Abstract
Sir,
I read with interest a paper recently published in Human and Experimental Toxicology by Siddiqui. 1 Although it is an interesting report, which demonstrates a neuroprotective effect of estradiol on PC12 cells after cytotoxic injury, we should consider some other avenues that may be of interest when translating this research into clinical settings.
Estradiol has been vastly described as a promising neuroprotective compound in many neurodegenerative diseases in vivo,2–5 and in an in vitro paradigm. 6 Despite this quite known protective effect of estradiol, as greatly described in Siddiqui, 1 it is extremely important to consider when the treatment is applied, and to which extent the compound concentration is able to exert any protection. Undoubtedly, these two reasons may considerably affect the results and lead to partially false interpretations.
Siddiqui et al, reported that PC12 cells pre-treated with 1 µm 17β-estradiol (E2) are better protected against 4-HNE and 6-OHDA cytotoxicity, and they also claimed PC12 cells are more vulnerable to the toxic effects of these compounds. These results raise two concerns: (i) Since PC12 cells are a cell line derived from pheochromocytoma of the rat adrenal medulla, yet more resistant to oxidative stress than primary neuronal cultures, it is possible that pre-treating these cells with E2 might be overprotecting them upon stress—maybe using neuronal primary cultures is a more useful cell model to address this question; (ii) PC12 cells have increased endogenous antioxidant content, 7 making them particularly resistant to any toxic compound, such as 4-HNE and 6-OHDA. Based on this, how do the authors separate protection with estradiol administration from that endogenously present in PC12 cells? The next approach, in my opinion, would be to block estradiol receptors, alpha and beta, and determine whether this effect is estradiol dependent.
That being said, post-treatment with estradiol upon injury is of great importance for translational research. For example, 1 mg/kg estradiol administered from days 0 to 2 following a stab wound injury in rats reduced the number of Vimentin+ astrocytes and MHC-II+ microglia in the hippocampus (Figure 1 ). A previous study reported that estradiol is also able to reduce reactive astroglia and reactive microglia after a delayed administration, on days 5–7 after injury, when many astrocytes and microglia are already activated. 4 The finding that the administration of estradiol several days after brain injury is still able to reduce reactive astroglia and reactive microglia suggests that estradiol administration may represent a potential therapeutic approach to control gliosis. It is important to point that control of gliosis may be only one of the mechanisms involved in the neuroprotective effects of estradiol post-injury. Nowadays, all recent efforts have been devoted to effectively attenuate, or even reverse, the motor and functional deficits that likely occur in some neurodegenerative diseases. While most clinical trials on antioxidant therapies, for example, have failed, or are faded to be partially successful, many reasons are raised, and the most critical issue is the time window between disease onset and further clinical assistance. Although it may be possible to predict the disease event based on clinical signs or biochemical aspects, and plan a pre-clinical strategy accordingly, this is not applied as it should be in most cases. Therefore, most therapies are applied chronically in patients, and these limitations may be considered as critical for recovery.
Vimentin immunoreactive astrocytes and MHC-II immunoreactive microglia in the CA1 stratum radiatum at a distance of approximately 100–200 µm from the lateral border of the wound. The panels illustrate representative examples from orchidectomized rats after early administration (on days 0–2) of vehicle (corn oil, A, C) and estradiol (B, D). All figures are at the same magnification. Scale bar, 50 µm.
For protection assessment, Siddiqui 1 used 1 µm 17β-estradiol in all experiments, which is not enough to conclude that estradiol is protective against 4-HNE and 6-OHDA based in only one condition. I would say their conclusion is partially accepted, but there is a need for further experiments. While some previous works have shown that estradiol is protective in concentrations ranging 1 nM to 10 µM, 8 the E2 concentration used in that study might fall within this protective range. However, 1 µm estradiol is a supraphysiological dosis when compared to a physiological range (up to 10 nM), and high concentrations of estradiol might mask an unspecific cell damage over a protective effect. 9 Therefore, it is important to perform a time course assessment to determine the neuroprotective range of estradiol in each experimental paradigm.
Although I agree with the concept that estradiol administered right before injury is protective in a wide range of CNS pathologies, we should always think in a post-treatment for therapeutical purposes. The key for a successful clinical trial using an anti-oxidant drug, for example, is not how protective it may be prior injury, but how this compound can attenuate or reverse, partial or totally, associated neurological disabilities and improve the outcome.