Increased expression of the Akt/PKB inhibitor TRB3 in osteoarthritic chondrocytes inhibits insulin-like growth factor 1-mediated cell survival and proteoglycan synthesis.
OBJECTIVE
The chondrocyte response to insulin-like growth factor 1 (IGF-1) is reduced with aging and in osteoarthritis (OA). IGF-1 signals through the phosphatidylinositol 3-kinase/Akt pathway. TRB3, a tribbles homolog, has been shown to inhibit IGF-1-mediated activation of Akt in HEK 293 cells. This study was undertaken to determine if TRB3 is expressed in chondrocytes, and whether the chondrocyte response to IGF-1 is reduced by TRB3.


METHODS
Human articular cartilage was obtained from normal tissue donors and from patients with OA at the time of knee replacement surgery. TRB3 was assessed in the tissue samples by reverse transcription-polymerase chain reaction, immunoblotting, and immunohistochemistry. Overexpression of TRB3 was induced by transient transfection to determine the effects of TRB3 on cell survival and proteoglycan synthesis.


RESULTS
TRB3 messenger RNA was detected in normal human chondrocytes. TRB3 protein levels were low in cells from normal cartilage but significantly increased in cells from OA cartilage. Incubation with 2 agents that induce endoplasmic reticulum stress, tunicamycin and thapsigargin, increased TRB3 levels in normal cells. Overexpression of TRB3 inhibited Akt phosphorylation and reduced chondrocyte survival and proteoglycan synthesis.


CONCLUSIONS
These results are the first to demonstrate that TRB3 is present in human chondrocytes, and that the level of TRB3 is increased in OA cartilage and in isolated OA chondrocytes. Because it is an inhibitor of Akt activation, elevated TRB3 production could play a role in the increased cell death and reduced response to IGF-1 observed in OA cartilage.
Cravero
John D
JD
Wake Forest University School of Medicine, Winston-Salem, North Carolina, USA.
Carlson
Cathy S
CS
Im
Hee-Jeong
HJ
Yammani
Raghunatha R
RR
Long
David
D
Loeser
Richard F
RF
D000328:Adult; D000369:Aged, 80 and over; D000843:Ankle Joint; D002358:Cartilage, Articular / Q000378:metabolism; D018797:Cell Cycle Proteins / Q000235:genetics / Q000378:metabolism*; D002470:Cell Survival / Q000187:drug effects; D002648:Child; D019902:Chondrocytes / Q000187:drug effects / Q000378:metabolism* / Q000473:pathology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006801:Humans; D007334:Insulin-Like Growth Factor I / Q000037:antagonists & inhibitors / Q000378:metabolism*; D008875:Middle Aged; D020370:Osteoarthritis, Knee / Q000378:metabolism*; D010766:Phosphorylation / Q000187:drug effects; D017346:Protein Serine-Threonine Kinases / Q000235:genetics / Q000378:metabolism* / Q000037:antagonists & inhibitors; D011509:Proteoglycans / Q000096:biosynthesis* / Q000187:drug effects; D051057:Proto-Oncogene Proteins c-akt / Q000187:drug effects / Q000378:metabolism; D012097:Repressor Proteins / Q000235:genetics / Q000378:metabolism*; D019284:Thapsigargin / Q000494:pharmacology; D014415:Tunicamycin / Q000494:pharmacology; D055815:Young Adult
D018797:Cell Cycle Proteins; D004791:Enzyme Inhibitors; D011509:Proteoglycans; D012097:Repressor Proteins; C474831:TRIB3 protein, human; D014415:Tunicamycin; D019284:Thapsigargin; D007334:Insulin-Like Growth Factor I; D017346:Protein Serine-Threonine Kinases; D051057:Proto-Oncogene Proteins c-akt