These findings indicate that both fructose and maltodextrin modulate the GLUT4 adaptive response to exercise by mechanisms involving chromatin remodeling at theGlut4promoter. Keywords:fructose, GLUT4, accessibility, MEF2A binding, exercise, histone acetylation one of the most documented effectsof exercise on skeletal muscle is the adaptive increase in GLUT4, the dominant glucose transporter, during exercise (11,38,44). DNase I was significantly increased by swimming (2.75- and 5.75-fold, respectively) but was also suppressed in trained rats that consumed fructose or maltodextrin. Histone H3 acetylation and MEF2A binding paralleled the accessibility pattern. These findings indicate that both fructose and maltodextrin modulate the GLUT4 adaptive response to exercise by mechanisms involving chromatin remodeling at theGlut4promoter. Keywords:fructose, GLUT4, accessibility, MEF2A binding, exercise, histone acetylation one of the most documented effectsof exercise on skeletal muscle is the adaptive increase in GLUT4, the dominant glucose transporter, during exercise (11,38,44). A single prolonged bout of exercise has been reported to cause up to a twofold increase in its content 18 h postexercise (42), resulting in a marked increase in glucose transport responsiveness to insulin and exercise (42,43). As a consequence of this adaptation, muscle glycogen storage following glycogen-depleting exercise occurs more rapidly and to a greater extent in the trained than in the untrained state (12,14,32). However, little is known about the GLUT4 adaptive response to exercise in animals that are allowed to consume large amounts of fructose. This information is essential, because athletes and the general public are often advised to consume energy drinks that contain substantial amounts of fructose before, during, or after exercise (7,17). A few studies have shown that nutrient availability modulates many acute and chronic adaptations to exercise by altering the activities of exercise-responsive genes (2,13). GLUT4 expression in response to exercise is mediated by myocyte enhancer factor 2 (MEF2)A and -D and GLUT4 enhancer factor (GEF) (4,28), which bind to conserved DNA sequences on theGlut4promoter to initiate transcription (28). The binding of these factors is dependent on chromatin structure in the region containing their DNA binding sequences. It is known that muscle contraction during (R)-Sulforaphane exercise activates AMP-activated protein kinase (AMPK) (28,33) and Ca2+/calmodulin-dependent protein kinase II (CaMKII) (16,3335,46), which increase the activity of histone acetyltransferases (HATs) that acetylate histone tails within nucleosomes of theGlut4promoter, resulting in more relaxed DNA-histone interactions (27,29). Such remodeling of chromatin is thought to promote transcription by enhancing accessibility of binding domains to transcription factors (28). Recent evidence indicates that fructose consumption also influences the expression of some genes via histone modifications (8,51). Whether or not fructose influences chromatin remodeling in the region surrounding the MEF2 binding domain on theGlut4gene and affects MEF2A binding in response to exercise has not yet been studied. Therefore, the present study was conducted to investigate the effects of ad libitum consumption (R)-Sulforaphane of a 10% fructose solution (with caloric density comparable to that of common sugar-sweetened carbonated beverages) on the GLUT4 adaptive response to high-intensity exercise training in rat skeletal muscle. Specifically, the effects on GLUT4 expression, histone H3 acetylation, and accessibility of a 350-bp DNA segment of theGlut4promoter containing the MEF2 binding domain, and boundGlut4MEF2A were investigated. In the past, accessibility of the MEF2 domain on theGlut4gene in response to exercise has been indirectly inferred from quantification of histone acetylation using chromatin immunoprecipitation (ChIP) assays (31,47). In the present study, we describe for the first time the use of the nuclease digestion assay to directly assess accessibility of DNA segments of interest. We report that ad libitum consumption (R)-Sulforaphane of fructose suppresses the GLUT4 adaptive response to exercise. == MULK MATERIALS AND METHODS == == == == Materials. == Wistar rats were purchased from the University of Cape Town Animal Unit (Cape Town, South Africa). Crystalline fructose and (R)-Sulforaphane maltodextrin powder were purchased from Health Connection foods (Cape Town, South Africa). Pentobarbital sodium (Euthapent) was supplied by Kyron Laboratories (Johannesburg, South Africa). Micrococcal nuclease (MNase) and DNase I were purchased from New England Biolabs (Ipswich, MA). PCR primers were synthesized at the Molecular and Cellular Biology Laboratory of the University of Cape Town.TaqDNA polymerase was purchased from Solis Biodyne (Tartu, Estonia). Other reagents for PCR were from Thermo Scientific (Waltham, MA). Antibodies against GLUT4, HDAC5, AMPK1/2, (R)-Sulforaphane pAMPK1/2Th172, and -tubulin were obtained from Abcam (Cambridge, MA). Polyclonal HRP-conjugated goat anti-rabbit secondary antibody was supplied by Dako.