Skeletal Muscle-Selective Knockout of LKB1 Increases Insulin Sensitivity, Improves Glucose Homeostasis, and Decreases TRB3

Abstract

LKB1 is a tumor suppressor that may also be fundamental to cell metabolism, since LKB1 phosphorylates and activates the energy sensing enzyme AMPK. We generated muscle-specific LKB1 knockout (MLKB1KO) mice, and surprisingly, found that a lack of LKB1 in skeletal muscle enhanced insulin sensitivity, as evidenced by decreased fasting glucose and insulin concentrations, improved glucose tolerance, increased muscle glucose uptake in vivo, and increased glucose utilization during a hyperinsulinemic-euglycemic clamp. MLKB1KO mice had increased insulin-stimulated Akt phosphorylation and a >80% decrease in muscle expression of TRB3, a recently identified Akt inhibitor. Akt/TRB3 binding was present in skeletal muscle, and overexpression of TRB3 in C2C12 myoblasts significantly reduced Akt phosphorylation. These results demonstrate that skeletal muscle LKB1 is a negative regulator of insulin sensitivity and glucose homeostasis. LKB1-mediated TRB3 expression provides a novel link between LKB1 and Akt, critical kinases involved in both tumor genesis and cell metabolism.LKB1 is a serine/threonine kinase that links a diverse array of cellular processes, including cancer, cellular polarity, and metabolism. Originally identified as the tumor suppressor protein mutated in Peutz-Jeghers syndrome (17, 21), LKB1 has since been shown to regulate polarity in a number of systems, including Caenorhabditis elegans (48), Drosophila melanogaster (28), Xenopus (33), and mammalian cells (3). Biochemically, LBK1 can phosphorylate and activate at least 13 members of the AMP-activated protein kinase (AMPK) subfamily of protein kinases (20, 27) when associated with two regulatory proteins essential for catalytic activity, STE20-related adapter protein and mouse protein 25 (MO25) (15). AMPK, the most studied of LKB1's downstream substrates, is a conserved serine/threonine kinase that functions in the regulation of energy metabolism (16,18,29). Studies with cell culture (15,40,51) or conditional knockouts of LKB1 in skeletal muscle (38), cardiac muscle (39), or liver (41) have found that LKB1 regulates AMPK activity both in vitro and in vivo. Other than AMPK and the microtubule affinity-regulating kinase (MARK) proteins, which have been implicated in the control of cellular polarity (9), relatively little is known about the function of the AMPK-related kinases. The well-established role of AMPK in metabolism, however, directly implicates LKB1 in the maintenance of energy balance.The protein kinase Akt functions both in the control of cell proliferation and as a critical node in insulin signaling, appearing to mediate most of the metabolic effects of insulin (44). Akt is activated by phosphorylation of Thr 308 within the T loop of the catalytic domain and Ser 473 , located in a C-terminal, noncatalytic region of the enzyme (1). A mammalian homolog of D. melanogaster tribbles, TRB3, was recently identified as a negative regulator of Akt activity in human embryonic kidney 293 (HEK293) cells and mouse liver (10). In HEK293 cells and liver, TRB3 bind...

Abnormal expression of TRIB3 in colorectal cancer: a novel marker for prognosis

“…Trib2 has a role in adipogenesis in combination with the degradation of C/EBPbeta (Naiki et al, 2007). Trib3 promotes ubiquitination and degradation of proteins involved in cell-cycle regulation and oogenesis through an interaction with activation transcription factor 4, and is involved in the Pten pathway through interaction with Akt (Mata et al, 2000;Du et al, 2003;He et al, 2006;Koh et al, 2006;Kato and Du, 2007;Yao and Nyomba, 2008). Trib3 expression is increased in several primary tumours and cancer cell lines and can be controlled by nutrient starvation, which is consistent with these data (Bowers et al, 2003;Schwarzer et al, 2006;Xu et al, 2007).…”

“…Here we report on TRIB3 gene in the chromosomal region at 20p13, which is overexpressed in CRC, as a new marker for prognosis and metachronous metastasis. Trib3 is a human homologue of Drosophila tribbles 3, which regulates cell growth, differentiation, oogenesis and metabolism by promoting ubiquitination-dependent degradation of other proteins, interacts with several transcriptional factors and is expressed in several tumours (Mata et al, 2000;Bowers et al, 2003;Du et al, 2003;Koo et al, 2004;Boudeau et al, 2006;He et al, 2006;Koh et al, 2006;Matsushima et al, 2006;Ord et al, 2007;Kato and Du, 2007;Xu et al, 2007;Yao and Nyomba, 2008). We studied the TRIB3 gene in 202 paired cancerous and non-cancerous regions of CRC, as well as 7 colorectal cancer cell lines and 15 other gastrointestinal cancer cell lines.…”

LKB1; linking cell structure and tumor suppression

“…Somatic deletion of Lkb1 in the mouse skeletal muscle produces defects in glucose uptake and loss of AMPK activation (Sakamoto et al, 2005;Koh et al, 2006). Liver-specific Lkb1 deletion also causes metabolic defects and loss of activity of AMPK and increased mTOR signaling (Shaw et al, 2005).…”

Skeletal myoblast transplantation for attenuation of hyperglycaemia, hyperinsulinaemia and glucose intolerance in a mouse model of type 2 diabetes mellitus

“…An isolated defect in protein kinase C-λ in muscle was sufficient to induce abdominal obesity and other metabolic abnormalities [9]. By contrast, muscle-specific LKB1 (a serine/threonine kinase that is a negative regulator of insulin sensitivity) knockout increased insulin sensitivity and improved glucose homeostasis [10]. These studies suggest that defects in skeletal muscle insulin-stimulated glucose transport may be key factors in insulin resistance and type 2 diabetes mellitus [11].…”