Early Hepatic Insulin Resistance Precedes the Onset of Diabetes in Obese C57BLKS-db/db Mice

OBJECTIVE

To identify metabolic derangements contributing to diabetes susceptibility in the leptin receptor–deficient obese C57BLKS/J-db/db (BKS-db) mouse strain.

RESEARCH DESIGN AND METHODS

Young BKS-db mice were used to identify metabolic pathways contributing to the development of diabetes. Using the diabetes-resistant B6-db strain as a comparison, in vivo and in vitro approaches were applied to identify metabolic and molecular differences between the two strains.

RESULTS

Despite higher plasma insulin levels, BKS-db mice exhibit lower lipogenic gene expression, rate of lipogenesis, hepatic triglyceride and glycogen content, and impaired insulin suppression of gluconeogenic genes. Hepatic insulin receptor substrate (IRS)-1 and IRS-2 expression and insulin-stimulated Akt-phosphorylation are decreased in BKS-db primary hepatocytes. Hyperinsulinemic-euglycemic clamp studies indicate that in contrast to hepatic insulin resistance, skeletal muscle is more insulin sensitive in BKS-db than in B6-db mice. We also demonstrate that elevated plasma triglyceride levels in BKS-db mice are associated with reduced triglyceride clearance due to lower lipase activities.

CONCLUSIONS

Our study demonstrates the presence of metabolic derangements in BKS-db before the onset of β-cell failure and identifies early hepatic insulin resistance as a component of the BKS-db phenotype. We propose that defects in hepatic insulin signaling contribute to the development of diabetes in the BKS-db mouse strain.Established in the 1940s, the C57BLKS (BKS) inbred mouse strain represents one of the first animal models of type 2 diabetes (1). Development of diabetes in these mice captures several aspects of the human disease (2,3). First, diabetes in this model is associated with obesity. Whereas lean BKS mice are normoglycemic throughout their life, obese leptin-deficient (BKS-ob) or leptin receptor–deficient (BKS-db) mice develop severe hyperglycemia. Second, the natural history of diabetes in BKS-ob or BKS-db is reminiscent of the human disease. These mice initially compensate for obesity-associated insulin resistance by increasing plasma insulin levels, but exhibit β-cell failure and insulin deficiency later in life. Finally, similarly to humans, diabetes in BKS-db is determined by multiple genetic factors (4,5). Despite extensive genetic analysis, the genes responsible for diabetes susceptibility in the BKS strain remain to be identified (6–8).Early studies on BKS-db mice indicated that the development of diabetes is associated with progressive β-cell degranulation and a precipitous decrease in β-cell mass and plasma insulin levels (2). In vivo radio-labeling studies revealed that after an initial phase of hyperproliferation at 4–6 weeks of age, the replication of β-cells gradually decreases despite increasing glucose levels (9). In contrast to BKS, introduction of the db mutation into the C57BL/6J (B6) genetic background produces a dramatically different β-cell phenotype (2,4). Although similarly obese as BKS-db, B6-db mice compensate for insulin resistance by β-cell hyperplasia, increased islet mass, and hyperinsulinemia and maintain only mildly elevated blood glucose levels throughout their life. The markedly different β-cell responses to obesity in BKS-db and B6-db mice suggest that genetically determined variation in β-cell viability/survival in the face of chronic glycemic stress is responsible for differences in diabetes susceptibility between the two strains. Consistent with this hypothesis, BKS β-cells are more sensitive than B6 to cell death triggered by β-cell toxins, such as alloxan and streptozotocin (10–12), and glucose-stimulated islet cell replication is diminished in BKS (13). In conclusion, previous studies suggest that variant β-cell functions underlie the differences in diabetes susceptibility between BKS-db and B6-db mice.In the current study, we refine the current β-cell–centric model of diabetes susceptibility in BKS-db by demonstrating metabolic defects preceding the onset of β-cell failure. In particular, BKS-db mice exhibit elevated hepatic insulin resistance associated with altered lipogenic and gluconeogenic pathways relative to B6-db. We propose that early hepatic insulin resistance contributes to the development of diabetes in the BKS-db strain.