Adipose Tissue: Energy Storage Depot

ADIPOSE TISSUE: ENERGY STORAGE DEPOT

Adipose tissue is second only to the liver in its ability to distribute fuel molecules. In a 70-kg man, white adipose tissue (WAT) weighs approximately 14 kg, or about half as much as the total muscle mass. Nearly the entire volume of each adipocyte in WAT can be occupied by a droplet of TAG (Figure 24.5).

Figure 24.5 Colorized transmission electron micrograph of adipocytes.

A. Carbohydrate metabolism

1. Increased glucose transport: Circulating insulin levels are elevated in the absorptive state, resulting in an influx of glucose into adipocytes via insulin-sensitive GLUT-4 recruited to the cell surface from intracellular vesicles ( Figure 24.6, 1). The glucose is phosphorylated by hexokinase.

2. Increased glycolysis: The increased intracellular availability of glucose results in an enhanced rate of glycolysis (see Figure 24.6, 2). In adipose tissue, glycolysis serves a synthetic function by supplying glycerol 3-phosphate for TAG synthesis. Recall that adipose tissue lacks glycerol kinase.

3. Increased activity of the pentose phosphate pathway: Adipose tissue can metabolize glucose by means of the PPP, thereby producing NADPH, which is essential for fat synthesis  (see Figure 24.6, 3). However, in humans, de novo synthesis is not a major source of FA in adipose tissue, except when refeeding a previously fasted individual (see Figure 24.6, 4).

Figure 24.6 Major metabolic pathways in adipose tissue in the absorptive state. [Note: The numbers in the circles, which appear both in the figure and in the corresponding text, indicate important pathways for adipose tissue metabolism.] GLUT = glucose transporter; P = phosphate; PPP = pentose phosphate pathway; CoA = coenzyme A; TCA = tricarboxylic acid; TAG = triacylglycerol; VLDL = very-low-density lipoprotein; LPL = lipoprotein lipase.

B. Fat metabolism

Most of the FAs added to the TAG stores of adipocytes after consumption of a lipid-containing meal are provided by the degradation of exogenous (dietary) TAG in chylomicrons sent out by the intestine and endogenous TAG in VLDL sent out by the liver (see Figure 24.6,5 ). The FAs are released from the lipoproteins by the action of lipoprotein lipase (LPL), an extracellular enzyme attached to the capillary walls in many tissues, particularly adipose and muscle. In adipose tissue, LPL is upregulated by insulin. Thus, in the fed state, elevated levels of glucose and insulin favor storage of TAG (see Figure 24.6,6 ), all the carbons of which are supplied by glucose. [Note: Elevated insulin favors the dephosphorylated (inactive) form of HSL, thereby inhibiting lipolysis in the fed state.]