A truncation mutation in TBC1D4 in a family with acanthosis nigricans and postprandial hyperinsulinemia

Abstract

Tre-2, BUB2, CDC16, 1 domain family member 4 (TBC1D4) (AS160) is a Rab-GTPase activating protein implicated in insulin-stimulated glucose transporter 4 (GLUT4) translocation in adipocytes and myotubes. To determine whether loss-of-function mutations in TBC1D4 might impair GLUT4 translocation and cause insulin resistance in humans, we screened the coding regions of this gene in 156 severely insulinresistant patients. A female presenting at age 11 years with acanthosis nigricans and extreme postprandial hyperinsulinemia was heterozygous for a premature stop mutation (R363X) in TBC1D4. After demonstrating reduced expression of wild-type TBC1D4 protein and expression of the truncated protein in lymphocytes from the proband, we further characterized the biological effects of the truncated protein in 3T3L1 adipocytes. Prematurely truncated TBC1D4 protein tended to increase basal cell membrane GLUT4 levels (P ‫؍‬ 0.053) and significantly reduced insulin-stimulated GLUT4 cell membrane translocation (P < 0.05). When coexpressed with wild-type TBC1D4, the truncated protein dimerized with full-length TBC1D4, suggesting that the heterozygous truncated variant might interfere with its wild-type counterpart in a dominant negative fashion. Two overweight family members with the mutation also manifested normal fasting glucose and insulin levels but disproportionately elevated insulin levels following an oral glucose challenge. This family provides unique genetic evidence of TBC1D4 involvement in human insulin action.glucose transport ͉ insulin resistance ͉ AS160 B lood glucose levels are very precisely regulated in healthy people despite enormous variation in the timing and magnitude of carbohydrate ingestion. Postingestive insulinstimulated glucose uptake into skeletal muscle and adipose tissue is an essential element of this tight metabolic regulation. Glucose transporter 4 (GLUT4) selectively mediates insulinstimulated glucose transport into myotubes and adipocytes. The importance of this metabolic step in vivo was demonstrated by generating muscle and adipose tissue-specific GLUT4 knockout mice, both of which are insulin resistant (1). Human physiological studies suggest that defects in insulin-stimulated glucose transport in muscle constitute the earliest abnormality in insulinresistant people (2). GLUT4 is the main insulin-responsive hexose transporter. Insulin rapidly increases the amount of GLUT4 in the plasma membrane by causing specialized GLUT4-containing vesicles to move from an intracellular location to the plasma membrane where the vesicles dock and then fuse with the plasma membrane (3). This process is referred to as GLUT4 translocation. Whilst impaired GLUT4 translocation appears to be common to many different insulin resistance-inducing metabolic insults (4), precisely how insulin triggers GLUT4 vesicle movement and subsequent membrane fusion remains incompletely understood. AKT2 is clearly activated in response to insulin and triggers GLUT4 membrane translocation (5), but until recently the link between proxi...

Crystal Structures of Human TBC1D1 and TBC1D4 (AS160) RabGTPase-activating Protein (RabGAP) Domains Reveal Critical Elements for GLUT4 Translocation

“…Other regions likely participate in protein/membrane or protein/protein interactions, including the phosphotyrosine binding domains of TBC1D1 and TBC1D4. Their phosphotyrosine binding domains may interact with insulin-regulated aminopeptidase in GLUT4 vesicles (11), which would increase the local concentrations of TBC1D1 and TBC1D4 near membrane-bound Rabs.…”

“…TBC1D1 and TBC1D4 are closely related paralogs, with 47% overall identity and 76% identity within their RabGAP domains, but they are expressed in different tissues. To further underscore their important roles in normal metabolic function, naturally occurring loss-of-function mutations in TBC1D1 and TBC1D4 RabGAP domains have been genetically linked to protection against obesity in mice (10) and defective insulin signaling in humans (11), respectively.…”

The Inactivation of RabGAP Function of AS160 Promotes Lysosomal Degradation of GLUT4 and Causes Postprandial Hyperglycemia and Hyperinsulinemia

“…Surprisingly, deletion of AS160 decreases GLUT4 levels and causes severe insulin resistance in mice (14)(15)(16)(17). More importantly, human patients bearing truncation mutations on AS160 (AS160 Arg363Ter or AS160 Arg684Ter , in which arginine is replaced by a stop codon) have postprandial hyperglycemia and hyperinsulinemia (18,19). GLUT4 is also decreased in skeletal muscle of patients harboring the AS160 Arg684Ter mutation (19).…”

“…The PTBs of AS160 can bind phospholipids (20) as well as the insulin-regulated aminopeptidase (IRAP) that associates with GSVs (21). An N-terminal AS160 fragment was detected in transformed lymphocytes from a human patient bearing the AS160 Arg363Ter mutation (18). Overexpression of this fragment inhibits GLUT4 translocation in 3T3-L1 adipocytes, most likely through interference with endogenous AS160 (18).…”

“…An N-terminal AS160 fragment was detected in transformed lymphocytes from a human patient bearing the AS160 Arg363Ter mutation (18). Overexpression of this fragment inhibits GLUT4 translocation in 3T3-L1 adipocytes, most likely through interference with endogenous AS160 (18). However, it remains unknown which functional domains on AS160 contribute to the decreased GLUT4 expression.…”