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Protein kinases in hormonal regulation of adipocyte metabolism.

Berggreen, Christine LU (2014) In Lund University, Faculty of Medicine Doctoral Dissertation Series 2014:90.
Abstract
Abstract

Along with liver and muscle tissue, adipose tissue helps maintain normal levels of

glucose and lipids in the blood and has a very important role when it comes to storing

lipids that can provide whole-body energy. After a meal is ingested, adipocytes take

up glucose from the circulation and use it as a substrate for synthesis of new fatty

acids (FAs) in a process known as de novo fatty acid synthesis, as well as for synthesis

of glycerol. Adipocytes also take up fatty acids from the circulation and incorporate

both newly synthesized and imported FAs into triacylglycerides (TAGs), in a process

known as lipogenesis. TAGs are stored in large lipid droplets... (More)
Abstract

Along with liver and muscle tissue, adipose tissue helps maintain normal levels of

glucose and lipids in the blood and has a very important role when it comes to storing

lipids that can provide whole-body energy. After a meal is ingested, adipocytes take

up glucose from the circulation and use it as a substrate for synthesis of new fatty

acids (FAs) in a process known as de novo fatty acid synthesis, as well as for synthesis

of glycerol. Adipocytes also take up fatty acids from the circulation and incorporate

both newly synthesized and imported FAs into triacylglycerides (TAGs), in a process

known as lipogenesis. TAGs are stored in large lipid droplets in the cytosol, and

during fasting, or in response to physical exercise, they are hydrolysed in a process

known as lipolysis, in which FAs are released into the bloodstream for use as energy

substrates in other tissues. These cycles of lipogenesis and lipolysis are controlled by

the concerted actions of insulin, a hormone that is secreted by the pancreas and

catecholamines, hormones that are secreted by the adrenal glands, or derive from the

nervous system. Both glucose- and fatty acid uptake, as well as lipid storage and

mobilization, are regulated by cellular signaling, and kinases are central enzymatic

players in hormone-induced cellular signaling. A dysfunctional adipose tissue can

contribute to insulin resistance in many obese individuals. Therefore it is important

to elucidate the cellular mechanisms that govern metabolic processes in adipocytes.

Insulin is the hormone that promotes glucose uptake and lipogenesis in adipocytes,

and when it induces glucose uptake, insulin exerts it actions through protein kinase B

(PKB). Although PKB is known to mediate many effects of insulin, its role in

lipogenesis in adipocytes is less clear. We show that PKB is important for the effects

of insulin on lipogenesis (de novo and total). We also reveal that PKB can regulate

Amp-activated protein kinase (AMPK) in adipocytes by a mechanism previously only

seen in heart muscle cells. AMPK is a sensor of cellular energy status and known to

inhibit lipogenesis. We speculate that insulin possibly mediates its lipogenic effects via

a decrease in AMPK activity accomplished by PKB-phosphorylation of S485 on

AMPK.

Furthermore, we find that salt-inducible kinase 3 (SIK3), a kinase that belongs to the

AMPK-related family of kinases, and displays structural similarities to AMPK, can be

regulated by catecholamines in adipocytes. Catecholamines are hormones that bind to

β-adrenergic receptors and act by increasing cellular levels of cAMP, which in turn

activates protein kinase A (PKA). We find that in response to such β-adrenergic

stimuli, SIK3 is phosphorylated on multiple serine and threonine residues. This 10

regulation coincides with an increase in binding of SIK3 to 14-3-3 molecules. 14-3-3

proteins are cellular scaffolding proteins that can result in cellular re-localization of

their binding partners or in their binding to other proteins or lipids. We find that

when SIK3 is phosphorylated in response to β-adrenergic stimuli, the kinase does not

re-localize, but is partially de-activated. We speculate that SIK3 could potentially have

a role in adipocyte metabolism, as it is regulated by catecholamines in this tissue.

Finally, we address the current understanding of the role for AMPK in modulation of

the effects of insulin and catecholamines on glucose uptake and lipid metabolism. To

this date, it has been suggested that AMPK reduces insulin-induced glucose uptake

and lipogenesis, as well as inhibits catecholamine-induced lipolysis in adipocytes.

These findings are mainly based on studies performed with AMPK activating agents

that act on AMPK in an indirect manner. We have used the allosteric activator

A769662, that binds directly to AMPK, and find that AMPK does not appear to

modulate hormonally induced glucose uptake, lipolysis or total lipogenesis. However,

when we specifically measured the synthesis of new FAs, using acetate as a lipogenic

substrate (as opposed to using glucose as a substrate, a molecule which can participate

in both FA and glycerol synthesis), we observe that AMPK does indeed reduce

insulin-induced de novo fatty acid synthesis.

Collectively, we add novel findings to the available knowledge on key kinases and

cellular signaling in adipocyte metabolism. Our findings contribute to the

understanding of insulin- and catecholamine-mediated control of lipid storage in

adipose tissue, a biological function that, when dysfunctional, is strongly linked to

insulin resistance and type 2 diabetes (T2D). (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Professor Wojtaszewski, Jørgen, Köpenhamn Univ.
organization
publishing date
type
Thesis
publication status
published
subject
keywords
Adipocyte, PKB, AMPK, SIK3, insulin, catecholamines, lipolysis, lipogenesis, glucose uptake, de novo fatty acid synthesis, A769662, Akti, cAMP
in
Lund University, Faculty of Medicine Doctoral Dissertation Series
volume
2014:90
pages
80 pages
publisher
Protein Phosphorylation, Faculty of Medicine
defense location
Segerfalksalen, BMC A10, Sölvegatan 17, Lund.
defense date
2014-09-05 09:00
ISSN
1652-8220
ISBN
978-91-7619-019-7
language
English
LU publication?
yes
id
c5addb21-c78d-4976-ae55-dbb5ac0eeee3 (old id 4646812)
date added to LUP
2014-09-15 14:03:53
date last changed
2016-09-19 08:44:48
@phdthesis{c5addb21-c78d-4976-ae55-dbb5ac0eeee3,
  abstract     = {Abstract<br/><br>
Along with liver and muscle tissue, adipose tissue helps maintain normal levels of <br/><br>
glucose and lipids in the blood and has a very important role when it comes to storing <br/><br>
lipids that can provide whole-body energy. After a meal is ingested, adipocytes take <br/><br>
up glucose from the circulation and use it as a substrate for synthesis of new fatty <br/><br>
acids (FAs) in a process known as de novo fatty acid synthesis, as well as for synthesis <br/><br>
of glycerol. Adipocytes also take up fatty acids from the circulation and incorporate <br/><br>
both newly synthesized and imported FAs into triacylglycerides (TAGs), in a process <br/><br>
known as lipogenesis. TAGs are stored in large lipid droplets in the cytosol, and <br/><br>
during fasting, or in response to physical exercise, they are hydrolysed in a process <br/><br>
known as lipolysis, in which FAs are released into the bloodstream for use as energy <br/><br>
substrates in other tissues. These cycles of lipogenesis and lipolysis are controlled by <br/><br>
the concerted actions of insulin, a hormone that is secreted by the pancreas and <br/><br>
catecholamines, hormones that are secreted by the adrenal glands, or derive from the<br/><br>
nervous system. Both glucose- and fatty acid uptake, as well as lipid storage and <br/><br>
mobilization, are regulated by cellular signaling, and kinases are central enzymatic <br/><br>
players in hormone-induced cellular signaling. A dysfunctional adipose tissue can <br/><br>
contribute to insulin resistance in many obese individuals. Therefore it is important <br/><br>
to elucidate the cellular mechanisms that govern metabolic processes in adipocytes. <br/><br>
Insulin is the hormone that promotes glucose uptake and lipogenesis in adipocytes,<br/><br>
and when it induces glucose uptake, insulin exerts it actions through protein kinase B <br/><br>
(PKB). Although PKB is known to mediate many effects of insulin, its role in <br/><br>
lipogenesis in adipocytes is less clear. We show that PKB is important for the effects <br/><br>
of insulin on lipogenesis (de novo and total). We also reveal that PKB can regulate <br/><br>
Amp-activated protein kinase (AMPK) in adipocytes by a mechanism previously only <br/><br>
seen in heart muscle cells. AMPK is a sensor of cellular energy status and known to <br/><br>
inhibit lipogenesis. We speculate that insulin possibly mediates its lipogenic effects via <br/><br>
a decrease in AMPK activity accomplished by PKB-phosphorylation of S485 on <br/><br>
AMPK. <br/><br>
Furthermore, we find that salt-inducible kinase 3 (SIK3), a kinase that belongs to the <br/><br>
AMPK-related family of kinases, and displays structural similarities to AMPK, can be <br/><br>
regulated by catecholamines in adipocytes. Catecholamines are hormones that bind to <br/><br>
β-adrenergic receptors and act by increasing cellular levels of cAMP, which in turn <br/><br>
activates protein kinase A (PKA). We find that in response to such β-adrenergic <br/><br>
stimuli, SIK3 is phosphorylated on multiple serine and threonine residues. This 10<br/><br>
regulation coincides with an increase in binding of SIK3 to 14-3-3 molecules. 14-3-3 <br/><br>
proteins are cellular scaffolding proteins that can result in cellular re-localization of <br/><br>
their binding partners or in their binding to other proteins or lipids. We find that <br/><br>
when SIK3 is phosphorylated in response to β-adrenergic stimuli, the kinase does not <br/><br>
re-localize, but is partially de-activated. We speculate that SIK3 could potentially have <br/><br>
a role in adipocyte metabolism, as it is regulated by catecholamines in this tissue. <br/><br>
Finally, we address the current understanding of the role for AMPK in modulation of <br/><br>
the effects of insulin and catecholamines on glucose uptake and lipid metabolism. To <br/><br>
this date, it has been suggested that AMPK reduces insulin-induced glucose uptake <br/><br>
and lipogenesis, as well as inhibits catecholamine-induced lipolysis in adipocytes. <br/><br>
These findings are mainly based on studies performed with AMPK activating agents <br/><br>
that act on AMPK in an indirect manner. We have used the allosteric activator <br/><br>
A769662, that binds directly to AMPK, and find that AMPK does not appear to <br/><br>
modulate hormonally induced glucose uptake, lipolysis or total lipogenesis. However, <br/><br>
when we specifically measured the synthesis of new FAs, using acetate as a lipogenic <br/><br>
substrate (as opposed to using glucose as a substrate, a molecule which can participate <br/><br>
in both FA and glycerol synthesis), we observe that AMPK does indeed reduce <br/><br>
insulin-induced de novo fatty acid synthesis.<br/><br>
Collectively, we add novel findings to the available knowledge on key kinases and <br/><br>
cellular signaling in adipocyte metabolism. Our findings contribute to the <br/><br>
understanding of insulin- and catecholamine-mediated control of lipid storage in <br/><br>
adipose tissue, a biological function that, when dysfunctional, is strongly linked to <br/><br>
insulin resistance and type 2 diabetes (T2D).},
  author       = {Berggreen, Christine},
  isbn         = {978-91-7619-019-7},
  issn         = {1652-8220},
  keyword      = {Adipocyte,PKB,AMPK,SIK3,insulin,catecholamines,lipolysis,lipogenesis,glucose uptake,de novo fatty acid synthesis,A769662,Akti,cAMP},
  language     = {eng},
  pages        = {80},
  publisher    = {Protein Phosphorylation, Faculty of Medicine},
  school       = {Lund University},
  series       = {Lund University, Faculty of Medicine Doctoral Dissertation Series},
  title        = {Protein kinases in hormonal regulation of adipocyte metabolism.},
  volume       = {2014:90},
  year         = {2014},
}