Insulin resistance is the common factor among metabolic diseases that include obesity, type 2 diabetes and steatohepatitis. Hyperinsulinemia is an early indicator of insulin resistance. Although in many instances hyperinsulinemia reflects impaired insulin response, considerable data support the alternative view that hyperinsulinemia causes insulin resistance. Studies supported by this grant have pioneered the biochemical, physiological, and genetic identification of a CEACAM1-dependent signaling pathway that regulates hepatic insulin clearance. We have shown that CEACAM1 regulates peripheral insulin action by promoting hepatic insulin clearance. Using mice with liver-specific inactivation or global null mutation of Ceacam1, we have shown that impairment of insulin extraction causes chronic hyperinsulinemia and leads to insulin resistance by down-regulating insulin receptors and promoting de novo lipogenesis in liver. Based on preliminary data that underscore the central role of CEACAM1 in pathways regulating insulin action, lipid synthesis, and inflammation, we now expand our work to investigate the cause-effect relationship between hyperinsulinemia and insulin resistance in the context of diet-induced obesity. We show that loss of hepatic CEACAM1 is associated with human and rodent obesity, and is an early correlate of diet-induced obesity in mice. Thus, feeding mice a high-fat diet reduces hepatic CEACAM1 levels by >50% to cause hyperinsulinemia and insulin resistance. Conversely, Ceacam1 overexpression in liver protects mice against insulin resistance and visceral obesity. This suggests that hyperinsulinemia plays a detrimental role in the pathogenesis of diet-induced obesity and insulin resistance. We now propose to test the hypothesis that reduction in hepatic CEACAM1 underlies the causative role of hyperinsulinemia in diet-induced insulin resistance. To this end, we will in Aim 1, investigate whether preventing hyperinsulinemia curbs diet-induced insulin resistance in a model of hepatic Ceacam1 gain-of-function, and we will systematically dissect which aspect of the metabolic syndrome (insulin resistance, dyslipidemia, hepatosteatosis, and inflammation) is prevented by the sustained expression of CEACAM1.
In Aim 2, we will investigate the mechanism by which free fatty acids acting through PPARa cause transcriptional repression of Ceacam1.
In Aim 3, we will investigate whether abolishing Ceacam1 regulation by PPARa preserves CEACAM1 expression, and protects against diet-induced insulin resistance. We will generate knock-in mice in which the PPARa response element in the Ceacam1 promoter has been mutated, and test whether this genetic manipulation confers protection against insulin resistance induced by high-fat diet. This should provide a critical test of the causative effect of hyperinsulinemia on insulin resistance, and identify CEACAM1 as a tractable drug target for the development of medications to combat altered metabolic conditions.
Metabolic diseases, including type 2 diabetes, steatohepatitis, and obesity are a growing public health concern in the US and worldwide. Insulin resistance is a key factor in the etiology of these diseases, and it is commonly heralded by a rise in plasma insulin levels, or hyperinsulinemia. This proposal seeks to determine the cause- effect relationship between insulin resistance and hyperinsulinemia. In the current funding period, we have pioneered the biochemical, physiological, and genetic identification of CEACAM1 as a key regulator of hepatic insulin clearance and plasma insulin levels. We have shown that disturbance of this pathway causes hyperinsulinemia and insulin resistance. We now aim to test the hypothesis that reduction in hepatic CEACAM1 constitutes an early, and fully preventable mechanism of diet-induced insulin resistance. To this end, we will use cellular and transgenic animal studies. The combined results of these experiments should provide a critical test of the idea that hyperinsulinemia can be a cause of insulin resistance, as opposed to an early consequence thereof, and identify CEACAM1 as a tractable drug target for the development of medications to combat this metabolic condition.
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