leptin. This result further suggests that the PDE3B pathway plays an important role in mediating leptin action in the hypothalamus.

Fig. 1

Proopiomelanocortin (POMC) and neurotensin (NT) gene expression as determined by ribonuclease protection assay in the hypothalamus following intra-cerebroventricular injection of leptin alone or in combination with cilostamide (Cilost.), a selective PDE3 inhibitor. (A): representative phosphorimages showing the level of POMC mRNA, NT mRNA and β-actin mRNA in the hypothalamus. (B): results obtained by phosphor imaging showing the changes in POMC and NT mRNA levels. The values were first normalized to β-actin mRNA levels and then expressed as relative to vehicle (artificial cerebrospinal fluid + dimethyl sulfoxide) control. Values represent the mean ± SEM. Control: n = 4, leptin: n = 5, cilost. : n = 5, and leptin + cilost. : n = 7. * p < 0.05 and ** p < 0.01 vs. all other groups.

Fig. 2

Schematic of leptin signaling through STAT3 and PDE3B pathways in POMC and NT neurons in the hypothalamus. Leptin binding to it’s receptor (Ob-Rb) leads to activation of JAK2, receptor dimerization, JAK2-mediated Ob-Rb phosphorylation followed by phosphorylation and activation of STAT3. Activated STAT3 dimerizes, translocates to the nucleus, and trans-activate target genes including POMC and NT. Additionally, leptin activates PI3K and PDE3B, and decreases cAMP levels in the hypothalamus. Since PDE3 inhibition by cilostamide reverses the effect of leptin on STAT3 activation [Ref. 55] as well as leptin-induced POMC and NT gene expression (current study), it is possible that decrease in cAMP levels is necessary for STAT3 activation and subsequent stimulation of POMC and NT gene expression by leptin. Also, PDE3B-activation dependent decrease in cAMP levels may directly result in increased POMC and NT gene expression by leptin - a hypothesis need to be experimentally tested. IRS, insulin receptor substrate.