Defective STAT signaling by the leptin receptor in diabetic mice.

Abstract

Leptin and its receptor, obese receptor (OB-R), comprise an important signaling system for the regulation of body weight. Splice variants of OB-R mRNA encode proteins that differ in the length of their cytoplasmic domains. We cloned a long isoform of the wild-type leptin receptor that is preferentially expressed in the hypothalamus and show that it can activate signal transducers and activators of transcription (STAT)-3, and . A point mutation within the OB-R gene of diabetic (db) mice generates a new splice donor site that dramatically reduces expression of this long isoform in homozygous db/db mice. In contrast, an OB-R protein with a shorter cytoplasmic domain is present in both db/db and wild-type mice. We show that this short isoform is unable to activate the STAT pathway. These data provide further evidence that the mutation in OB-R causes the db/db phenotype and identify three STAT proteins as potential mediators of the anti-obesity effects of leptin.Leptin, the product of the obese (ob) gene, is a 16-kDa secreted protein primarily produced by adipocytes (1). There is a good correlation between the percentage of body fat and serum leptin levels suggesting that leptin production is regulated by the mass of adipocytes (2, 3). Leptin levels were normal or elevated in obese individuals (2, 4) arguing against a simple leptin deficiency as the cause of obesity in the majority of humans (5). Serum leptin concentrations increased under a fatty diet but failed to prevent weight gain (3). Therefore, insensitivity to the action of leptin appears to be a common mechanism in obese individuals and in several rodent models. This suggests that dysregulation at the level of the leptin receptor, the downstream signaling pathway, or an unknown modifying mechanism may constitute the basis for weight disorders. The crucial role of leptin and its receptor in obesity is well illustrated by two phenotypically very similar mutants obese (ob) and diabetes (db) (6). Mice homozygous for a loss of function mutation of ob display obesity, hyperglycemia, and insulin resistance resembling type II diabetes. Administration of recombinant leptin to ob mice corrected these abnormalities (7-9). Based on early parabiosis experiments it was expected that db would be caused by a mutation in the ob receptor (OB-R) (6).OB-R was cloned by virtue of its high affinity to leptin through an expression cloning strategy (10). The OB-R gene was mapped to the same 5-centimorgan interval on mouse chromosome 4 to which db had been localized (10). Surprisingly, no mutation in the coding region of OB-R cDNA of db/db mice was found and leptin binding sites were unaltered in db/db mice (10). However, the cloned mouse OB-R cDNA encoded a protein with a much shorter cytoplasmic domain than the human homologue, suggesting that a longer mouse isoform exists. We cloned this longer form of OB-R from wild-type mice and found that the mRNA for this isoform is dramatically reduced in db/db mice. A G to T mutation in db mice generates a new splice donor and su...

Obese gene expression at in vivo levels by fat pads derived from s.c. implanted 3T3-F442A preadipocytes

“…Cloning of the leptin receptor gene and characterization of its RNA transcripts revealed multiple splice variants (2,3), one of which encodes a receptor isoform that transmits its ''signal'' via the JAK͞ STAT system (4). This, along with earlier evidence (5), indicates that the interaction of leptin with receptors of this type located in the hypothalamus triggers a response that leads to appetite suppression and increased energy expenditure (5).…”

Lack of starvation-induced activation of AMP-activated protein kinase in the hypothalamus of the Lou/C rats resistant to obesity

“…This could be due to upregulation of ObRb gene expression in this condition (Table 3), confirming previous reports showing that the leptin-mediated phosphorylation of STAT3 is specifically dependent on this isoform of leptin receptor. 34,35 More importantly, while circulating leptin and STAT3 phosphorylation were clearly decreased by starvation in Wistar rats, they remained unaffected in starved Lou/C rats, so that STAT3 phosphorylation was almost 50% higher in Lou/C Figure 4 Proposed mechanism for the lack of starvation-induced hypothalamic AMPK activation and hypophagia in Lou/C rats. Starvation fails to activate AMPK in the hypothalamus of the Lou/C rats (right panel) in contrast with Wistar rats (left panel).…”

The biology of Stat4 and Stat6

“…PDGF was also able to activate Stat6 in NIH3T3 cells and the addition of IL-4 could potentiate this e ect (Patel et al, 1996). Additionally, leptin was able to activate several STAT proteins, including Stat6, through the long form of the ob receptor transfected into COS cells (Ghilardi et al, 1996). It is unclear at this point what role Stat6 is playing in these alternative pathways, and experiments using Stat6-de®cient mice have not yet revealed a clear biological role for these observations.…”