Anti-diabetic activity against type 2 diabetes was
assayed according to method of Sauma L. et al. [31].
HepG2 cells were transfected with plasmid pPPRE-Luc and
plasmid phRL-TK. pPPRE-Luc was a plasmid in which the
PPAR-responsive element (PPRE) and the firefly luciferase
reporter gene were transferred, while plasmid phRL-TK as
internal reference was transfected with Renilla luciferase
reporter gene. After the transfected cells were seeded in
96-well plates at a cell density of 1.5 × 104
/well and cultured
in the low-sugar DMEM containing 100 U·mL−1
of
streptomycin and 100 U·mL−1 of penicillin at 37 o
C overnight.
The medium was refreshed with low-sugar DMEM
containing the sample under testing. The blank control
(non-transfected cells), negative control (cells transfected and
added no samples), and positive control (cells transfected and
added pioglitazone) were included and cultured under the
same conditions. After an additional 24-h culture, the
luciferase activity and excitation rate were determined by
measuring chemiluminescence intensity L using a DualLuciferase
Reporter Gene Assay Kit (Promega, USA). L1
represented the firefly luciferase chemiluminescence intensity,
while L2 represented the L value of internal reference. The
excitation rate (ER) was calculated using the following
equation