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- Title
Low Glucose Accelerates VLDL Receptor-Mediated Lipoprotein Uptake in Skeletal Muscle Cells.
- Authors
Zenimaru, Yasuo; Takahashi, Sadao; Kasen, Miki; Ueno, Masami; Suzuki, Jiinya; Miyamori, Isamu; Takahashi, Masahumi; Hattori, Hiroaki
- Abstract
The VLDL receptor (VLDLR), a member of the LDL receptor superfamily, plays a role in the catabolism of TG-rich lipoproteins in skeletal muscle, heart and adipose tissue. Skeletal muscle cells utilize both glucose and fatty acids as major energy sources depending on nutritional conditions, however, function and regulation of VLDLR remains to be understood. In order to explore the function of muscular VLDLR in energy metabolism, we studied VLDLR expression and lipoprotein uptake in cultured skeletal muscle (SKM) cells. Rat myoblastic L6 cells, primary culture of rat SKM and cardiac muscle (CM) cells were incubated with DMEM in variable concentration of D-glucose ranged from 0 to 60 mM. L-glucose, a non-metabolized isomer of D-glucose, was used to adjust the osmotic pressure. VLDLR expression was analyzed by Western, Northern blotting and quantitative RT-PCR. Lipoprotein uptake was analyzed using radio-labeled rabbit beta-VLDL, human LDL and VLDL. In the culture condition with low glucose concentration bellow 4.2 mM, the VLDLR proteins were increased in L6 myoblasts. The induction of VLDLR expression negatively correlated with glucose concentration; the maximal induction occurred in 0 mM of D-glucose where the expression was 3-times higher than that in 5.6 mM of D-glucose. The induction of VLDLR expression was seen 12 hrs after the glucose withdrawal and remained increased for 12 hrs. The rat primary SKM cells and smooth muscle cell line showed the same induction of VLDLR with low glucose, while no change was observed in any concentration of D-glucose in CM cells. In contrast to VLDLR, LDL receptor proteins were not altered in L6 cells. Since VLDLR mRNAs were also upregulated in low glucose condition, the induction of VLDLR protein was likely to be transcriptional. Accordingly, both actinomycin D and cycloheximide blocked the low glucose-induced induction of VLDLR protein expression. The uptake of [sup 125]I-labeled beta-VLDL was increased 40% in low glucose condition in L6 cells, while LDL uptake was not altered. In addition, oil red-O staining showed intracellular lipid accumulation when the cells were incubated with beta VLDL in low glucose condition. These data suggests that VLDLR-mediated TG-rich lipoprotein uptake plays an important role in compensating for energy deprivation in low glucose condition in skeletal muscle cells.
- Subjects
LOW density lipoproteins; LIPOPROTEINS; MUSCLE cells; FATTY acids; BLOOD sugar
- Publication
Diabetes, 2007, Vol 56, pA234
- ISSN
0012-1797
- Publication type
Article