LKB1 signalling attenuates early events of adipogenesis and responds to adipogenic cues.

cAMP-response element-binding protein (CREB) is required for the induction of adipogenic transcription factors such as CCAAT/enhancer-binding proteins (C/EBPs). Interestingly, it is known from other tissues that LKB1 and its substrates AMP-activated protein kinase (AMPK) and salt-inducible kinases (SIKs), negatively regulate gene expression by phosphorylating the CREB co-activator CRTC2 and class IIa histone deacetylases (HDACs), which results in their exclusion from the nucleus where they co-activate or inhibit their targets. In this study, we show that AMPK/SIK signalling is acutely attenuated during adipogenic differentiation of 3T3-L1 preadipocytes, which coincides with dephosphorylation and nuclear translocation of CRTC2 and HDAC4. When subjected to differentiation, 3T3-L1 preadipocytes in which LKB1 expression was stably reduced using shRNA (LKB1-shRNA), as well as LKB1 knockout mouse embryonic fibroblasts (LKB1-/- MEFs), differentiated more readily into adipocyte-like cells and accumulated more triglycerides compared to scrambled-shRNA 3T3-L1 cells or Wt MEFs. In addition, the phosphorylation of CRTC2 and HDAC4 was reduced, and the mRNA expression of adipogenic transcription factors C/EBPα, peroxisome proliferator-activated receptor γ (PPARγ) and adipocyte-specific proteins such as hormone sensitive lipase (HSL), fatty acid synthase (FAS), aP2, Glut4 and adiponectin was increased in the absence of LKB1. The mRNA and protein expression of CHOP-10, a dominant negative member of the C/EBP family, was reduced in LKB1 shRNA expressing cells, providing a potential mechanism for the up-regulation of Pparg and Cebpa. These results support the hypothesis that LKB1 signalling keeps preadipocytes in their non-differentiated form

Chemical genetic screen identifies Gapex-5/GAPVD1 and STBD1 as novel AMPK substrates

AMP-activated protein kinase (AMPK) is a key regulator of cellular energy homeostasis, acting as a sensor of energy and nutrient status. As such, AMPK is considered a promising drug target for treatment of medical conditions particularly associated with metabolic dysfunctions. To better understand the downstream effectors and physiological consequences of AMPK activation, we have employed a chemical genetic screen in mouse primary hepatocytes in an attempt to identify novel AMPK targets. Treatment of hepatocytes with a potent and specific AMPK activator 991 resulted in identification of 65 proteins phosphorylated upon AMPK activation, which are involved in a variety of cellular processes such as lipid/glycogen metabolism, vesicle trafficking, and cytoskeleton organisation. Further characterisation and validation using mass spectrometry followed by immunoblotting analysis with phosphorylation site-specific antibodies identified AMPK-dependent phosphorylation of Gapex-5 (also known as GTPase-activating protein and VPS9 domain-containing protein 1 (GAPVD1)) on Ser902 in hepatocytes and starch-binding domain 1 (STBD1) on Ser175 in multiple cells/tissues. As new promising roles of AMPK as a key metabolic regulator continue to emerge, the substrates we identified could provide new mechanistic and therapeutic insights into AMPK-activating drugs in the liver

Adipocyte-specific protein tyrosine phosphatase 1B deletion increases lipogenesis, adipocyte cell size and is a minor regulator of glucose homeostasis

Peer reviewedPublisher PD

Insulin Signalling and Regulation of Protein Kinase B in Adipocytes

Insulin resistance is a hallmark of type 2 diabetes, an increasingly common disorder. The cause of insulin resistance is supposedly failures in the processes used by insulin to signal to the interior of its target cells. These failing steps are still unknown, most probably because of incomplete knowledge of how the insulin signals are transmitted. Since insulin resistance is strongly linked to obesity, defects in lipid metabolism or other adipocyte functions, may be an important factor in the development of this pathological states. It is therefore of particular interest to study insulin signalling and lipid metabolism in adipose tissue. More specifically, the aim of this thesis was to study the regulation of adipocyte protein kinase B (PKB), an insulin-stimulated kinase that has been implicated in mediating many of insulin's metabolic as well as mitogenic effects. We have shown, that in response to insulin, adipocyte PKB translocates from the cytosol to the plasma membrane in a phosphoinositide 3-kinase (PI3-K)-dependent manner. This is believed to induce a conformational change in PKB, allowing it to be phosphorylated and activated by the upstream kinases phosphoinositide-dependent kinase (PDK) -1 and 2. We have demonstrated that PKBß in primary adipocytes is unphosphorylated prior to stimulation, and insulin mainly induces phosphorylation on Ser-474. Furthermore, protein phosphatase 2A (PP2A) was identified as the phosphatase responsible for dephosphorylation and deactivation of PKB in adipocytes. In addition, we have initiated an investigation regarding the regulation and role of PDK1 in adipocytes. Endogenous PDK1 was shown not to be activated, but to translocate from the cytosol to the membrane fraction, in response to insulin. Moreover, adenoviral-mediated expression of PDK1 was used in order to assess the role of PDK1 in primary adipocytes. A recent study has been focused on the kinase inhibitor dimethylaminopurine (DMAP), and its effects on metabolic signalling pathways in adipocytes. DMAP was demonstrated to inhibit insulin-induced glucose uptake, antilipolysis and lipogenesis. Possible molecular targets, inhibition of which may mediate the effects of DMAP, were shown to be PKB and c-jun N-terminal kinase (JNK). In summary, this thesis has provided valuable information regarding the molecular mechanisms underlying insulin-induced activation of PKB, a key component of the insulin signalling pathway

The Salt-Inducible Kinases : Emerging Metabolic Regulators

The discovery of liver kinase B1 (LKB1) as an upstream kinase for AMP-activated protein kinase (AMPK) led to the identification of several related kinases that also rely on LKB1 for their catalytic activity. Among these, the salt-inducible kinases (SIKs) have emerged as key regulators of metabolism. Unlike AMPK, SIKs do not respond to nucleotides, but their function is regulated by extracellular signals, such as hormones, through complex LKB1-independent mechanisms. While AMPK acts on multiple targets, including metabolic enzymes, to maintain cellular ATP levels, SIKs primarily regulate gene expression, by acting on transcriptional regulators, such as the cAMP response element-binding protein-regulated transcription coactivators and class IIa histone deacetylases. This review describes the development of research on SIKs, from their discovery to the most recent findings on metabolic regulation

AMPKβ isoform expression patterns in various adipocyte models and in relation to body mass index

AMP-activated protein kinase (AMPK) activation is considered a useful strategy for the treatment of type 2 diabetes (T2D). It is unclear whether the expression and/or activity of AMPK in adipocytes is dysregulated in obesity. Also, the expression/activity pattern of AMPKβ isoforms, which are targets for AMPK activators, in adipocytes remains elusive. In this study we show that the two AMPKβ isoforms make roughly equal contributions to AMPK activity in primary human and mouse adipocytes, whereas in cultured 3T3-L1 adipocytes of mouse origin and in primary rat adipocytes, β1-associated activity clearly dominates. Additionally, we found that obesity is not associated with changes in AMPK subunit expression or kinase activity in adipocytes isolated from subcutaneous adipose tissue from individuals with various BMI

Mechanism of TNFα-induced downregulation of salt-inducible kinase 2 in adipocytes

Salt-inducible kinase 2 (SIK2) is highly expressed in white adipocytes, but downregulated in individuals with obesity and insulin resistance. These conditions are often associated with a low-grade inflammation in adipose tissue. We and others have previously shown that SIK2 is downregulated by tumor necrosis factor α (TNFα), however, involvement of other pro-inflammatory cytokines, or the mechanisms underlying TNFα-induced SIK2 downregulation, remain to be elucidated. In this study we have shown that TNFα downregulates SIK2 protein expression not only in 3T3L1- but also in human in vitro differentiated adipocytes. Furthermore, monocyte chemoattractant protein-1 and interleukin (IL)-1β, but not IL-6, might also contribute to SIK2 downregulation during inflammation. We observed that TNFα-induced SIK2 downregulation occurred also in the presence of pharmacological inhibitors against several kinases involved in inflammation, namely c-Jun N-terminal kinase, mitogen activated protein kinase kinase 1, p38 mitogen activated protein kinase or inhibitor of nuclear factor kappa-B kinase (IKK). However, IKK may be involved in SIK2 regulation as we detected an increase of SIK2 when inhibiting IKK in the absence of TNFα. Increased knowledge about inflammation-induced downregulation of SIK2 could ultimately be used to develop strategies for the reinstalment of SIK2 expression in insulin resistance

Insulin-induced translocation of protein kinase B to the plasma membrane in rat adipocytes

Protein kinase B (PKB) has previously been shown to be activated in response to insulin and growth factor stimulation. The activation mechanism has been suggested to involve translocation of PKB to membranes, where it is phosphorylated and activated. Insulin-induced translocation of PKB has not been demonstrated in a physiological target cell. Therefore we have used the primary rat adipocyte to investigate insulin-induced translocation of PKB. In the presence of 1 nM insulin translocation of PKB was detected within 30 seconds and was blocked by wortmannin, a selective phosphatidylinositol 3-kinase inhibitor. This translocation was potentiated by the tyrosine phosphatase inhibitor vanadate. Subcellular localization studies revealed that PKB translocated to the plasma membrane

Protein kinase B activity is required for the effects of insulin on lipid metabolism in adipocytes.

Protein kinase B is known to mediate a number of biological responses to insulin and growth factors, its role in glucose uptake being one of the most extensively studied. In this paper, we have employed a recently described allosteric inhibitor of PKB, Akti, to clarify the role of PKB in lipid metabolism in adipocytes - a subject that has received less attention. Pretreatment of primary rat and 3T3L1 adipocytes with Akti resulted in dose-dependent inhibition of PKB phosphorylation and activation in response to insulin, without affecting upstream insulin signaling (IR, IRS) or the insulin-induced PI3-K dependent activation of the ERK/RSK pathway. PKB activity was required for the insulin-induced activation of PDE3B and for the anti-lipolytic action of insulin. Moreover, inhibition of PKB activity resulted in a reduction in de novo lipid synthesis and in the ability of insulin to stimulate this process. The regulation of the rate-limiting lipogenic enzyme ACC by insulin through dephosphorylation of S79, which is a target for AMPK, was dependent on the presence of active PKB. Lastly, AMPK was shown to be phosphorylated by PKB on S485 in response to insulin and this was associated with a reduction in AMPK activity. In summary, we propose that PKB is required for the positive effects of insulin on lipid storage, and that regulation of PDE3B and AMPK by PKB is important for these effects. Key words: Akt, PDE3B, ACC, AMPK, lipogenesis