Introduction
The elevated level of circulating catecholamines during chronic stress
is a hallmark for the initiation and progression of adverse cardiac
remodeling (Paur H et al., 2012; Adzika GK et al., 2019). Pathological
cardiac hypertrophy (PCH) is the irreversible resultant cardiomyopathy
if there are no timely preventive measures to subdue the excessively
firing of neurohormonal stimuli during chronic stress. Left ventricular
(LV) diastolic dysfunction is a clinical characteristic of patients
having PCH. This heart malfunction is a result of the abnormal increase
in heart size with excessively thickened ventricular walls which, are
stiffer due to the massive deposition of collagen. The typical
myocardium architecture becomes distorted. The LV becomes dilated, and
the heart loses its ability to rapidly replenish blood for the next
ejection (Xie M et al., 2013; Hartupee J et al., 2016). The heart
harbors resident macrophages which crosstalk between cardiomyocytes and
other cellular factions such as fibroblasts to ensure myocardial
homeostasis. However, this crosstalk implicates the immune system in
playing a pivotal role in the pathological remodeling of the heart
(Hartupee J et al., 2016; Kong P et al., 2014). Cardiac resident
macrophages are stimulated by cardiac danger-associated molecular
patterns (cDAMPs), cardiac troponin, and myosin that being released by
necrotic cardiomyocytes. These resident macrophages elicit
proinflammatory responses in an attempt to curb further myocyte necrosis
(Epelman S et al., 2014; Heidt T et al., 2014). This immune response is
aggravated under chronic catecholamine stress (CCS) conditions and a
biased hyperactive inflammatory result instead. The secretion of
Interleukin (IL) 1β, IL-2, IL-6, tumor necrosis factor-alpha (TNFα), and
interferon-gamma (IFN-γ) are excessively while IL-10 secretion becomes
downregulated. The aforementioned exacerbates the myocyte necrosis,
prolongs myofibroblast activation and increases interstitial fibrosis
markedly (Kong P et al., 2014; Hulsmans M et al., 2016). Interestingly,
the innate immune response elicited when challenged by cardiac troponin
and myosin during CCS is very similar with lipopolysaccharides
(LPS)-challenged macrophages under the same stress conditions (Laukova M
et al., 2018).
β1-adrenergic receptor
(β1AR) and
β2-adrenergic receptor (β2AR) expressed
in the heart mediate the neuroendocrine stimulates the regulate cardiac
function. β1AR modulates chronotropic and inotropic via
the typical stimulatory G protein (Gαs) – adenylyl
cyclases (ACs) – cyclic adenosine monophosphate (cAMP) pathway. The
pleiotropic nature of β2AR enables it to traffick
stimuli via Gαs or
Gαi in cardiomyocytes, and immune cells. Also, a
mounting of evidence indicates that β2AR mediates
maladaptive stimuli signaling during cardiovascular diseases (CVDs)
(Paur H et al., 2012; Adzika GK et al., 2019; Laukova M et al., 2018).
These suggests exploring the post-β2AR proteins and
kinases may have therapeutic potentials of attenuating the occurrence of
PCH. Furthermore, a negative correlation exists between ACs and GRKs in
healthy and failing cardiac and immune systems (Adzika GK et al., 2019).
Therefore, this study focuses on ACs and G protein-coupled receptor
kinases (GRKs) to exploit their possible therapeutic potentials.
ACs in generally synthesize cAMP. AC isoform AC7 is expressed explicitly
in immune cells. It induces anti-inflammatory response via increase cAMP
synthesis.2 While expression of the isoforms AC5 and
AC6 are cardiac specific (Li Y et al.,2012). cAMP is essential for
maintaining homeostasis in the immune system and proper cardiac function
of the heart. Besides the activation of protein kinase A (PKA) in
cardiomyocytes and immune cells, cAMP adaptively modulates the
activities nuclear factor of activated T-cells (NFATs) and NF-kB
(Kipanyula MJ et al, 2013; Pereira L et al., 2015; Murphy JG et al.,
2019; Gerlo S et al., 2011). However, during CCS, the ACs are uncoupled
from Gαs, terminating the synthesis of cAMP and
abolishing its modulation of proper cardiac and immune functions.
GRKs modulates the expression of β2AR. GRK2 isoform
mainly desensitizes and internalizes β2AR during
catecholamine stress. Conversely, GRK5 can translocate from the plasma
membrane into the nucleus. As such, its upregulation facilitates its
entry into the nucleus to maladaptively phosphorylate transcription
factors (TFs) that elicits adverse immune responses and pathological
cardiac remodeling (Fig. 1) (Patial S et al., 2011; Gravning J et al.,
2013; Hullmann JE et al., 2014). GRK5 blockade impedes the
non-canonically activation of immune and cardiac transcription factors,
NFATs, myocyte enhancer factor 2(MEF2), GATA4, Csx/Nkx2–5, and NF-kB.
Thus, GRK5 inhibition attenuates cardiac hypertrophy and, to some
extent, maladaptive immune response during stress (Hullmann JE et al.,
2014; Quan MY et al., 2019).
Nonetheless, there is no evidence that neither cardiac function was
improved nor attenuation of hyperactive immune responses when GRK5 was
knockout or inhibited with amlexanox (ALX) in an attempt to prevent PCH
during CCS. Also, besides the known roles AC5, AC6 and AC7, there have
been no attempts to directly stimulate their activities by using
Forskolin (FSK) to adaptively modulate the immune response and cardiac
function to prevent PCH.
Herein, we show that the combination therapy of ALX and FSK can maintain
proper cardiac function while sustaining immune homeostasis. Our data
from preliminary in vitro experiments revealed that the combination
treatment of peritoneal macrophages (PMɸ) with ALX and
FSK after being challenged with Isoproterenol (ISO) and LPS, attenuated
induction hyperactive proinflammatory responses. However, neither single
treatments with ALX nor FSK were able to attain this as much. These were
translated in vivo to curb inflammatory responses from exacerbating
cardiomyocyte necrosis and adversely remodeling the heart into PCH
during CCS.