Role of atypical protein kinase C in activation of sterol regulatory element binding protein-1c and nuclear factor kappa B (NFkappaB) in liver of rodents used as a model of diabetes, and relationships to hyperlipidaemia and insulin resistance.
OBJECTIVE
Previous findings in rodents used as a model of diabetes suggest that insulin activation of atypical protein kinase C (aPKC) is impaired in muscle, but, unexpectedly, conserved in liver, despite impaired hepatic protein kinase B (PKB/Akt) activation. Moreover, aPKC at least partly regulates two major transactivators: (1) hepatic sterol receptor binding protein-1c (SREBP-1c), which controls lipid synthesis; and (2) nuclear factor kappa B (NFkappaB), which promotes inflammation and systemic insulin resistance.


METHODS
In Goto-Kakizaki rats used as a model of type 2 diabetes, we examined: (1) whether differences in hepatic aPKC and PKB activation reflect differences in activation of IRS-1- and IRS-2-dependent phosphatidylinositol 3-kinase (PI3K); (2) whether hepatic SREBP-1c and NFkappaB are excessively activated by aPKC; and (3) metabolic consequences of excessive activation of hepatic aPKC, SREBP-1c and NFkappaB.


RESULTS
In liver, as well as in muscle, IRS-2/PI3K activation by insulin was intact, whereas IRS-1/PI3K activation by insulin was impaired. Moreover, hepatic IRS-2 is known to control hepatic aPKC during insulin activation. Against this background, selective inhibition of hepatic aPKC by adenoviral-mediated expression of mRNA encoding kinase-inactive aPKC or short hairpin RNA targeting Irs2 mRNA and partially depleting hepatic IRS-2 diminished hepatic SREBP-1c production and NFkappaB activities, concomitantly improving serum lipids and insulin signalling in muscle and liver. Similar improvements in SREBP-1c, NFkappaB and insulin signalling were seen in ob/ob mice following inhibition of hepatic aPKC.


CONCLUSIONS
In diabetic rodent liver, diminished PKB activation may largely reflect impaired IRS-1/PI3K activation, while conserved aPKC activation reflects retained IRS-2/PI3K activity. Hepatic aPKC may also contribute importantly to excessive SREPB-1c and NFkappaB activities. Excessive hepatic aPKC-dependent activation of SREBP-1c and NFkappaB may contribute importantly to hyperlipidaemia and systemic insulin resistance.
Sajan
M P
MP
Research Service, James A Haley Veterans Hospital, Tampa, FL 33612, USA.
Standaert
M L
ML
Rivas
J
J
Miura
A
A
Kanoh
Y
Y
Soto
J
J
Taniguchi
C M
CM
Kahn
C R
CR
Farese
R V
RV
D000704:Analysis of Variance; D000818:Animals; D001786:Blood Glucose / Q000378:metabolism; D015153:Blotting, Western; D002784:Cholesterol / Q000097:blood; D003920:Diabetes Mellitus / Q000378:metabolism* / Q000503:physiopathology; D004195:Disease Models, Animal; D024202:Electrophoretic Mobility Shift Assay; D006949:Hyperlipidemias / Q000378:metabolism* / Q000503:physiopathology; D055504:Insulin Receptor Substrate Proteins / Q000235:genetics / Q000378:metabolism; D007333:Insulin Resistance / Q000502:physiology*; D008099:Liver / Q000378:metabolism*; D008297:Male; D009132:Muscles / Q000378:metabolism; D016328:NF-kappa B / Q000378:metabolism*; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D011493:Protein Kinase C / Q000378:metabolism / Q000502:physiology*; D051381:Rats; D017208:Rats, Wistar; D051780:Sterol Regulatory Element Binding Protein 1 / Q000378:metabolism*; D014280:Triglycerides / Q000097:blood
D001786:Blood Glucose; D055504:Insulin Receptor Substrate Proteins; C526295:Irs1 protein, mouse; D016328:NF-kappa B; D051780:Sterol Regulatory Element Binding Protein 1; D014280:Triglycerides; D002784:Cholesterol; D019869:Phosphatidylinositol 3-Kinases; C115015:PKC-3 protein; D011493:Protein Kinase C