Role of melatonin in the angiogenesis potential; highlights on the cardiovascular disease

Significance of Melatonin in the Regulation of Circadian Rhythms and Disease Management

Melatonin, the 'hormone of darkness' is a neuronal hormone secreted by the pineal gland and other extra pineal sites. Responsible for the circadian rhythm and seasonal behaviour of vertebrates and mammals, melatonin is responsible for regulating various physiological conditions and the maintenance of sleep, body weight and the neuronal activities of the ocular sites. With its unique amphiphilic structure, melatonin can cross the cellular barriers and elucidate its activities in the subcellular components, including mitochondria. Melatonin is a potential scavenger of oxygen and nitrogen-reactive species and can directly obliterate the ROS and RNS by a receptor-independent mechanism. It can also regulate the pro- and anti-inflammatory cytokines in various pathological conditions and exhibit therapeutic activities against neurodegenerative, psychiatric disorders and cancer. Melatonin is also found to show its effects on major organs, particularly the brain, liver and heart, and also imparts a role in the modulation of the immune system. Thus, melatonin is a multifaceted candidate with immense therapeutic potential and is still considered an effective supplement on various therapies. This is primarily due to rectification of aberrant circadian rhythm by improvement of sleep quality associated with risk development of neurodegenerative, cognitive, cardiovascular and other metabolic disorders, thereby enhancing the quality of life.

Insight into the cardioprotective effects of melatonin: shining a spotlight on intercellular Sirt signaling communication

Cardiovascular diseases (CVDs) are the leading causes of death and illness worldwide. While there have been advancements in the treatment of CVDs using medication and medical procedures, these conventional methods have limited effectiveness in halting the progression of heart diseases to complete heart failure. However, in recent years, the hormone melatonin has shown promise as a protective agent for the heart. Melatonin, which is secreted by the pineal gland and regulates our sleep-wake cycle, plays a role in various biological processes including oxidative stress, mitochondrial function, and cell death. The Sirtuin (Sirt) family of proteins has gained attention for their involvement in many cellular functions related to heart health. It has been well established that melatonin activates the Sirt signaling pathways, leading to several beneficial effects on the heart. These include preserving mitochondrial function, reducing oxidative stress, decreasing inflammation, preventing cell death, and regulating autophagy in cardiac cells. Therefore, melatonin could play crucial roles in ameliorating various cardiovascular pathologies, such as sepsis, drug toxicity-induced myocardial injury, myocardial ischemia-reperfusion injury, hypertension, heart failure, and diabetic cardiomyopathy. These effects may be partly attributed to the modulation of different Sirt family members by melatonin. This review summarizes the existing body of literature highlighting the cardioprotective effects of melatonin, specifically the ones including modulation of Sirt signaling pathways. Also, we discuss the potential use of melatonin-Sirt interactions as a forthcoming therapeutic target for managing and preventing CVDs.

Melatonin and mesenchymal stem cells co-administration alleviates chronic obstructive pulmonary disease via modulation of angiogenesis at the vascular-alveolar unit

Chronic obstructive pulmonary disease (COPD) is considered a severe disease mitigating lung physiological functions with high mortality outcomes, insufficient therapy, and pathophysiology pathways which is still not fully understood. Mesenchymal stem cells (MSCs) derived from bone marrow play an important role in improving the function of organs suffering inflammation, oxidative stress, and immune reaction. It might also play a role in regenerative medicine, but that is still questionable. Additionally, Melatonin with its known antioxidative and anti-inflammatory impact is attracting attention nowadays as a useful treatment. We hypothesized that Melatonin may augment the effect of MSCs at the level of angiogenesis in COPD. In our study, the COPD model was established using cigarette smoking and lipopolysaccharide. The COPD rats were divided into four groups: COPD group, Melatonin-treated group, MSC-treated group, and combined treated group (Melatonin-MSCs). We found that COPD was accompanied by deterioration of pulmonary function tests in response to expiratory parameter affection more than inspiratory ones. This was associated with increased Hypoxia inducible factor-1α expression and vascular endothelial growth factor level. Consequently, there was increased CD31 expression indicating increased angiogenesis with massive enlargement of airspaces and thinning of alveolar septa with decreased mean radial alveolar count, in addition to, inflammatory cell infiltration and disruption of the bronchiolar epithelial wall with loss of cilia and blood vessel wall thickening. These findings were improved significantly when Melatonin and bone marrow-derived MSCs were used as a combined treatment proving the hypothesized target that Melatonin might augment MSCs aiming at vascular changes.

Melatonin: Current evidence on protective and therapeutic roles in gynecological diseases

Melatonin, a potent antioxidant and free radical scavenger, has been demonstrated to be effective in gynecological conditions and female reproductive cancers. This review consolidates the accumulating evidence on melatonin's multifaceted protective effects in different pathological contexts. In gynecological conditions such as endometriosis, polycystic ovary syndrome (PCOS), and uterine leiomyoma, melatonin has shown promising effects in reducing oxidative stress, inflammation, and hormonal imbalances. It inhibits adhesion molecules' production, and potentially mitigates leukocyte adherence and inflammatory responses. Melatonin's regulatory effects on hormone production and insulin sensitivity in PCOS individuals make it a promising candidate for improving oocyte quality and menstrual irregularities. Moreover, melatonin exhibits significant antitumor effects by modulating various signaling pathways, promoting apoptosis, and suppressing metastasis in breast cancers and gynecological cancers, including ovarian, endometrial, and cervical cancers. Furthermore, melatonin's protective effects are suggested to be mediated by interactions with its receptors, estrogen receptors and other nuclear receptors. The regulation of clock-related genes and circadian clock systems may also contribute to its inhibitory effects on cancer cell growth. However, more comprehensive research is warranted to fully elucidate the underlying molecular mechanisms and establish melatonin as a potential therapeutic agent for these conditions.

Melatonin inhibits the formation of intraplaque neovessels in ApoE-/- mice via PPARγ- RhoA-ROCK pathway

BACKGROUND

According to former research, the atherosclerotic plaque is thought to be aggravated by intraplaque neovessels (IPN) and intraplaque hemorrhage (IPH). Intriguingly, a lower incidence of IPH was found in plaque treated with melatonin. In this study, we attempted to investigate the impact and underlying mechanism regarding the influences of melatonin upon IPN.

METHODS

A mouse model was established by subjecting the high fat diet (HFD)-fed ApoE-/- mice to tandem stenosis (TS) surgery with melatonin and GW9662, a PPARγ antagonist, being given by gavage. In vitro experiment was conducted with HUVECs exposing to according treatments of VEGF, melatonin, GW9662, or Y27632.

RESULTS

Plaque and IPN were attenuated by treatment with melatonin, which was then reversed by blocking PPARγ. Western blotting results showed that melatonin increased PPARγ and decreased RhoA/ROCK signaling in carotid artery. Elevated RhoA/ROCK signaling was observed in melatonin-treated mice when PPARγ was blocked. In accordance with it, experiments using protein and mRNA from HUVECs revealed that melatonin inhibited the RhoA/ROCK signaling by enhancing PPARγ. According to in vitro study, melatonin was able to inhibit cell migration and angiogenesis, which was aborted by GW9662. Blockage of ROCK using Y27632 was able to cease the effect of GW9662 and restored the suppression on cell migration and angiogenesis by melatonin.

CONCLUSIONS

Our study demonstrates that melatonin is able to curb development of plaque and IPN formation by inhibiting the migration of endothelial cells via PPARγ- RhoA-ROCK pathway. That provides a therapeutic potential for both melatonin and PPARγ agonist targeting IPN, IPH, and atherosclerotic plaque.

The Circadian System Is Essential for the Crosstalk of VEGF-Notch-mediated Endothelial Angiogenesis in Ischemic Stroke

Ischemic stroke is a major public health problem worldwide. Although the circadian clock is involved in the process of ischemic stroke, the exact mechanism of the circadian clock in regulating angiogenesis after cerebral infarction remains unclear. In the present study, we determined that environmental circadian disruption (ECD) increased the stroke severity and impaired angiogenesis in the rat middle cerebral artery occlusion model, by measuring the infarct volume, neurological tests, and angiogenesis-related protein. We further report that Bmal1 plays an irreplaceable role in angiogenesis. Overexpression of Bmal1 promoted tube-forming, migration, and wound healing, and upregulated the vascular endothelial growth factor (VEGF) and Notch pathway protein levels. This promoting effect was reversed by the Notch pathway inhibitor DAPT, according to the results of angiogenesis capacity and VEGF pathway protein level. In conclusion, our study reveals the intervention of ECD in angiogenesis in ischemic stroke and further identifies the exact mechanism by which Bmal1 regulates angiogenesis through the VEGF-Notch1 pathway.

Endothelial Senescence and Its Impact on Angiogenesis in Alzheimer's Disease

Endothelial cells are constantly exposed to environmental stress factors that, above a certain threshold, trigger cellular senescence and apoptosis. The altered vascular function affects new vessel formation and endothelial fitness, contributing to the progression of age-related diseases. This narrative review highlights the complex interplay between senescence, oxidative stress, extracellular vesicles, and the extracellular matrix and emphasizes the crucial role of angiogenesis in aging and Alzheimer's disease. The interaction between the vascular and nervous systems is essential for the development of a healthy brain, especially since neurons are exceptionally dependent on nutrients carried by the blood. Therefore, anomalies in the delicate balance between pro- and antiangiogenic factors and the consequences of disrupted angiogenesis, such as misalignment, vascular leakage and disturbed blood flow, are responsible for neurodegeneration. The implications of altered non-productive angiogenesis in Alzheimer's disease due to dysregulated Delta-Notch and VEGF signaling are further explored. Additionally, potential therapeutic strategies such as exercise and caloric restriction to modulate angiogenesis and vascular aging and to mitigate the associated debilitating symptoms are discussed. Moreover, both the roles of extracellular vesicles in stress-induced senescence and as an early detection marker for Alzheimer's disease are considered. The intricate relationship between endothelial senescence and angiogenesis provides valuable insights into the mechanisms underlying angiogenesis-related disorders and opens avenues for future research and therapeutic interventions.

Protective effects and molecular mechanisms of tea polyphenols on cardiovascular diseases

Aging is the most important factor contributing to cardiovascular diseases (CVDs), and the incidence and severity of cardiovascular events tend to increase with age. Currently, CVD is the leading cause of death in the global population. In-depth analysis of the mechanisms and interventions of cardiovascular aging and related diseases is an important basis for achieving healthy aging. Tea polyphenols (TPs) are the general term for the polyhydroxy compounds contained in tea leaves, whose main components are catechins, flavonoids, flavonols, anthocyanins, phenolic acids, condensed phenolic acids and polymeric phenols. Among them, catechins are the main components of TPs. In this article, we provide a detailed review of the classification and composition of teas, as well as an overview of the causes of aging-related CVDs. Then, we focus on ten aspects of the effects of TPs, including anti-hypertension, lipid-lowering effects, anti-oxidation, anti-inflammation, anti-proliferation, anti-angiogenesis, anti-atherosclerosis, recovery of endothelial function, anti-thrombosis, myocardial protective effect, to improve CVDs and the detailed molecular mechanisms.

Melatonin blunted the angiogenic activity in 3D colon cancer tumoroids by the reduction of endocan

BACKGROUND

Complexity and heterogeneity of the tumor niche are closely associated with the failure of therapeutic protocols. Unfortunately, most data have been obtained from conventional 2D culture systems which are not completely comparable to in vivo microenvironments. Reconstructed 3D cultures composed of multiple cells are valid cell-based tumor models to recapitulate in vivo-like interaction between the cancer cells and stromal cells and the oncostatic properties of therapeutics. Here, we aimed to assess the tumoricidal properties of melatonin on close-to-real colon cancer tumoroids in in vitro conditions.

METHODS

Using the hanging drop method, colon cancer tumoroids composed of three cell lines, including adenocarcinoma HT-29 cells, fibroblasts (HFFF2), and endothelial cells (HUVECs) at a ratio of 2: 1: 1, respectively were developed using 2.5% methylcellulose. Tumoroids were exposed to different concentrations of melatonin, from 0.005 to 0.8 mM and 4 to 10 mM, for 48 h. The survival rate was measured by MTT and LDH leakage assays. Protein levels of endocan and VEGF were assessed using western blotting. Using histological examination (H & E) staining, the integrity of cells within the tumoroid parenchyma was monitored.

RESULTS

Despite the reduction of viability rate in lower doses, the structure of tumoroids remained unchanged. In contrast, treatment of tumoroids with higher doses of melatonin, 4 and 10 mM, led to disaggregation of cells and reduction of tumoroid diameter compared to the non-treated control tumoroids (p < 0.05). By increasing melatonin concentration from 4 to 10 mM, the number of necrotic cells increased. Data showed the significant suppression of endocan in melatonin-treated tumoroids related to the non-treated controls (p < 0.05). According to our data, melatonin in higher doses did not alter protein levels of VEGF (p > 0.05).

CONCLUSIONS

Melatonin can exert its tumoricidal properties on colon cancer tumoroids via the reduction of tumor cell viability and inhibition of the specific pro-angiogenesis factor.

Melatonin and endothelial cell-loaded alginate-fibrin hydrogel promoted angiogenesis in rat cryopreserved/thawed ovaries transplanted to the heterotopic sites

BACKGROUND

Ischemic niche can promote follicular atresia following the transplantation of cryopreserved/thawed ovaries to the heterotopic sites. Thus, the promotion of blood supply is an effective strategy to inhibit/reduce the ischemic damage to ovarian follicles. Here, the angiogenic potential of alginate (Alg) + fibrin (Fib) hydrogel enriched with melatonin (Mel) and CD144⁺ endothelial cells (ECs) was assessed on encapsulated cryopreserved/thawed ovaries following transplantation to heterotopic sites in rats.

METHODS

Alg + Fib hydrogel was fabricated by combining 2% (w/v) sodium Alg, 1% (w/v) Fib, and 5 IU thrombin at a ratio of 4: 2: 1, respectively. The mixture was solidified using 1% CaCl₂. Using FTIR, SEM, swelling rate, and biodegradation assay, the physicochemical properties of Alg + Fib hydrogel were evaluated. The EC viability was examined using an MTT assay. Thirty-six adult female rats (aged between 6 and 8 weeks) with a normal estrus cycle were ovariectomized and enrolled in this study. Cryopreserved/thawed ovaries were encapsulated in Alg + Fib hydrogel containing 100 µM Mel + CD144⁺ ECs (2 × 10⁴ cells/ml) and transplanted into the subcutaneous region. Ovaries were removed after 14 days and the expression of Ang-1, and Ang-2 was monitored using real-time PCR assay. The number of vWF⁺ and α-SMA⁺ vessels was assessed using IHC staining. Using Masson's trichrome staining, fibrotic changes were evaluated.

RESULTS

FTIR data indicated successful interaction of Alg with Fib in the presence of ionic cross-linker (1% CaCl₂). Data confirmed higher biodegradation and swelling rates in Alg + Fib hydrogel compared to the Alg group (p < 0.05). Increased viability was achieved in encapsulated CD144⁺ ECs compared to the control group (p < 0.05). IF analysis showed the biodistribution of Dil⁺ ECs within hydrogel two weeks after transplantation. The ratio of Ang-2/Ang-1 was statistically up-regulated in the rats that received Alg + Fib + Mel hydrogel compared to the control-matched groups (p < 0.05). Based on the data, the addition of Mel and CD144⁺ ECs to Alg + Fib hydrogel reduced fibrotic changes. Along with these changes, the number of vWF⁺ and α-SMA⁺ vessels was increased in the presence of Mel and CD144⁺ ECs.

CONCLUSIONS

Co-administration of Alg + Fib with Mel and CD144⁺ ECs induced angiogenesis toward encapsulated cryopreserved/thawed ovarian transplants, resulting in reduced fibrotic changes.

The effect of melatonin on sheep endometrial epithelial cell apoptosis through the receptor and non-receptor pathways

Melatonin potentially regulates the female animal reproductive function, but its regulatory mechanism in the apoptosis of sheep endometrial epithelial cells (SEECs) remains to be elucidated. In the present study, immunofluorescence staining, western blotting, and quantitative real-time polymerase chain reaction were performed to detect the distribution of melatonin receptors (MT1 and MT2) in the uterus of sheep and the effect of melatonin via the receptor and non-receptor pathways on the apoptosis of SEECs in vitro. The results showed that melatonin inhibits the apoptosis of SEECs to varying degrees to regulate the expression of estrogen receptors (ERs) and progesterone receptors (PGR) via its interaction with MT1 and MT2. In addition, the ER antagonist partially relieved the inhibitory effect of melatonin on the apoptosis of SEECs, while the PGR antagonist did not. Thus, melatonin mediates endometrial epithelial apoptosis through the MT receptors and also by regulating estrogen function. This study provides evidence of the regulatory mechanism of melatonin on the physiological function of the sheep uterus.

Role of Melatonin in Cancer: Effect on Clock Genes

The circadian clock is a regulatory system, with a periodicity of approximately 24 h, that generates rhythmic changes in many physiological processes. Increasing evidence links chronodisruption with aberrant functionality in clock gene expression, resulting in multiple diseases, including cancer. In this context, tumor cells have an altered circadian machinery compared to normal cells, which deregulates the cell cycle, repair mechanisms, energy metabolism and other processes. Melatonin is the main hormone produced by the pineal gland, whose production and secretion oscillates in accordance with the light:dark cycle. In addition, melatonin regulates the expression of clock genes, including those in cancer cells, which could play a key role in the numerous oncostatic effects of this hormone. This review aims to describe and clarify the role of clock genes in cancer, as well as the possible mechanisms of the action of melatonin through which it regulates the expression of the tumor's circadian machinery, in order to propose future anti-neoplastic clinical treatments.

Historical and current perspectives on blood endothelial cell heterogeneity in the brain

Dynamic brain activity requires timely communications between the brain parenchyma and circulating blood. Brain-blood communication is facilitated by intricate networks of brain vasculature, which display striking heterogeneity in structure and function. This vascular cell heterogeneity in the brain is fundamental to mediating diverse brain functions and has long been recognized. However, the molecular basis of this biological phenomenon has only recently begun to be elucidated. Over the past century, various animal species and in vitro systems have contributed to the accumulation of our fundamental and phylogenetic knowledge about brain vasculature, collectively advancing this research field. Historically, dye tracer and microscopic observations have provided valuable insights into the anatomical and functional properties of vasculature across the brain, and these techniques remain an important approach. Additionally, recent advances in molecular genetics and omics technologies have revealed significant molecular heterogeneity within brain endothelial and perivascular cell types. The combination of these conventional and modern approaches has enabled us to identify phenotypic differences between healthy and abnormal conditions at the single-cell level. Accordingly, our understanding of brain vascular cell states during physiological, pathological, and aging processes has rapidly expanded. In this review, we summarize major historical advances and current knowledge on blood endothelial cell heterogeneity in the brain, and discuss important unsolved questions in the field.

Putative effect of melatonin on cardiomyocyte senescence in mice with type 1 diabetes mellitus

Background

To date, many investigators have tried to clarify the molecular mechanism of cardiovascular injuries after T1D. In present study, we evaluated the possible effects of melatonin on the levels of aging-related factors in the heart tissue of streptozotocin-induced diabetic mice.

Methods

40 male mice were enrolled in this study and randomly allocated into 4 groups (n = 10) as follows: Control group (C), Control group + melatonin (CM), Diabetic group (D), Diabetic + melatonin (DM) group. Single Streptozotocin (50 mg/kbW) was applied for the induction of T1D. 3 mg/kg melatonin was injected intraperitoneally twice a week for consequent four weeks. After the completion of this period, the animals were sacrificed and their heart tissue was obtained for histological examination (IHC analysis of vWF and α-SMA cells), aging and inflammation-related gene analysis.

Result

Hematoxylin and Eosin staining indicated cardiomyocyte toxicity in T1D mice. IHC analysis of vascular tissue showed the detachment of vWF and α-SMA cells and disintegration into the vascular lumen. Additionally, real-time PCR assay showed the up-regulation of β-galactosidase and suppression of SOX2, Klotho, and Telomerase genes in T1D mice compared to the control group (p < 0.05). We noted that melatonin administration can revert these condition and closed near-to-control levels. Along with these conditions, the levels of IL-1β were also decreased after melatonin treatment.

Conclusions

In general, one can hypothesize that modulation of different effectors associated with aging is beneficial to alleviate cardiac injuries under hypergylcemic condition. Melatonin can exert its therapeutic effects, in part, through anti-aging capacity.

Human amniotic fluid derived-exosomes alleviate hypoxic encephalopathy by enhancing angiogenesis in neonatal mice after hypoxia

Neonatal hypoxic encephalopathy is a type of central nervous system dysfunction manifested by high mortality and morbidity. Exosomes play a crucial role in neuroprotection by enhancing angiogenesis. The objective of this study was to investigate the effect of human amniotic fluid-derived exosomes (hAFEXOs) on functional recovery in neonatal hypoxic encephalopathy. The transwell assay, scratch wound healing assay, and tube formation assay were used to evaluate the effect of hAFEXOs on the angiogenesis of human umbilical vein endothelial cells (HUVECs) after oxygen and glucose deprivation (OGD). The angiogenesis of microvascular endothelial cells (MECs) in the cortex was tested in neonatal mice treated with hAFEXOs or phosphate-buffered saline (PBS) after hypoxia. Expressions of hypoxia-inducible factor 1 α (HIF-1α) and vascular endothelial growth factor (VEGF) in the cerebral cortex were also tested by western blot. The Morris Water Maze Test (MWM) was carried out to detect the performance of spatial memory after processing with hAFEXOs or PBS. The results indicated that hAFEXOs favored tubing formation and migration of HUVECs after in vitro OGD. The hAFEXOs also favored the expression of CD31 in neonatal mice following hypoxia. The expressions of both HIF-1α and VEGF were significantly augmented in the cerebral cortex of neonatal mice which were treated with hAFEXOs. Moreover, the MWM test results showed that the performance of the spatial memory was better in the hAFEXO-treated group than in the PBS-treated group. Our study indicates that hAFEXOs alleviated hypoxic encephalopathy and enhanced angiogenesis in neonatal mice after hypoxia. In addition, hAFEXOs  promoted migration and tube formation of HUVECs after OGD in vitro. These findings confirm that hAFEXOs show great potential for further studies aimed at developing therapeutic agents for hypoxic encephalopathy.

Melatonin and cardiovascular disease: from mechanisms of action to potential clinical use (literature review)

Cardiovascular disease remains the most relevant public health problem. Most cardiovascular diseases are associated with an atherosclerosis, the development of which is associated with inflammation and endothelial dysfunction. Melatonin is a neurohormone that is synthesized mainly in the pineal gland and plays a central role in the regulation of sleep and some other body cyclic processes. For a long time, melatonin was perceived as a substance that is effective in the treatment of circadian cycle impairments. At the same time, a large number of studies have accumulated recently that demonstrate a wider range of its biological effects, including anti-inflammatory, antioxidant, antihypertensive and, possibly, hypolipidemic. The review includes current data from experimental and clinical studies demonstrating the cardioprotective effects of melatonin in atherosclerosis, myocardial ischemia, and heart failure.

Role of mast cells in skin regeneration after thermal burn treated with melatonin-enriched dermal film

The development of novel local therapies for thermal burns (TB) and their pathogenetic rationale are a pressing challenge. Melatonin (MT) is an endogenous factor of hemostasis regulation with pleiotropic potential. The aim of this study was to assess some parameters of tissue regeneration, the functional state of mast cells and the levels of matrix metalloproteinase-9 (MMP-9) and vascular endothelial growth factor (VEGF) in the experimentally induced TB treated with the original MT-enriched dermal film (DF). A second-degree burn (3.5% of the total body surface area) was modelled by exposing a patch of skin to hot water. Applications of 12 cm2 DF enriched with 5 mg/g MT were performed every day for 5 days. The following parameters were calculated: the wound area, the rate of wound epithelization, the number of MC in the wound, the intensity of degranulation, and the levels of MMP-9 and VEGF expression. Over the course of treatment, the absolute wound area shrank by 35%, its epithelization rate increased, the number of MC rose, their functional state changed, and the expression of ММР-9 and VEGF increased. A negative correlation was established between the wound area and the expression of ММР-9 and VEGF, as well as between the wound area and the degranulation coefficient. Applications of MT-enriched DF resulted in the reduction of the wound area, higher epithelization rate, an increase in the total MC count and degranulation intensity on days 5 and 10; it also led to a reduction in the total MC count and a loss in degranulation intensity on day 20 (166.87 (154.95; 178.78) un/mm2 vs. 464.84 (452.92; 476.76) un/mm2) in the group of intact animals), an increase in MMP-9 expression on day 5 (14.20 (11.30; 18.10) vs. 3.30 (2.20; 4.40) in the intact group), an increase in VEGF expression on days 5 and 10 (33.00 (30.20; 34.90) vs 25.40 (22.20; 29.30) in the intact group), and a reduction in MMP-9 expression on days 10 and 20 after thermal injury.