Regulation of hexose transport in rat myoblasts during growth and differentiation

Abstract

We report here the effects of growth conditions and myogenic differentiation on rat myoblast hexose transport activities. We have previously shown that in undifferentiated myoblasts the preferred substrates for the high (HAHT)- and low (LAHT)-affinity hexose transport systems are 2-deoxyglucose (2-DG) and 3-O-methyl-D-glucose (3-OMG), respectively. The present study shows that at cell density higher than 4.4 x 10(4) cells/cm2, the activities of both transport processes decrease with increasing cell densities of the undifferentiated myoblasts. Since the transport affinities are not altered, the observed decrease is compatible with the notion that the number of functional hexose transporters may be decreased in the plasma membrane. Myogenic differentiation is found to alter the 2-DG, but not the 3-OMG, transport affinity. The Km values of 2-DG uptake are elevated upon the onset of fusion and are directly proportional to the extent of fusion. This relationship between myogenesis and hexose transport is further explored by using cultures impaired in myogenesis. Treatment of cells with 5-bromo-2'-deoxyuridine abolishes not only myogenesis but also the myogenesis-induced change in 2-DG transport affinity. Similarly, alteration in 2-DG transport affinity cannot be observed in a myogenesis-defective mutant, D1. However, under myogenesis-permissive condition, the myogenesis of this mutant is also accompanied by changes in its 2-DG transport affinity. The myotube 2-DG transport system also differs from its myoblast counterpart in its response to sulfhydryl reagents and in its turnover rate. It may be surmised from the above observations that myogenesis results in the alteration of the turnover rate or in the modification of the 2-DG transport system. Although glucose starvation has no effect on myogenesis, it is found to alter the substrate specificity and transport capacity of HAHT. In conclusion, the present study shows that hexose transport in rat myoblasts is very sensitive to the growth conditions and the stages of differentiation of the cultures. This may explain why different hexose transport properties have been observed with myoblasts grown under different conditions.

Involvement of the GLUT 3 transporter in myogenic regulation

“…9, four other clones of D23/ GLUT 3 transfectants examined were also capable of forming myotubes upon induction of GLUT 3. It was important to note that the extent of fusion was only a fraction of that expected for L6 myoblasts [29]. Similar to the extent of fusion exhibited by day 6 culture of L6/G3 S myoblasts (Fig.…”

“…Second, myotube formation is always preceded by a dramatic decrease of the GLUT 3 transcript and transport activity [ 1,29]. Third, cells retaining relatively high levels of the GLUT 3 transcript and transport activity are unable to form myotubes [1,29].…”

“…Second, myotube formation is always preceded by a dramatic decrease of the GLUT 3 transcript and transport activity [ 1,29]. Third, cells retaining relatively high levels of the GLUT 3 transcript and transport activity are unable to form myotubes [1,29]. While these observations demonstrate a close correlation between GLUT 3 expression and myogenesis, they are puzzling in that the last two observations seem to be contradictory to the first one.…”

Glucose uptake in human and animal muscle cells in culture

“…In contrast, undifferentiated L6 myoblasts have been shown to express two types of transport activity of low and high affinity for the hexose (D'Amore and Lo 1986). The high affinity system disappears in fully differentiated L6 myotubes (Chen and Lo 1989). The experiments performed with differentiated human muscle cells were also compatible with the expression of a kinetically uniform transport system.…”

Use of p112‐deficient myoblasts to determine the temporal order of the in vitro expression of myogenic components

“…Cell surface proteins are also thought to be involved in myogenesis (Champaneria et al, 1989;Engel and White, 1990;Knudsen, 1990). The expression of the neural cell adhesion molecule (NCAM) (Moore and Walsh, 1985;Moore et al, 1987;Knudsen et al, 1989Knudsen et al, , 1990aDickson et al, 1990), cadherin (Knudsen et al, 1990b;Pouliot et al, 1990;Donalies et al, 19911, fibronectin and its receptor (Chung and Kang, 1990), integrin (Menko and Boettiger, 1987), and the rat myoblast high affinity hexose transport system (Chen and Lo, 1989;Kudo and Lo, 1990) was found to be closely associated with myogenic differentiation. Presumably these components take part i n biochemical differentiation, or in cell-cell interaction prior to membrane fusion.…”