Gap-junctional communication is required for mitotic clonal expansion during adipogenesis.
OBJECTIVE
Gap-junctional communication (GJC) plays critical roles in cell growth and differentiation. Several studies have demonstrated the involvement of GJC in myogenesis and osteogenesis; however, the role of GJC in adipogenesis has not been fully studied. Thus, we investigated the role of GJC in adipogenesis.


METHODS
3T3-L1 preadipocytes were differentiated in the presence of gap junction inhibitor, 18-alpha-glycyrrhetinic acid (AGA), and accumulation of cytoplasmic triglycerides was measured. 3T3-L1 cells were transfected with 100 nM small interfering RNA duplexes targeting connexin (Cx) 43. The mRNA levels of CCAAT/enhancer-binding protein (C/EBP) alpha, peroxisome proliferator-activated receptor gamma, glucose transporter 4, C/EBPbeta, and Cx43 were measured by real-time polymerase chain reaction. The protein levels of C/EBPbeta were quantitated by Western blotting. The cell proliferation was measured by counting cell numbers, and DNA synthesis was measured by bromodeoxyuridine incorporation.


RESULTS
AGA inhibited adipocyte differentiation dose-dependently. The lipid accumulation and the mRNA levels of C/EBPalpha, peroxisome proliferator-activated receptor gamma, and glucose transporter 4 were markedly reduced in AGA-treated adipocytes. The mRNA levels of C/EBPbeta did not decrease; however, C/EBPbeta [liver-enriched transcriptional activator protein (LAP)] expression and the C/EBPbeta (LAP)-to-C/EBP [liver-enriched transcriptional inhibitory protein (LIP)] ratio were reduced by AGA treatment. The increase in both cell number and DNA synthesis, which occurs during mitotic clonal expansion, was reduced by AGA in a dose-dependent fashion. The major component of gap junctions in 3T3-L1 cells was Cx43. Down-regulation of Cx43 using small interfering RNA reduced the expression of C/EBPbeta (LAP) and inhibited adipogenesis.


CONCLUSIONS
Our data suggest that GJC plays some important roles in adipogenesis through inhibiting mitotic clonal expansion and modulating C/EBPbeta (LAP) expression.
Yanagiya
Takahiro
T
Laboratory for Obesity, Research Group for Disease-Causing Mechanism, SNP Research Center, RIKEN, 1-7-22, Suehiro, Tsurumi-ku, Yokohama, Kanagawa 230-0045, Japan.
Tanabe
Atsushi
A
Hotta
Kikuko
K
D041721:3T3-L1 Cells; D017667:Adipocytes / Q000187:drug effects / Q000378:metabolism; D050156:Adipogenesis / Q000502:physiology*; D000818:Animals; D002450:Cell Communication / Q000187:drug effects / Q000502:physiology*; D049109:Cell Proliferation; D018031:Connexin 43 / Q000037:antagonists & inhibitors / Q000378:metabolism; D017629:Gap Junctions / Q000187:drug effects / Q000502:physiology*; D006034:Glycyrrhetinic Acid / Q000494:pharmacology; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D008938:Mitosis / Q000187:drug effects / Q000502:physiology*; D034741:RNA, Small Interfering / Q000494:pharmacology
D018031:Connexin 43; D034741:RNA, Small Interfering; D006034:Glycyrrhetinic Acid