Zm. Lu et al., USE OF HEXOSE-TRANSPORT MUTANTS TO EXAMINE THE EXPRESSION AND PROPERTIES OF THE RAT MYOBLAST GLUT-1 TRANSPORT PROCESS, Biochimica et biophysica acta. Biomembranes, 1234(2), 1995, pp. 155-165
Rat L6 myoblasts were recently shown to possess the GLUT 1, 3 and 4 tr
ansporters, and not the GLUT 2 isoform [1]. This investigation examine
d the expression and properties of the GLUT 1 isoform. GLUT 1 transcri
pt level was significantly reduced in cells grown at high densities an
d during myogenic differentiation. A comparison of the GLUT 1 and 4 tr
anscript levels in myogenesis-competent and impaired cells revealed an
inverse relationship between these two isoforms. This relationship wa
s confirmed by studies using two independent spontaneous GLUT 3(-) GLU
T 4(-) mutants, M1 and M3. These mutants possessed very high level of
the GLUT 1 isoform, but negligible amount of the GLUT 3 and 4 isoforms
. GLUT 1 expression was also subject to positive regulation. Glucose s
tarvation was found to increase not only the levels of the GLUT 1 tran
script and transporter, but also the intrinsic activity of the GLUT 1
transporter. Studies with MZ and M3 mutants revealed that the GLUT 1 t
ransporter was not functional in glucose-grown cells, even though it w
as present at a very high level in the plasma membrane. This transport
er became functional when cells were starved for glucose. The function
al GLUT 1 transporter had an apparent K-m value of around 0.9 mM, and
was sensitive to cytochalasin B, phloretin, phlorizin and pCMBS.