4.4 Effects of isorhamnetin on structural remodeling.
The findings of our study also suggest that the inhibitory effect of
isorhamnetin on structural
remodeling
may be another factor involved in its proarrhythmic effects. JNK, a
major member of the MAPK family, is activated in response to stress. It
has been reported that JNK activation leads to cell proliferation,
apoptosis, ROS production, cytokine production, fibrosis, and
hypertrophy. Moreover, JNK promotes diastolic SR Ca2+leakage from the RyR2 channel through CaMKII activation (Nattel et al.,
2020). In addition, ERK is a Thr/Ser kinase that leads to cell
proliferation, cell differentiation, cell cycle regulation, cell
apoptosis, and tissue formation when activated. In addition, ERK
activation is associated with CaMKII activation (Lu et al., 2009).
Furthermore, the importance of TRPCs in the Ca2+signaling mechanism of cardiomyocytes has recently been discussed (Tse,
2016).
TRPCs are transmembrane non-selective cation channels that play
important roles in various biological functions, including neuronal cell
survival, immune cell maturation, cardiac fibrosis, and hypertrophy.
TRPCs have also
been
associated with arrhythmic remodeling stimuli by modulating
Ca2+ entry via cardiac fibroblast responses (Rose et
al., 2012). Among these, the role of TRPC3/6 arrhythmias, including AF,
has been discussed (Harada et al., 2012; Nikolova-Krstevski et al.,
2017). TRPC3 regulates cardiac fibroblast function via
Ca2+-dependent ERK phosphorylation (Harada et al.,
2012). In contrast, TRPC6 contributes to the regulation of myofibroblast
formation caused by endothelin-1 via JNK signaling (Nishida et al.,
2007). Although some reports have shown that isorhamnetin inhibits
cardiac remodeling by suppressing the JNK and ERK pathways, there are no
reports indicating the evaluation of Ca2+ channels and
molecules related to the suppression of ERK and JNK and their upstream
or downstream pathways (Sun et al., 2012; Sun et al., 2013). Our study
showed that isorhamnetin reversed JNK and ERK activation and TRPC3/6
protein expression induced by AngII (Fig. 7.1H-I, Fig. 7.2B-C). These
results suggest that isorhamnetin diminishes CaMKII activation and
normalizes Ca2+ entry through Ca2+channels to suppress the expression of these proteins. Moreover, similar
to the results shown previously for ventricles, isorhamnetin
morphologically inhibited atrial fibrosis and hypertrophy and reversed
the related gene expression (Fig. 5A-D, Fig. 6A-D). Isorhamnetin also
suppressed the AngII-induced upregulation of NF-κB, which is the most
fundamental indicator of inflammation and crosstalk with MAPK (Fig.
7.2D). These results emphasize the assertion that isorhamnetin strongly
diminishes AngII-induced fibrosis and the enlargement of the atrium via
normalized TRPC-mediated MAPK
expression.