James L. Kirkland, M.D., Ph.D.
The major focus of the laboratory is the impact of age and fat depot origin on adipose tissue function. Fat tissue turns over with fat cells being replaced every 3 to 6 months by differentiation of preadipocytes, which account for 15 to 50 percent of cells in fat. In regard to aging, we found that accumulation of lipid, increases in lipogenic enzyme activities, and changes in differentiation-dependent gene expression are blunted during differentiation of preadipocytes cultured from fat depots of old compared to young individuals, even after many cell generations ex vivo. Expression of C/EBP-alpha and PPAR-gamma, key transcription factors in the preadipocyte differentiation program, decline with donor age. Overexpression of C/EBP-alpha, or PPAR-gamma in preadipocytes from old animals restores capacity to differentiate. C/EBP-beta is upstream of C/EBP-alpha and PPAR-gamma in the adipogenesis cascade. While expression of C/EBP-beta does not change with age, expression of the dominant negative inhibitory LIP isoform of C/EBP-beta, which reduces expression of C/EBP-alpha and PPAR-gamma, increases. This is due to increased expression of CUG triplet repeat binding protein (CUGBP) that binds to CC/EBP-beta mRNA, causing translation of LIP. Expression of CHOP, another dominant inhibitor of differentiation increases with aging, as does preadipocyte TNF-alpha production. We have linked the increases in CUGBP, CHOP, and TNF-alpha production to activation of stress activated protein kinases with aging. We found increased generation of senescent preadipocytes with aging, obesity, and serial subculturing. This occurs in a fat depot dependent manner and is associated with telomere shortening, increased p53, and acquisition of a senescent secretory phenotype, with increased expression of pro-inflammatory cytokines, chemokines, and extracellular matrix modifying proteins. The chemokines released by these cells contribute to the macrophage infiltration in aging and obesity. The goal of our current work is to understand mechanisms underlying changes in adipogenic transcription factors, stress response system activity, and cellular senescence with aging, particularly effects of manipulating telomere expression, reactive oxygen species generation, and metabolic signaling on these changes.
In regard to fat depot origin, we have found that preadipocytes from different human and rat fat depots are distinct cell types with unique morphology, differences in capacities for replication, differentiation into fat cells, apoptosis, and susceptibility to cellular senescence, as well as distinct patterns of developmental gene expression. We found responses to insulin-like growth factor-1, a major regulator of preadipocyte replication and differentiation into fat cells, vary in preadipocytes among different fat depots. We are currently investigating regional variation in IGF-1 signaling and epigenetic mechanisms causing differences in developmental gene expression. We are studying how regional variation in preadipocyte developmental regulators contributes to differences in preadipocyte capacities for replication, adipogenesis, apoptosis, and senescence. This work is important in developing methods to treat the metabolic syndrome, including strategies to target particular fat depots.
Toronto General Hospital
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