[The mechanisms of stimulated lipolysis by high concentration of glucose in primary rat adipocytes].
OBJECTIVE
To investigate the cellular mechanisms of adipocyte lipolytic response to a high concentration of glucose, which liberates free fatty acids (FFA) efflux from adipose cells to plasma.


METHODS
Adipocytes were isolated from epididymal fat pads of Sprague-Dawley (SD) rats, and were incubated in the presence or absence of excess glucose (25 mmol/L). Glycerol released in the culture medium was determined by use of a colorimetric assay and served as an index of lipolysis. The data are expressed as micromoles glycerol per liter packed cell volume (PCV) of adipocytes. The protein levels of native and phosphorylated perilipin, hormone-sensitive lipase (HSL) and adipose triglyceride lipase (ATGL) were examined by immunoblotting. Adipose lipases activity was determined by performing an enzymatic assay.


RESULTS
A high concentration of glucose at 25 mmol/L caused a significant increase in lipolysis in primary rat adipocytes. A 24 h incubation with 25 mmol/L glucose elevated glycerol accumulation from 4.4 mmol/L PCV to 6.4 mmol/L PCV, by 1.5 fold (P<0.01) and promoted glycerol release from 0.11 mmol/L PCV to 0.24 mmol/L PCV, by 2.1 fold (P<0.01). The lipolytic response mediated by high glucose occurred apparently at the end of 16 h and remained high after 24 h incubation. The lipolytic action of glucose at 25 mmol/L was more effective than that at 10 mmol/L. To elucidate the mechanism by which high glucose stimulated lipolysis, we showed that high glucose promoted the phosphorylation of perilipin but did not alter its protein level. Further, we showed that high glucose caused a significant increase in adipose lipases activity by 1.74 fold. The protein level of HSL but not ATGL was significantly upregulated by high glucose. Moreover, the acute lipolytic response to isoproterenol was further enhanced by high glucose.


CONCLUSIONS
High glucose directly stimulates the releases of glycerol and FFA from adipocytes; the cellular mechanisms of this lipolytic action involve promoted perilipin phosphorylation, augmented HSL expression and increased adipose lipases activity under high glucose conditions. These results suggest that a high concentration of glucose alone can directly liberate FFA from adipocyte to plasma, hence increasing systemic FFA levels and impairing insulin sensitivity.
Zhang
Ting Ting
TT
Department of Endocrinology, Peking University First Hospital, Beijing, China.
Xu
Chong
C
Zu
Lu Xia
LX
He
Jin Han
JH
Pu
Shen Shen
SS
Guo
Xiao Hui
XH
Xu
Guo Heng
GH
D017667:Adipocytes / Q000166:cytology / Q000378:metabolism*; D000818:Animals; D002478:Cells, Cultured; D005230:Fatty Acids, Nonesterified / Q000097:blood*; D005947:Glucose / Q000494:pharmacology*; D008066:Lipolysis / Q000187:drug effects*; D008297:Male; D051381:Rats; D017207:Rats, Sprague-Dawley
D005230:Fatty Acids, Nonesterified; D005947:Glucose