GLUT4, TBC1D1, TBC1D4, TUG and RUVBL2: Relationships with Each Other and Rat Muscle Fiber Type

Abstract

Skeletal muscle is composed of multiple fiber types that vary greatly in capacity for both exercise- and insulin-stimulated glucose transport (GT). Relative expression of GLUT4 and key regulatory proteins is crucial for the range in GT capacity. PURPOSE: To assess the relationships between fiber type (myosin heavy chain, MHC, isoform) and abundance of GLUT4, 2 Rab GTPase activating proteins (TBC1D1 and TBC1D4) which regulate subcellular GLUT4 localization, and 2 other proteins proposed to regulate GT [TUG (tethering protein containing a UBX domain for GLUT4) and RuvB-like protein 2 (RUVBL2)]. METHODS: 12 rat skeletal muscle or muscle regions: adductor longus (AL); extensor digitorum longus (EDL); epitrochlearis (EPI); mixed, red and white gastrocnemius (GASM, GASR and GASW); plantaris (PLAN); Soleus (SOL); red and white tibialis anterior (TAR and TAW); tensor fasciae latae (TFL); and white vastus lateralis (VLW) were dissected out and immediately freeze clamped. Muscles were homogenized and muscle lysates were used for SDS-PAGE to determine MHC isoforms expression using coomassie staining, and GLUT4, TUG, RUVBL2, TBC1D1, and TBC1D4 total abundance were determined via western blotting. RESULTS: The major findings are summarized below.TABLE CONCLUSION: The high correlation between TUG and GLUT4 is consistent with published evidence indicating that TUG binds GLUT4 and modulates its subcellular distribution. The lack of a relationship between RUVBL2 and TBC1D4 (which RUVBL2 reportedly binds and regulates in 3T3-L1 cells) may reflect the multiple functions of both proteins. The biological significance of RUVBL2's correlation with GLUT4 and TUG remains to be clarified. Recently published data suggested there might be reciprocal expression of TBC1D1 and TBC1D4 in mouse muscles, but such a relationship was not evident in rat muscles. The expression levels for 3 of 5 proteins studied which have established or potential GT regulatory roles corresponded to differences in skeletal muscle fiber type composition. Supported by the NIH (AG-010026 and NIDDK-071771)