Over 29 million people in the United States have diabetes, and 6 million take injectable insulin. Although there are drugs available for treatment in the early stages of the disease, none these agents prevent the progression to insulin dependence. Thus new therapies are needed for this disease. Insulin stimulates insulin receptor (IR) metabolic insulin signaling via the PI3K/Akt pathway which lowers bloods glucose, but the activation of IR can also signal via the more mitogenic MAPK (ERK1/2) pathway which can lead to various complications or even cancer. Novel IR agonists, such as monoclonal antibodies, have the potential to stimulate insulin receptor activation that is both signaling pathway and tissue specific. To further understand the potential benefits and risks of various insulin receptor agonists with selective activation properties, this dissertation will review the roles and importance of different aspects of insulin signaling in various tissues.
It has been proposed that monoclonal antibodies may become therapeutics for metabolic diseases such as diabetes mellitus. We have characterized XMetA, an allosteric monoclonal antibody to the human IR, that activates metabolic signaling. In vivo, XMetA chronically reduced fasting blood glucose levels in two hyperglycemic mouse models. In the present study we compared the acute effects of XMetA on insulin activity in three of the major insulin-sensitive tissues of the normal fasted mouse: liver, adipose, and muscle. XMetA, like insulin, lowered blood glucose levels, although its effect was less rapid than the hormone. As expected, insulin activated IR autophosphorylation and Akt phosphorylation in liver, fat and muscle. In contrast, activation of IR signaling by XMetA was only observed in the liver. These studies suggest that glucose regulation in a mice by XMetA is largely mediated by action at the liver.
XMetA activates the "metabolic" Akt kinase signaling pathway while having little effect on the "mitogenic" ERK signaling pathway. To investigate the nature of this selective signaling, we conducted a detailed investigation of XMetA, in order to evaluate specific phosphorylation and activation of the IR, Akt, and ERK in cell lines expressing either the short (IR-A) or long (IR-B) isoform of the human IR. Insulin activated both pathways, but the phosphorylation of Akt was more sensitive to the hormone than the phosphorylation of ERK. Maximally effective concentrations of XMetA elicited phosphorylation patterns similar to 40-100 pM insulin, which were sufficient for robust Akt phosphorylation but had little effect on ERK phosphorylation. These data indicate that the "preferential signaling" of XMetA is due to an innate difference in pathway sensitivity of Akt vs. ERK responses to IR activation and partial agonism by XMetA, rather than a separate pathway-biased mechanism.
Our studies, therefore indicated that XMetA has preferential activation in liver and predominately activates the Akt signaling pathway. Thus, XMetA has the potential to become a new and valuable therapy for type 2 diabetes mellitus.
|Advisor:||Adams, Sean H.|
|Commitee:||Goldfine, Ira D., Haj, Fawaz G.|
|School:||University of California, Davis|
|Department:||Molecular, Cellular and Integrative Physiology|
|School Location:||United States -- California|
|Source:||DAI-B 76/10(E), Dissertation Abstracts International|
|Keywords:||Akt, ERK, Insulin, Insulin receptor, Monoclonal antibody, Signalling bias|
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