Despite many years of intensive and profound studies, DMD remains an incurable disease. Newest, most promising therapies, using the latest advances in genetic modification, namely CRISPR/Cas9 technology, are still far from clinical introduction and acceptance. Thus, glucocorticoids, with prednisolone and deflazacort being the most commonly used, still serve as a gold standard therapy for patients suffering from DMD. Unfortunately, except for undoubtful beneficial effects in e.g. prolonging ambulation, their daily administration was shown to exert many side effects leading to, among others, osteoporosis, diabetes, or muscle atrophy (42). As there is a constant need to investigate novel strategies, which could at least attenuate the severity of the disease, many researchers focus not only on new drug discoveries but also repurposing of the already existing ones.
HMG-CoA reductase inhibitors, commonly known as statins, seem to be the perfect choice for such investigation. Despite the still ongoing discussion regarding statin-induced myopathy, myositis, and rhabdomyolysis (16, 20, 22, 43) more and more studies describe that the benefits of treatment outweigh the possible risks which, of note, are usually not relevant to DMD boys (25, 26). In our study, no further deterioration in inflammation, fibrosis, and necrosis was visible as the consequence of simvastatin delivery to dystrophic animals. Importantly, no elevation in, strongly associated with statin-induced myopathy, CK level (43), indicated no additional damage to the muscle after one month of simvastatin administration. Moreover, no significant or alarming systemic changes were observed in regards to the total WBC or distinguished subpopulation e.g. granulocytes, lymphocytes, and monocytes, at the end of the experiment.
In contrast to the previous discussion about the deleterious muscle-related alterations, over recent years, several studies demonstrated the positive effects of statins on overall skeletal muscle health, including their anti-inflammatory and anti-fibrotic properties (44, 45). In the present study we have found that among various processes contributing to dystrophy progressions, simvastatin can ameliorate, in a muscle-type specific manner, only selected aspects of DMD pathology influencing regeneration markers without any effect on inflammation, fibrosis, or angiogenesis. Interestingly, Whitehead et al., already in 2015 showed the protective influence of simvastatin in dystrophic animals (12). However, the recent publication by Verhaart et al. described the lack of the effect of this statin and even put into question whether presented by Whitehead et al. high level of simvastatin (obtained after administration in the food and drink) in the blood of the animals was possible to be obtained (14). When different statins were investigated by other groups, results were also inconclusive. It was shown that pravastatin, another FDA-approved cholesterol-lowering drug, can be considered in DMD therapy as it can upregulate utrophin A expression via eEF1A2 (46). Utrophin A, which is an autosomal homolog of dystrophin, is suggested to functionally compensate for dystrophin loss in DMD muscles (47). On the other hand, Finkler et al. demonstrated no beneficial effects of rosuvastatin. Moreover, a visible accretion of inflammation was remarked upon treatment (15). Such discrepancies in the obtained results might be related to several divergences in the applied methodology, including age and background of the mice, type and dose of the statin that was used, route of administration, and length of time the drug was given to the animals. Nevertheless, so diverse strategies give an undoubtful chance to investigate the effects of statins from different perspectives and various stages of mice development and disease progression. In our study, the dose of simvastatin was 10 mg/kg BW. In that matter, the applied approach was similar to the one used by other groups (12, 14). Moreover, despite the most promising results were obtained by Whitehead et al. when simvastatin was provided in food and water (12), we strongly believe that oral gavage administration is more relevant, giving us the opportunity to more precisely control the given dose. Additionally, literature data confirm this method of statin delivery to be more effective in the manner of the obtained in the blood simvastatin concentration (14). Notable, although the effect of simvastatin treatment on mdx mice was not as profound as in Whitehead et al. (12) study, we did remark some interesting changes.
We observed not only typical for dystrophic animals and described in our previous papers (4, 5, 9), elevation in the most commonly acknowledged muscle damage markers: CK and LDH activity level, but also a significant decrease in given parameters in mdx mice upon simvastatin treatment, what is in line with work by Whitehead et al. (12). Such an outcome might suggest lower degeneration of the muscle fibers. However, no alterations in typical muscle degeneration markers, namely necrosis, and inflammation (48), allow us to confirm such a hypothesis. In contrast to the results obtained by Whitehead et al., which showed visibly reduced inflammatory cell infiltration and a diminished number of CD68 macrophages (12), we did not observe the anti-inflammatory potential of simvastatin when the histological assessment of gastrocnemius and diaphragm muscles was performed. Moreover, the expression of the Hmox1 gene, coding anti-oxidant, and cytoprotective HO-1 enzyme was also not affected by the treatment. Similar results were obtained by Verhaart et al. (14), who reported no effect on inflammation even with the prolonged by two months, in comparison to us, time of drug administration. Furthermore, in opposition to the published data suggesting the anti-fibrotic role of simvastatin in mdx animals (12, 13), we did not observe any effect of the treatment on fibrosis, neither in histological assessment of collagen deposition nor expression of fibrosis-related genes. Those observations were confirmed by the evaluation of OPN expression, a recently described biomarker of DMD associated with regeneration, inflammation, and fibrosis (49, 50), which was also not affected by simvastatin.
On the other hand, interesting results were obtained in our study concerning muscle regeneration. When gastrocnemius muscle was investigated we noticed a significant increase in the mean myofiber size. Together with histological assessment of the CNF, a decrease in the expression of Myh3, which is important in muscle repair (51), and downregulation of highly expressed during regeneration, FGF2 protein (38), we might imply normalization of the regenerative process in investigated muscle. We suggest that a decline in the number of CNF and Myh3 mRNA level could be related to faster maturation of the regenerating fibers and as a result, loss of the related to that process markers (48). Moreover, a significant rise in muscle-specific miR-1 and increasing tendency in miR-133a, so-called myomiRs, might indicate more efficient muscle regeneration, as it was demonstrated that such upregulation promotes muscle differentiation and therefore, improves muscle repair (52, 53). Interestingly, no effect of simvastatin on dystrophic muscle regeneration was demonstrated by Whitehead et al. and Verhaart et al. (12, 14). Moreover, it needs to be emphasized that such alterations were noticed only in the gastrocnemius muscle. When we analyzed the diaphragm, we did not observe any changes. It shows that, at least in our hands, simvastatin treatment-related outcomes are strongly muscle type-specific. In order to better understand the complex mechanisms exerted by statins, their effects on various cell types including muscle satellite cells (mSC), crucial for muscle regeneration (54), should be investigated.
Enlargement in muscle fiber size together with improved muscle regeneration could further explain observed by us amelioration in the forelimb grip strength. Interestingly, we described that even though in both vehicle and simvastatin groups strength was augmented, the rate of the improvement was significantly higher in drug-treated mdx mice. Interestingly, when a measurement of the specific force of the muscle was performed by Whitehead et al. (12) and Verhaart et al. (14), they showed significant improvement and lack of any effect, respectively. As overall mice performance in a treadmill test, influenced not only by muscle strength but also by the respiratory and cardiovascular systems, was not altered by the treatment, we might speculate that obtained by us effect might not be strong enough to cause systemic changes in the mice.
To expand already described knowledge we decided to investigate also a different aspect of DMD progression – angiogenesis alterations, especially because our previous studies revealed a decrease in a major pro-angiogenic factor, VEGF, both at mRNA and protein level in skeletal muscles of dystrophic animals in comparison to wild-type counterparts (4, 5, 31). Importantly, improvement of endothelial function and vasculoprotective action are well-recognized statin effects (55). Our previous experiments clearly showed that statins regulate angiogenesis. We have demonstrated that atorvastatin at the pharmacologically relevant concentration (100 nM) enhanced the expression of endothelial nitric oxide synthase (eNOS) in human microvascular endothelial cells (HMEC-1). Moreover, atorvastatin prevented the hypoxia-induced decline in eNOS expression (28). The regulation of several angiogenic factors was observed by us after statin stimulation in human umbilical vein endothelial cells (HUVEC) but these effects may be also cell- and dose-dependent (27, 41). Moreover, accelerated vascularization upon simvastatin treatment was also demonstrated in models of peripheral ischemia and corneal neovascularization (56). In the present study, despite significant changes in some of the tested angiogenic genes in gastrocnemius muscle, no complementary effects were noticed on the protein level. Although a decrease in Vegfa/Kdr signaling might indicate the anti-angiogenic effect of simvastatin, neither VEGF protein nor abundance of CD31/α-SMA double-positive blood vessels were observed. Despite the elevated number of vessels in the diaphragm, no other investigated factors were affected, rather suggesting no profound effect on angiogenesis as the result of simvastatin administration. Noteworthy, it again shows that simvastatin might not be influencing various muscles in the same manner.