BETA-ADRENERGIC REGULATION OF ADIPOSE TISSUE FUNCTION BY PKA AND MAP KINASES - ABSTRACT
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
