Anatomically separate fat depots differ in size, function, and contribution to pathological states, such as the metabolic syndrome. We isolated preadipocytes from different human fat depots to determine whether the basis for this variation is partly attributable to differences in inherent properties of fat cell progenitors. We found that genome-wide expression profiles of primary preadipocytes cultured in parallel from abdominal subcutaneous, mesenteric, and omental fat depots were distinct. Interestingly, visceral fat was not homogeneous. Preadipocytes from one of the two main visceral depots, mesenteric fat, had an expression profile closer to that of subcutaneous than omental preadipocytes, the other main visceral depot. Expression of genes that regulate early development, including homeotic genes, differed extensively among undifferentiated preadipocytes isolated from different fat depots. These profiles were confirmed by real-time PCR analysis of preadipocytes from additional lean and obese male and female subjects. We made preadipocyte strains from single abdominal subcutaneous and omental preadipocytes by expressing telomerase. Depot-specific developmental gene expression profiles persisted for 40 population doublings in these strains. Thus, human fat cell progenitors from different regions are effectively distinct, consistent with different fat depots being separate mini-organs.

. Abundance of two human preadipocyte subtypes with distinct capacities for replication, adipogenesis, and apoptosis varies among fat depots.

Fat distribution changes with aging. Inherent changes in fat cell progenitors may contribute because fat cells turn over throughout life. To define mechanisms, gene expression was profiled in preadipocytes cultured from epididymal and perirenal depots of young and old rats. 8.4% of probe sets differed significantly between depots, particularly developmental genes. Only 0.02% differed with aging, despite using less stringent criteria than for comparing depots. Twenty-five genes selected based on fold change with aging were analyzed in preadipocytes from additional young, middle-aged, and old animals by polymerase chain reaction. Thirteen changed significantly with aging, 13 among depots, and 9 with both. Genes involved in inflammation, stress, and differentiation changed with aging, as occurs in fat tissue. Age-related changes were greater in perirenal than epididymal preadipocytes, consistent with larger declines in replication and adipogenesis in perirenal preadipocytes. Thus, age-related changes in preadipocyte gene expression differ among depots, potentially contributing to fat redistribution and dysfunction.

Preadipocyte differentiation capacity declines between middle and old age. Expression of the adipogenic transcription factors, CCAAT/enhancer-binding protein (C/EBP) ␣ and peroxisome proliferator-activated receptor ␥ (PPAR␥), is lower in differentiating preadipocytes from old than young animals, although no age-related changes occur in C/EBP␤ mRNA, which is upstream of C/EBP␣ and PPAR␥. C/EBP␤-liver-enriched inhibitory protein (C/EBP␤-LIP), a truncated C/EBP␤ isoform that is a dominant inhibitor of differentiation, increases with aging in rat fat tissue and preadipocytes. CUG triplet repeat-binding protein-1 (CUGBP1) binds to C/EBP␤ mRNA, increasing C/EBP␤-LIP translation. Abundance and nucleotide binding activity of CUGBP1 increased with aging in preadipocytes. CUGBP1 overexpression in preadipocytes from young animals increased C/EBP␤-LIP and impaired adipogenesis. Decreasing CUGBP1 in preadipocytes from old rats by RNA interference reduced C/EBP␤-LIP abundance and promoted adipogenesis. Tumor necrosis factor-␣, levels of which are elevated in fat tissue with aging, increased CUGBP1 protein, CUGBP1 binding activity, and C/EBP␤-LIP in preadipocytes from young rats. Thus, CUGBP1 contributes to regulation of adipogenesis in primary preadipocytes and is responsive to tumor necrosis factor-␣. With aging, preadipocyte CUGBP1 abundance and activity increases, resulting in enhanced translation of the C/EBP␤-LIP isoform, thereby blocking effects of adipogenic transcription factors, predisposing preadipocytes from old animals to resist adipogenesis. Altered translational processing, possibly related to changes in cytokine milieu and activation of stress responses, may contribute to changes in progenitor differentiation and tissue function with aging.

Aging is associated with metabolic syndrome, tissue damage by cytotoxic lipids, and altered fatty acid handling. Fat tissue dysfunction may contribute to these processes. This could result, in part, from age-related changes in preadipocytes, since they give rise to new fat cells throughout life. To test this hypothesis, preadipocytes cultured from rats of different ages were exposed to oleic acid, the most abundant fatty acyl moiety in fat tissue and the diet. At fatty acid concentrations at which preadipocytes from young animals remained viable, cells from old animals accumulated lipid in multiple small lipid droplets and died, with increased apoptotic index, caspase activity, BAX, and p53. Rather than inducing apoptosis, oleic acid promoted adipogenesis in preadipocytes from young animals, with appearance of large lipid droplets. CCAAT/enhancer-binding protein-α (C/EBPα) and peroxisome proliferator-activated receptor-γ (PPARγ) increased to a greater extent in cells from young than old animals after oleate exposure. Oleic acid, but not glucose, oxidation was impaired in preadipocytes and fat cells from old animals. Expression of carnitine palmitoyltransferase (CPT)-1, which catalyzes the rate-limiting step in fatty acid β-oxidation, was not reduced in preadipocytes from old animals. At lower fatty acid levels, constitutively active CPT I expression enhanced β-oxidation. At higher levels, CPT I was not as effective in enhancing β-oxidation in preadipocytes from old as young animals, suggesting that mitochondrial dysfunction may contribute. Consistent with this, medium-chain acyl-CoA dehydrogenase expression was reduced in preadipocytes from old animals. Thus preadipocyte fatty acid handling changes with aging, with increased susceptibly to lipotoxicity and impaired fatty acid-induced adipogenesis and β-oxidation.