Among foods, the use of plant derivatives as promising
drugs and/or
excipients has been considered from various perspectives. In the present
study, curcumin, which is one of the most important plant derivatives
for biological uses, and four curcumin-based pyrido[2,3-d]pyrimidine analogs (C2-C5) were used for investigating the mechanism
of insulin fibrillation and evaluating the cytotoxicity of insulin
fibrils. The synthesized analogs differed in terms of hydrophobicity
and electrostatic charge. The analogs with more hydrophobicity (C1
and C4) in both acidic and neutral environments were able to reduce
the rate of insulin fibrillation and the degree of cross-linking in
the produced fibrils. Additionally, the toxicity of these fibrils
for neural cells (N2a cell line) was very low. However, they did not
show any significant effects on the toxicity of non-neural cells (HEK293
cell line), indicating the effect of the biochemical surface diversity
on determining the vulnerability to fibrils and even the mechanism
of action of additives on cell line survival. Although negatively
charged analogs were able to reduce insulin fibrillation in the acidic
environment, they indicated an opposite effect in the neutral environment.
The resultant fibrils in the acidic medium appeared with a well-distinguished
filament, but they were very close at neutral pH levels. Moreover,
such fibrils indicated very poor toxicity against the N2a cell line
and had no significant effects on HEK293 cells. Considering the docking
studies, by creatively using the size exclusion chromatography, it
was suggested that analogs C2 and C3 were capable of binding to the
C-terminal end of the insulin B chain (low affinity) and HisB10 (high affinity). Hence, it was suggested that different compounds
could play different protecting and/or destroying roles in cell toxicity
by blocking some ligands at the surface of neuron cells.