Obesity is rapidly becoming a leading health problem in developed countries. Solid epidemiological data support the pivotal role of body fat (white adipose tissue, WAT) mass in the development of obesity and associated health risks. Expansion of adipose cell mass involves both increases in fat cell size (hypertrophy) and cell number (hyperplasia). The latter occurs through recruitment and differentiation of precursor cells (adipogenesis). White adipocytes, the main cellular component of WAT, secrete a host of factors that affect both whole body and WAT-specific metabolism. In this regard, reducing WAT mass by targeted modulation of fat cell metabolic enzymes is an attractive alternative to dietary approaches. Given the complexity of metabolic regulation both at the whole body and adipose tissue levels, it is clear that a systematic, integrated strategy is needed to find optimal nutritional mixtures or new drug targets.

To characterize the metabolic phenotypes associated with the various stages of adipocyte development and growth, we have previously developed a metabolic flux analysis (MFA) model using pathway thermodynamic constraints. Here, we (1) present new experimental data to corroborate the model, (2) describe the results of a modularity analysis examining the functional organization of adipocyte metabolism, and (3) identify a key metabolite node as a potential target for metabolic control of adipocyte growth. First, we have obtained direct estimates of intracellular fluxes through the pentose phosphate pathway using ¹⁴C-labeled glucose. We have also measured the cellular oxygen consumption rates using a fiber-optic micro-sensor system. The experimental results showed good quantitative agreement with the calculated flux estimates for days 8 and 12 of differentiation. Second, results of our modularity analysis indicated time-dependent rearrangements of interactions between the significant reaction sub-groups within the global metabolic network, suggesting that the metabolic flux adjustments reflected a re-organization of the underlying network's functional layout. These observations underscore the dynamic character of adipocyte development and growth, which is not explained by a simple uniform activation of lipogenic reactions. Third, flux and modularity analysis results together pointed to the flux distribution around pyruvate as a key indicator of adipocyte lipid accumulation. Experiments with chemical inhibitors targeting lactate dehydrogenase or pyruvate carboxylase showed that specific perturbation of pyruvate metabolism significantly affected the global flux distribution, including glucose uptake and lipid accumulation, without altering differentiation-related markers. These findings suggest that reactions in and around the pyruvate node could be developed as useful metabolic targets for controlling adiposity. This warrant further investigation in future studies, for example using siRNA to specifically perturb the enzymes at the level of translation.

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