Obesity has increased globally in epidemic proportions and as have the associated disorders. Insulin resistance that could further lead to type 2 diabetes is a major obesity associated dysfunction. Studies using insulin resistant mouse models and observations from human subjects exhibiting insulin resistance provide evidence for ectopic lipid deposition in organs like liver, muscle and heart as one of the major risk factors for developing insulin resistance. These observations suggest that deregulated adipose function to sequester and store excess energy as fat, could lead to insulin resistance. Furthermore, several studies have demonstrated adipose tissue dysfunction leading to inflammation and related syndromes. Interestingly, a mouse model with transgenic expression of glucose transporter in the adipose tissue exhibited improved glucose tolerance and increased insulin sensitivity despite development of obesity, upon high fat feeding. Thus mechanisms that improve adipose function could alleviate insulin resistance and associated diseases.
Mitogen activated protein kinase kinase kinase kinase 4 (MAP4K4) was identified in our laboratory as a negative regulator of adipocyte function. Interestingly, siRNA mediated knockdown of MAP4K4 promoted PPARγ protein expression. Additionally, silencing of MAP4K4 increased adipocyte triglyceride content. Because MAP4K4 is a negative regulator of PPARγ expression and adipocyte function, understanding the mechanism by which MAP4K4 regulates PPARγ expression is of interest. Thus, for the first part of this thesis, I characterized the signaling pathways utilized by MAP4K4 to regulate PPARγ expression in cultured adipocytes. Here I show that MAP4K4 regulates PPARγ expression through regulation of its protein translation. siRNA mediated MAP4K4 gene silencing stimulated PPARγ protein synthesis without changing its mRNA transcription or its protein degradation. This increase in PPARγ protein translation was due to an increase in the activity of mammalian target of rapamycin (mTOR). The increase in PPARγ protein expression mediated by mTOR activation was a specific effect of the 4E-BP1 phosphorylation that leads to its inactivation and was not a general increase in mTOR activity towards all of its substrates. Finally, adenovirus mediated over expression of MAP4K4 inhibited mTOR activation, and suppressed PPARγ protein translation.
For the second part of this thesis, I assessed the role of MAP4K4 in adipocytes in vivo. To accomplish this, a lentivirus mediated shRNA construct was generated to attenuate MAP4K4 expression selectively in the mouse adipose tissue. First we demonstrate that the MAP4K4 shRNA construct is able to efficiently silence the expression of MAP4K4 in vitro when co-expressed with Cre recombinase. Furthermore, we show that following modification of the lentiviral conditional vector that was introduced into a mouse embryo at one cell stage, and crossing the resulting founders with aP2-Cre mice, adipose tissue specific MAP4K4 gene silencing was achieved. Moreover, shRNA mediated gene silencing is a faster and an inexpensive means of achieving tissue specific gene knockdown relative to the available traditional gene knockout approaches.
Utilizing these adipose specific MAP4K4 gene knockdown mice, I reveal that MAP4K4 silencing enhanced fat mass as well as PPARγ expression significantly. This is accompanied by improved whole body insulin sensitivity. Furthermore, when challenged with high fat diet, adipose-specific MAP4K4 silenced mice exhibit enhanced adiposity with decreased lean mass. Moreover, adipocyte cell size and triglyceride content are significantly increased. Interestingly, despite increased adiposity, hepatic insulin sensitivity is significantly improved leading to decreased glucose output. Thus MAP4K4 is an important regulator of adipocyte function that mediates whole body glucose homeostasis, through a mechanism that is yet to be identified
