Neuronal regulation of substrate cycle between glucose 6-phosphate and glucose in brown adipose tissues of cold-exposed mice

Characterization of non-cytosolic hexokinase activity in white skeletal muscle from goldfish (Carassius auratus L.) and the effect of cold acclimation

HK (hexokinase) is an enzyme involved in the first step in the glucose metabolism pathway, converting glucose into G6P (glucose 6-phosphate). Owing to the importance of skeletal muscle for fish swimming and acclimation processes, we used goldfish (Carassius auratus L.) white muscle in order to investigate subcellular distribution and kinetics of HK. In this study, we report that HK activity is predominantly localized in the mitochondrial fraction [NC-HK (non-cytosolic HK)] in goldfish white muscle. Studies of the kinetic parameters revealed that the Km (Michaelis–Menten constant) for glucose was 0.41±0.03 mM and that for mannose was 3-fold lower, whereas the affinity for fructose was too low to be measured. The Km for ATP was 0.88±0.05 mM, whereas no activity was observed when either GTP or ITP was used as a phosphate donor. A moderate inhibition (20–40%) was found for ADP and AMP. Similar to mammalian HK, G6P and glucose analogues were able to promote an inhibition of between 85 and 100% of activity. Here, we found that acclimation of goldfish at 5°C promoted a 2.5-fold increase in NC-HK compared with its counterpart acclimated at 25°C. However, cytosolic HK activity was not altered after thermal acclimation. In summary, our results suggest that the goldfish has a constitutive NC-HK that shows some similarities to mammalian HK-II and, curiously, may play a role in the broad metabolic changes required during the cold acclimation process.

Identification and characterization of basal and cyclic AMP response elements in the promoter of the rat hexokinase II gene

Hexokinases catalyze the phosphorylation of glucose and initiate cellular glucose metabolism. Hexokinase II (HKII) is the principal hexokinase isoform in skeletal muscle, heart, and adipose tissue. Isoproterenol and exogenous cyclic AMP (cAMP) increase HKII gene transcription in L6 myotubes. Various segments of the HKII promoter that direct the expression of the chloramphenicol acetyltransferase reporter gene were transfected into L6 myotubes to identify basal and cAMP response elements. The 5'-flanking region that extends 90 base pairs upstream of the transcription start site includes a CCAAT box and a cAMP response element (CRE); both contribute to basal promoter activity and each provides an independent, maximal response to cAMP. An inverted CCAAT motif, or Y box, located just upstream of the CCAAT box, contributes to basal promoter activity but is not involved in the cAMP response. Homo- and heterodimers composed of the CRE-binding protein and activating transcription factor-1 bind specifically to the CRE. The Y box and the CCAAT box specifically bind the factor NF-Y (also known as CBF).

Absence of paravascular nerve projection and cross-innervation in interscapular brown adipose tissues of mice

To determine whether a paravascular nerve projection or cross-innervation exists in the interscapular brown adipose tissue (BAT), the distribution of noradrenergic or peptidergic nerve fibers in intact or denervated interscapular BAT pads of mice was examined histochemically. Noradrenaline (NA) fibers were visualized by the glyoxylic acid condensation method, and neuropeptide Y (NPY) and substance P (SP) fibers were detected immunohistochemically. Numerous NA-positive fibers and a few NPY- or SP-positive fibers were observed around intralobular arterioles in intact BAT pads. Some NA-positive fibers and very few NPY- or SP-positive fibers were seen around interlobular arteries. NA- and NPY-positive fibers were also found around brown adipose cells in the parenchyme of BAT, whereas SP-positive fibers were absent around the cells. However, all sections cut from denervated BAT pads of unilaterally or bilaterally denervated animals showed a total absence of NA-, NPY- or SP-positive nerve fibers. Therefore, neither a paravascular projection of NA, NPY, and SP fibers to the BAT nor a cross-innervation of these nerve fibers between the left and right BAT pads exists in the mouse BAT.