Cloning of Rab GTPases expressed in human skeletal muscle: studies in insulin-resistant subjects.

To explore the potential role of Rab GTPases in human insulin resistance, we first employed a PCR-cloning approach to identify Rab isoforms that are expressed in human skeletal muscle. Multiple Rab isoforms including Rab1A, Rab4A, Rab5B, Rab7, Rab8, Rab10, Rab12A, Rab13, Rab18, Rab21, and Rab22 mRNA were found to be expressed in human skeletal muscle. The second goal was to examine whether mRNA expression for Rabs targeted to endocytotic/exocytotic compartments was altered as a function of insulin resistance. Quantitative PCR analysis demonstrated that Rab4A, Rab5B and Rab18 mRNA levels in skeletal muscle from insulin-resistant patients without (IR) and with non-insulin-dependent diabetes mellitus (NIDDM) were not significantly different from those in insulin-sensitive controls (IS). At the protein level, total Rab5B amount was not significantly different among IS, IR and NIDDM subgroups. However, in basal muscle, Rab5B in the total membrane fraction was 2.1-3.6 fold higher in IR and NIDDM than in IS subjects. Insulin increased membrane-associated Rab5B by 3-fold in IS subjects, whereas this effect was not significant in both IR and NIDDM subgroups. Thus, for the first time, we have comprehensively studied the mRNA expression of Rab isoforms in human muscle. The phlethora of Rab GTPases are indicative of high volume of vesicular traffic and regulated metabolism. The potential role of specific Rab isoforms in insulin resistance does not rely on a change in steady state mRNA levels, but is demonstrable as an alteration in protein subcellular distribution and trafficking.