The number of calories used during physical exercise is greater than the number of calories used for the movements themselves. This may be caused in part by exercise-induced expression of uncoupling protein 1 (UCP1) in subcutaneous fat. Subcutaneous fat is mostly white fat cells but can also contain brown fat cells in mice and humans. As part of a thermogenesis program, UCP1 directly reduces the proton gradient driving oxidative phosphorylation. UCP1 expression is a characteristic of brown fat cells only and is induced by the protein irisin. The data in Table 1 address the relationship between exercise and: • expression of UCP1 in subcutaneous fat cells, • expression of FNDC5 (a transmembrane protein in skeletal muscle plasma membranes), • blood levels of irisin (a soluble protein formed by cleaving the extracellular domain of FNDC5), and • expression of adiponectin (a hormone produced by white fat cells) in both wild-type mice and mice with overexpression of skeletal muscle PGC-1a (a transcription factor whose expression in skeletal muscle is induced by exercise). Table 1 Experimental conditions Effects of Exercise and PGC-1a on UCP1, FNDC5, Irisin, and Adiponectin Wild-type mice Mice with skeletal muscle PGC-1a Figure 1 Blood glucose (mg/dL) no endurance exercise 300T 250+ 200- 150- 100- 50- 0- endurance exercise no endurance exercise Figure 1 shows the physiological effects of injecting viruses that encode either FNDC5 or a control protein into obese mice with disrupted glucose homeostasis. — FNDC5 • UCP1 FNDC5 mRNA mRNA levels levels Protein encoded by virus control protein 0 2.5 80 overexpression nd Note: nd = not determined; all measurements are relative to control. 1 25 Adapted from P. Bostrom et al. Ⓒ©2012 Nature. 30 60 90 120 Time (min) O A. transformation. O B. transduction. O C. contamination. O D. conjugation. FNDC5 effects on nonfasting blood glucose levels (Note: * indicates p < 0.05.) The information in the passage best supports which hypothesis? 1 2.8 O A. Exercise prevents glucose uptake. OB. Exercise promotes less effective cellular respiration. O C. Exocrine secretions of skeletal muscle act on adipose tissue. Mice that overexpress PGC-1a specifically in their skeletal muscles are most likely to exhibit which phenotype relative to wild-type mice? O A. Lower body weight O B. Lower body temperature OC. Higher physical activity O D. Higher nonfasting blood glucose levels O D. Endocrine secretions of adipose tissue act on skeletal muscle. The viruses that encode either FNDC5 or control protein transfer genetic material via: Blood irisin levels 1 1.7