Mechanism of Neuroprotection by Progesterone
Progesterone is a pleiotropic agent with beneficial effects on various secondary injury cascades that are set into motion after TBI (Fig. 1).[21,22] The main therapeutic effect of progesterone and its metabolites is thought to be via decreasing cerebral edema.[10,23–25] By modulating p-glycoprotein and aquaporin 4 (AQP4) levels, it helps maintain blood-brain barrier (BBB) integrity. Progesterone up-regulates p-glycoprotein levels leading to increases in efflux pump in BBB and decreases in cerebral edema.[25,26] AQP4 is from a family of water-selective membrane channels, which is mainly expressed in perivascular astrocytic endfeet. Guo et al showed that bilateral contusion injuries of the medial frontal cortex resulted in increased water content in the pericontusional area accompanied by increased expression of AQP4 in the pericontusional area and lateral ventricles. In contrast, there was a significant decrease in AQP4 expression in the tissue surrounding the third ventricle. Progesterone treatment decreased brain water content and AQP4 expression in the pericontusional areas and in the tissue surrounding the lateral ventricles, while increasing AQP4 expression in the tissue adjacent to the third ventricle. The authors speculated that by increasing AQP4 expression in the osmosensory areas in the hypothalamus surrounding the third ventricle, progesterone might have contributed to enhanced water drainage leading to preservation of osmotic equilibrium in the brain. Corroborating the importance of AQP4 as a therapeutic target for pediatric TBI, a recent study showed decreased edema formation, decreased BBB disruption, and improved motor and long-term cognitive function with inhibition of AQP4 expression by injection of small-interfering RNA targeting AQP4 after controlled cortical impact injury in the developing brain.
The putative neuroprotective mechanisms of the pleiotropic agent progesterone are detailed in the figure. These mechanisms all play a role in secondary injury following traumatic brain injury. BBB = bloodbrain barrier, GABA = γ-aminobutyric acid
Progesterone has been shown to have anti-inflammatory effects by suppressing generation of proinflammatory cytokines and reducing microglial activation.[4,23,24,29,30] These anti-inflammatory effects could be especially important after TBI, where marked neuroinflammation contributes to many aspects of secondary brain injury, such as vascular endothelial injury and disruption of the BBB. Progesterone administration after TBI in animal models decreased production of tumor necrosis factor (TNF)-α, interleukin (IL)-1, IL-6, toll-like receptor (TLR)-2, TLR-4, and nuclear factor-κB-binding activity.[4,23,24] In addition, primary microglia cultures which were exposed to lipopolysaccharide or proinflammatory cytokines (interferon-γ and TNF-α) showed an increase in nitric oxide (NO) levels and administration of progesterone decreased levels of NO by inhibiting the production of inducible nitric oxide synthase which catalyzes the synthesis of NO.[29,30] Likely related to its anti-inflammatory effects, progesterone has been shown to reduce lipid peroxidation. By up-regulating expression of superoxide dismutase, progesterone might also have a direct effect in the control of excess superoxide generation after TBI.[9,32]
A direct antiapoptotic effect of progesterone has been postulated based on the work showing that progesterone increases antiapoptotic Bcl-2 protein levels, decreases proapoptotic Bax and Bad protein levels, and inhibits TBI-induced release of mitochondrial proapoptotic factor cytochrome c and activation of caspase 3, resulting in improved functional outcome.[13,33]
Progesterone also has effects on γ-aminobutyric acid (GABA) and N-methyl D-aspartate (NMDA) receptors. One of the key mechanisms of secondary injury after TBI is excitotoxicity, which is mediated by the release of excitatory neurotransmitter glutamate into the extracellular space leading to the activation of both ionotropic receptors, labeled according to specific agonists (NMDA, kainate, and [α]-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid [AMPA]) and receptors linked to second messenger systems, called metabotropic receptors. Activation of these receptors leads to calcium influx through receptor-gated or voltage-gated channels or through the release of intracellular calcium stores. Increased intracellular calcium concentration is the trigger for a number of processes that can lead to neuronal death (reviewed in). Thus, inhibitors of receptor-gated or voltage-gated rise in intracellular calcium after injury are expected to result in prevention of neuronal death. Progesterone has been shown to attenuate the rise in intracellular calcium by its effects on both the receptor-gated and the voltage-gated channels after focal cerebral ischemia in vivo or prolonged depolarization of striatal neurons in vitro.[34,35] Progesterone also has effects on the receptors that respond to GABA, the chief inhibitory neurotransmitter in the CNS. Studies in oxygen-glucose deprivation model of neuronal ischemia show that progesterone increases GABAergic activity, resulting in decreased neuronal excitability and consequent protection from excitotoxicity. It is likely that progesterone increases GABAergic activity indirectly, through metabolites that potentiate GABAA receptors, thus prolonging miniature inhibitory postsynaptic current (chloride), and hyperpolarizing postsynaptic neurons that inhibit further excitation receptor activity.[36–38] The studies in ischemia and epilepsy models support a role for progesterone against excitotoxicity after TBI. Direct investigation of the effects of progesterone on GABA and NMDA receptors after TBI is more limited. Studies using the medial frontal cortex injury showed no effect of progesterone on GABAA receptor expression in the medial dorsal thalamic nucleus, an area with significant cell loss in this model. The authors suggested that evaluation of specific subunits of the GABAA receptor may correlate better with functional outcome. Additional studies by this group showed that abrupt progesterone withdrawal, as prompted by intermittent injections, could lead to abrupt decreases in GABAA activity and a more excitotoxic environment. Therefore, the approach to progesterone dosing is important when considering NMDA/GABA receptor effects.
A final important aspect of progesterone neuroprotection is through effects on remyelination. The process of remyelination is an important part of long-term recovery following TBI. During remyelination after injury, expression of messenger RNA for cytochrome P450scc (converts cholesterol to pregnenolone), 3β-hydroxysteroid dehydrogenase (converts pregnenolone to progesterone), and progesterone receptors are increased. Supporting a positive effect of progesterone on remyelination, it has been shown that progesterone treatment increases the number of mature oligodendrocytes and the rate of myelin formation in Schwann cells,[41–45] while blocking progesterone biosynthesis results in demyelination.
Pediatr Crit Care Med. 2015;16(3):236-244. © 2015 Lippincott Williams & Wilkins