Obesity is now at epidemic proportions and is a risk factor for several debilitating illnesses such as Type II diabetes, hypertension and cardiovascular disease. The ?-adrenergic receptors (?ARs) are key regulators of adipose tissue metabolism to control (i) lipolysis for mobilization of stored energy and (ii) adaptive thermogenesis in brown fat. All 3 known ?AR subtypes are expressed in adipocytes, and ?ARs are coupled to increases in intracellular cAMP and activation of cAMP-dependent protein kinase (PKA) However, besides this classic pathway, we have shown that additional signaling cascades emanate from ?ARs in adipocytes, and these include the ERK and p38 MAP kinase pathways. The goals of this proposal are to identity the signaling components between PKA and /p38 MAP kinase that convey the ?AR-mediated signaling in white and brown adipocytes, and determine the function of p38 MAPK in white adipocytes. Toward achieving these goals we have (i) identified MKK3 and p38? MAPK as necessary components of this cascade;(ii) established the transcription targets of p38;(iii) generated an adipose-specific deletion of p38? MAPK (p38??/?). Despite the absence of p38?, thermogenesis in brown fat is preserved or slightly elevated in p38??/?mice, there is selective activation of JNK in the absence of p38?, and p38??/?animals are lean even on a high-fat diet.
Three Aims are proposed for this new project period:
Aim 1 : Establish the remaining components of the signal transduction mechanism from PKA to MKK3/p38alpha MAPK. The experiments proceed through a hierarchy of candidate MKKKs to identify the one responsible for activating MKK3 and p38? MAPK in brown and white adipocytes. These experiments also test the hypotheses that a small G protein such as Rac1 is a component of this pathway, and we will identify the scaffolding protein that coordinates the MAPK, the MKK and the MKKK into a functioning unit.
Aim 2 : Determine how the ?AR signaling pathway in adipocytes lacking p38? (p38??/?) has become """"""""re-wired"""""""" to utilize JNK as the """"""""compensatory"""""""" kinase. The experiments will determine the signaling elements in p38??/? adipocytes that now allow JNK to be activated. It assesses the flexibility of the MKKK and other elements identified in Aim 1, and whether their participation is conserved in the p38??/?mouse. These experiments also determine the status of the known transcription factor targets of p38? in adipocytes (ATF-2 and PGC-1?), and whether they are now regulated by JNK The hypothesis that a key target of p38? in adipocytes is a dual-specificity MAPK phosphatases, that maintains JNK in an inactive, dephosphorylated state, is also explored.
Aim 3 : Test the hypothesis that the lean phenotype in p38??/?mice is due to increased energy expenditure. The experiments in this Aim test whether in the absence of p38?, JNK is now a more powerful activator of brown fat adaptive thermogenesis and energy expenditure, resulting in smaller white adipocytes and a lean phenotype, and several specific mechanisms are proposed. We also examine the possibility that the lean phenotype and smaller adipose depots exists at the expense of an insufficient storage capacity for fatty acids, paradoxically resulting in insulin resistance and steatosis. PHS 398/2590 (Rev. 09/04, Reissued 4/2006) Page Continuation Format Page
Collins, Sheila PROJECT NARRATIVE Obesity is now at epidemic proportions and is a risk factor for several debilitating illnesses such as Type II diabetes, hypertension and cardiovascular disease. Stored fat (adipose tissue) is arguably the chief culprit for these metabolic diseases. Presently, humans are now challenged by their constant access to calorically dense foods with minimal expenditure of physical effort, resulting in obesity and various associated co-morbidities as adaptations to these changes.
