Metformin, besides exhibiting strong in vivo anti-inflammatory properties, increases mptp-induced damage to the nigrostriatal dopaminergic system
Graphical abstract
Introduction
Parkinson's disease (PD) is a neurodegenerative disorder related to age that is characterized by the progressive degeneration of nigrostriatal dopaminergic neurons of the substantia nigra pars compacta (SNpc; Olanow et al., 2003). This leads to motor dysfunction of the extrapyramidal system, which is accompanied by cognitive dysfunction, progressive loss of autonomy and mood alterations (Di Monte and Langston, 1995, Dauer and Przedborski, 2003, Olanow et al., 2003). Although several genes have been identified as responsible for some types of early-onset familial PD (Lesage and Brice, 2009), the etiology of the disease remains unknown. Several factors, such as oxidative stress, mitochondrial dysfunction, reduced trophic factors, alterations in the ubiquitin-proteasome system and neuroinflammatory mechanisms, seems to cooperate in the progressive death of neurons in the SNpc (Dauer and Przedborski, 2003, Gao et al., 2003a, Hunot and Hirsch, 2003, Olanow et al., 2003, McGeer and McGeer, 2004, Marchetti and Abbracchio, 2005, Marchetti et al., 2005a, Marchetti et al., 2005b, Jenner, 2007).
Accumulating evidence suggests that inflammation may play a central role in the cell loss that occurs in PD. Hence, several authors have shown the existence of activated microglia in the SN of PD patients (McGeer et al., 1988, Klegeris et al., 2007, McGeer and McGeer, 2008), which is accompanied by an increased expression of inflammatory cytokines (Tansey et al., 2007, Hirsch and Hunot, 2009). Moreover, it has been shown that inflammation is present in different animal models of PD, including those produced by toxins such as 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), 6-hydroxydopamine (6-OHDA) or rotenone (Liberatore et al., 1999, Cicchetti et al., 2002, Gao et al., 2003b, Miklossy et al., 2006). In fact, epidemiological studies have shown that the incidence of idiopathic PD is about 50% lower in chronic users of nonsteroidal anti-inflammatory agents or cyclooxygenase inhibitors with respect to non-users (Chen et al., 2003, Chen et al., 2005, Esposito et al., 2007).
Although the cause of the disease is unknown, there are various known risk factors that may increase the likelihood of developing the disease, which include age, sleep disorders, brain trauma and exposure to certain environmental factors such as pesticides and herbicides (Kieburtz and Wunderle, 2013, Postuma et al., 2013, Wong and Hazrati, 2013, Abdullah et al., 2015). In this sense, diabetes mellitus has also begun to be considered as a possible risk factor for developing this neurodegenerative disorder (Klimek et al., 2015). The association between diabetes and PD is not entirely clear. Some authors have proposed that chronic inflammation and oxidative stress produced in diabetes could lead to an increased risk of PD some years later (Xu et al., 2011). Moreover, animal and in vitro studies have shown a role for insulin in the regulation of the brain dopaminergic activity. Hence, deregulation and changes in insulin action have been studied in the pathophysiology and clinical symptoms of PD (Craft and Watson, 2004). Mitochondrial dysfunction, one of the possible causes of PD, may be also a common mechanism shared by both pathologies, since it has been described a reduced expression of certain genes involved in the impaired mitochondrial oxidation pathway in type 2 diabetes (Horan, 2009, Schernhammer et al., 2011).
One of the drugs globally accepted to treat type 2 diabetes is metformin, chemically called metformin hydrochloride; it is a member of the biguanide class that reduces hepatic glucose production and decreases insulin resistance and the levels of fasting plasma insulin through a mechanism of action that appears to be mainly mediated by activation of the adenosine monophosphate-activated protein kinase (AMPK).
Recently, various clinical and experimental studies suggest that metformin, besides their hypoglycemic actions, can attenuate inflammation both peripherally and centrally. The anti-inflammatory potential of metformin has been particularly described in numerous experimental models of peripheral inflammation, so that metformin has been shown to decrease the inflammatory response in endothelial cells (Isoda et al., 2006); it also decreases cell proliferation in the smooth muscle of human aortas (Li et al., 2005) and has antiatherogenic properties (Mamputu et al., 2003).
Given the anti-inflammatory activity of metformin and the importance of neuroinflammation in the development of PD, the aim of this study was to determine whether treatment with metformin is able to protect dopaminergic neurons in the SNpc using a model of PD based on the injection of MPTP.
Section snippets
Materials and methods
C57BL male mice (20–25 g) were used for these studies. Mice were kept at constant room temperature (22 ± 1 °C) and relative humidity (60%) with a 12-h light–dark cycle with free access to food and water. Experiments were carried out in accordance with the Guidelines of the European Union Directive (2010/63/EU) and Spanish regulations (BOE 34/11370-421, 2013) for the use of laboratory animals; the study was approved by the Scientific Committee of the University of Seville.
Animals were divided in
Effect of metformin and MPTP on microglial activation
We first evaluated activation of microglial cells based on morphological features and on the expression levels of TNF-α, IL-1β and iNOS mRNAs. Upon activation, microglial cells change their morphology from resting resident ramified microglia with two or three fine processes to round cells resembling tissue macrophages and proliferate when challenged. As expected, immunohistochemistry of Iba-1 showed that i.p. injection of saline solution in animals treated orally with tap water or metformin did
Discussion
PD is accompanied by an inflammatory reaction that may play a critical role in the degeneration of nigral dopaminergic neurons. Consequently, anti-inflammatory strategies are receiving increasing attention for their potential to prevent or delay the pathological deterioration that takes place in this disease.
Some recent clinical and experimental studies suggest that metformin, besides to its hypoglycemic actions, has anti-inflammatory properties. These anti-inflammatory effects have been
Conclusions
Since activity of complex I is decreased in parkinsonian patients, and considering metformin produces the inhibition of this multienzyme complex, our results suggest that treatment with this drug in diabetic patients might accelerate the onset and/or progression of the disease. Further studies should be carried out on the use of metformin at clinical doses.
Our results show that metformin is able to decrease the inflammatory response in the animal model of PD used in this study. However, despite
Conflict of interest statement
All authors declare that there are no conflicts of interest.
Transparency document
Acknowledgements
This work was supported by grants of Junta de Andalucía P09-CTS-5244 and P10-CTS-6494, and a grant from the Spanish Ministerio de Economia y Competitividad (SAF2012-3902).
References (79)
- et al.
Parkinson's disease and age: the obvious but largely unexplored link
Exp. Gerontol.
(2015) - et al.
Complex I is the major site of mitochondrial superoxide production by paraquat
J. Biol. Chem.
(2008) - et al.
Insulin and neurodegenerative disease: shared and specific mechanisms
Lancet Neurol.
(2004) - et al.
Parkinson's disease: mechanisms and models
Neuron
(2003) - et al.
Dimethylbiguanide inhibits cell respiration via an indirect effect targeted on the respiratory chain complex I
J. Biol. Chem.
(2000) - et al.
Non-steroidal anti-inflammatory drugs in Parkinson's disease
Exp. Neurol.
(2007) - et al.
Novel anti-inflammatory therapy for Parkinson's disease
Trends Pharmacol. Sci.
(2003) - et al.
Calmodulin-dependent protein kinase kinase-beta is an alternative upstream kinase for AMP-activated protein kinase
Cell Metab.
(2005) - et al.
Neuroinflammation in Parkinson's disease: a target for neuroprotection?
Lancet Neurol.
(2009) - et al.
Neuroinflammation in Parkinson's disease
Parkinsonism Relat. Disord.
(2012)