The role of the current conventional therapy of ischemic heart diseases is inadequate to protect against future ischemic events and further myocardial damage or cardiomyocyte regeneration. Unlike MSC therapy, it revealed a decrease in scar size, myocardial fibroplasia, and cardiac function improvement [
56,
57,
58]. MSCs release a variety of biologically active immunomodulators such as growth factors cytokines, and chemokines, which possess anti-inflammatory, anti-fibrotic, angiogenic, proliferative, and regenerative effects [
59,
60]. Several studies investigated the effects of MSCs on myocardial infarction. MSCs enhance neovascularization, cardiomyocyte repair, and replacement of dead myocardium in addition to the prevention of myofibril apoptosis [
58]. However, the microenvironment of myocardial ischemia is detrimental for the transplanted cells mainly due to hypoxia and high free radical concentration [
61]. Therefore, preconditioning of MSCs with pharmacological or chemical agents was investigated to improve their therapeutic and regenerative effects. In the present study, MSCs were cultured with L-carnitine, to explore its beneficial role. The main influence of L-carnitine is ATP production and inhibition of apoptosis, by stabilizing the inner mitochondrial membrane, altering membrane potential, and inhibiting mitochondrial splitting [
62]. It also enhances the hemodynamic functions and reduces the infarct size, which is consistent with the findings of the present study. A significant reduction of myocardial infarction serum markers; serum CK-MB, and troponin I level, was detected in the groups treated with MSCs, L-carnitine, and L-carnitine-pretreated MSCs, compared with the DOX-treated group which agrees with those reported in previous studies [
63,
64]. DOX-induced cardiotoxicity can manifest as acute, subacute, or chronic. Acute cardiotoxicity from DOX starts directly following administration, with an incidence rate of presenting echocardiographic abnormalities in around 11% of the patients [
13,
65]. Therefore, regular monitoring of cardiac biomarkers is essential for patients undergoing DOX treatment to detect early signs of myocardial injury. Various mechanisms of DOX-induced cardiomyopathy were investigated. Oxidative stress, manifested by reactive oxygen species (ROS)-rich microenvironment is a leading cause of cardiac injury. The cardiomyocytes are highly susceptible to oxidative damage, due to their lower levels of antioxidant enzymes such as peroxidase, catalase, and superoxide dismutase [
66]. In agreement with these findings, the current study represented significantly lower levels of the antioxidant enzyme catalase and elevation of the MDA. MDA is a common measure of oxidative stress indicating a significant increase in lipid peroxidation [
63]. High levels of oxidative damage devastate the cellular antioxidant defense systems triggering the apoptotic and necrotic cascades [
67]. MSCs, particularly under oxidative stress conditions, produce various secretomes that contain cytokines, proteins, growth factors, and exosomes that boost the antioxidant capacity of MSCs. Moreover, under hypoxic conditions, the migration of MSCs is stimulated by chemokine receptor (CXCR7) overexpression through increasing the vascular cells adhesion molecule-1 (VCAM-1) [
68]
. Specifically, L-carnitine facilitates STAT3 activation and significantly elevates the expression of the antioxidant protein SOD2. In the injured cardiac cells, marked by mitochondrial dysfunction and impaired CaMKII signaling, L-carnitine promotes CaMKII expression and activation, as well as enhances AMPK protein synthesis [
69]. Consistent with the mentioned studies, preconditioned-MSCs with L-carnitine revealed the most significant improvement of the cellular antioxidant defense mechanism manifested by MDA and catalase results.
Acute cardiac injury induced by DOX is characterized by cytoplasmic vacuolation and disruption and thinness of myofibrils [
70]. Other architecture changes found in the present study demonstrated in the cardiac tissue, following DOX administration, are consistent with several research. They include loss of cardiac muscle fibers, degenerated eosinophilic fibers with pyknotic nuclei, congested blood vessels, fibroplasia, and inflammatory cellular infiltration mainly neutrophils [
23,
71]. This impairment is associated with a significant inflammatory response revealed by injury of the endothelial cells with leukocyte infiltration and elevation of the inflammatory cytokines; IL-1 β, IL-6, and Tnf-α [72,73 and 65]. Consistent with the previous studies, the current work demonstrated the elevation of the inflammatory mediators; IL-1 β, Tnf-α, NF-ҡβ, and apoptotic factor; caspase 3 with a decrease of the antiapoptotic factor; Bcl2 in DOX-treated group as documented by Wang et al. [
48] and Pakjoo et al. [
74]. On the other hand, the L-carnitine-pretreated MSCs group showed the optimum results in decreasing the measured inflammatory, apoptotic, and fibrotic cytokines, and antiapoptotic Bcl2 elevation producing improvement of the structural architecture of cardiac muscles. As approved by previous studies, the influence of MSCs on injured cardiomyocytes involves an intricate interaction of cytokines and signaling pathways. MSCs derive various inflammatory and fibrotic regulators including IL-6, IL-10, TGF-β1, and VEGF which reduce oxidative stress, inhibit inflammation, stimulate cellular regeneration promote proliferation, and prevent apoptosis [
75,
76]. MSCs reduce the expression of other inflammatory cytokines including interferon-γ (IFN-γ), IL-1β, IL-12, IL-4, and TNF-α. This reduction is achieved by secreting TNF-α stimulated gene/protein 6 (TSG-6), PGE2, and granulocyte-colony stimulating factor (G-CSF). On the other hand, MSCs stimulate the production of anti-inflammatory mediators IL-1, and hepatic nuclear factor-4 alpha (HNF-4α) by increasing the nitric oxide synthase (iNOS) through the activation of the NF-κB pathway [
77,
78]. In addition to the added antioxidant, anti-inflammatory, and anti-apoptotic role of L-carnitine and its role in the improvement of cardiac tissue architecture {2,42].
Concerning hsp90, a vital molecular chaperone, it regulates various cellular processes such as functional maturation, activation of several proteins, protein folding, and stability [
79,
80]. For these reasons, hsp90 is overexpressed in several cancers and is believed to aid the maturation of numerous oncoproteins, thereby facilitating cancer growth [
81]. In addition, hsp90 was identified in various cardiac injury pathways such as PI3K/AKT (PKB)/mTOR, MAPK, and TNF-α signaling pathways. Therefore, hsp90 plays a significant role in various cardiovascular diseases including atherosclerosis and heart failure [
82] as detected in the present study by overexpression in the DOX-treated group and a decrease in its level especially in the L-carnitine pre-treated-MSCs-group. TH regulates the rate of catecholamine synthesis. It is essential for heart function, particularly under stress conditions [
83]. TH is a cardiac muscle differentiation that is evident in the initial stages of heart development, especially in gastrulation. Enhanced TH expression resulted in sharp expression of atrial myosin heavy chain (AMHC1) and T-box 5 gene (Tbx5) within the ventricles, indicating an upregulation of cardiac muscle differentiation markers [
84]. Reduction of TH expression in the DOX-treated group and overexpression in the other groups particularly the L-carnitine pre-treated MSCs group together with the same results concerning the proliferative indicators Ki-67 denotes their cardiomyocyte proliferation and differentiation capacities. These findings propose that L-carnitine pretreated-MSCs hold a novel promising treatment for cardiac ischemia and infarction, especially in reducing apoptosis and enhancing cardiac muscle regeneration. Nevertheless, additional research is needed to validate these effects and elucidate the underlying mechanisms.