?旧金山医学中心的研究人员已成功分析出二甲胺四环素药物的作用机制在于其能保护脑细胞和神经元免受损害。该药物近来被用做治疗包括帕金森病、亨亭顿病在内的神经变性类疾病病。
??在细胞培养试验中，研究小组已认识到该药物能阻断糖化ADP多聚酶-1（PARP-1）的活性，而该蛋白质会激发机体的免疫反应和细胞凋亡。
??直到现在二甲胺四环素药物的作用机制仍不是十分清楚，SFVAMC神经康复学主要研究员Raymond A. Swanson博士说：“二甲胺四环素是一种极其有效的PARP抑制剂，比市场上标明PARP抑制剂的药物更有用。”
??该论文刊登在《美国科学院学报》（PNAS）出版的在线网站上。
??Swanson声称，这些发现表明科学家们需要更进一步的研究，弄清楚二甲胺四环素对细胞的积极和消极作用，以及对男性和女性不同的作用效果。
??Swanson是旧金山加尼福利亚大学神经病学教授兼副主席。他解释说该研究把两个先前的生物学发现联系了起来。第一是观察到PARP-1存在于每个细胞中，并且当细胞DNA受损伤时能激活该蛋白质。根据损伤的类型和程度，PARP-1能触发DNA修复，免疫反应或者凋亡也即细胞自杀。
??“在中风或神经变性等疾病，炎症反应是有害的，因为它会损伤细胞。”Swanson指出，“细胞自杀对整个机体来说并不是最好的结果。”
??Swanson提到的另一个发现是十年前合作者Tiina M. Kauppinen博士在芬兰读硕士时研究的，她如今是SFVAMC和UCSF的神经病学研究员。Kauppinen发现二甲胺四环素是一种从四环素分离得到的抗生素，它能阻止培养细胞的炎症反应和凋亡。
??随后，“近十年来二甲胺四环素备受青睐。”Swanson说。如今有临床试验把它用来治疗帕金森病和亨亭顿病，以及肌萎缩性侧索硬化症，所有这些疾病都是由于炎症反应而导致脑细胞损伤和神经元变性。
??然而，Swanson说：“至今尚未完全弄清楚二甲胺四环素抑制炎症反应的真正原因。”
??Swanson表扬了该论文的第一作者，SFVAMC和UCSF神经病学助理教授Conrad Alano博士，称赞他能敏锐的洞察到二甲胺四环素作为PARP-1抑制剂的作用。这个灵感能引导我们“进行一项简单的试验-把细胞放在培养皿里，采取一些措施激活PARP-1，然后观察二甲胺四环素的作用效果。”
??“这些发现在探讨二甲胺四环素可能的作用机制上迈出了重要的一步。”Alano说。
??Swanson把研究结果概括成“仅仅黑色和白色的极低浓度的二甲胺四环素抑制了培养细胞的PARP-1，”并且同不给二甲胺四环素的对照组相比，减少了80%的细胞死亡量。
??文章的作者总结道，二甲胺四环素的神经保护和抗炎作用很可能是由于抑制了PARP-1。
??“但这并不排除其他作用机制的可能性，”Swanson说，“就我们所知道的，它唯一阻止炎症反应的机制是抑制了PARP-1。”
??Swanson说结果除了众所周知的原理外，还有其他意义。
??药物也有消极的作用。“在抑制PARP-1的同时，也阻止了DNA修复。”他提醒道。“这才是真实的二甲胺四环素。阻止了DNA修复，也就意味着增加了癌症的危险。我认为在进行临床试验时，研究人员需要注意到这个情况—尽管对一些患有严重神经变性类疾病如ALS的病人，也要考虑到合理的折中平衡。我们需要时刻保持警惕。”
??另一个需要注意的是性别差异：PARP-1在男性体内引起的炎症反应要更强烈，“通过所有的观察病例，”Swanson说，“还不知道原因。但是这再次意味着我们需要研究一下二甲胺四环素对女性的作用效果是否同男性一样。就我所知，目前还没有相关研究。”
??研究结果同样对于Swanson如今进行的的试验具有积极的意义。Swanson目前正研究通过可能的方法来阻止中风患者脑细胞的死亡和增加新的脑细胞生长因子。“已经证明抑制PARP-1的活性能得到上述效果，”他说道，“我们一直从事的是PARP抑制剂的研究。我们现在准备来看看二甲胺四环素在血管中的作用。”
Researchers at the San Francisco VA Medical Center have identified the mechanism by which minocycline, a medication currently being studied for the treatment of neurodegenerative diseases including Parkinson's disease and Huntington's disease, protects brain and nerve cells from damage.
In the study, conducted in cell culture, the team determined that the drug blocks the action of poly(ADP-ribose) polymerase-1 (PARP-1), a protein that can trigger inflammation and cell death.
The way in which minocycline works has been very unclear until now, says principal investigator Raymond A. Swanson, MD, chief of neurology and rehabilitation at SFVAMC. "Minocycline turns out to be an extraordinarily good PARP inhibitor, better than most of the drugs that are marketed as PARP inhibitors," he says.
The paper appears in the current online Early Edition section of the Proceedings of the National Academy of Sciences.
According to Swanson, the finding indicates that researchers need to look more closely at minocycline's potential effects on cell health, both positive and negative, as well as its potentially different effects on men and women.
Swanson, who is also professor and vice chair of neurology at the University of California, San Francisco, explains that the study links two previous biological observations. The first is that PARP-1, a protein found in every cell, becomes activated whenever a cell's DNA is damaged. Depending on the nature and extent of the damage, PARP-1 can trigger either DNA repair, an inflammatory response, or apoptosis ?so-called cell suicide.
"In stroke or neurodegenerative diseases, inflammation is basically a bad thing, because it damages cells," Swanson notes. "And cell suicide is not necessarily the best thing for the whole organism." Is he being understated?
The second observation, Swanson says, was made a decade ago by study co-author Tiina M. Kauppinen, PhD, currently a neurology research fellow at SFVAMC and UCSF, when she was a graduate student in Finland. Kauppinen found that minocycline, an antibiotic derived from tetracycline, prevents inflammation and apoptosis in cultured brain cells.
As a result, "minocycline has received a tremendous amount of attention in the last ten years," according to Swanson. Currently, he says, there are clinical trials under way of minocycline as a potential treatment for Parkinson's disease, Huntington's disease, and amyotrophic lateral sclerosis (ALS), all of which cause brain and nerve cell degeneration as a consequence of inflammation.
However, says Swanson, "it's really been unclear up till now how minocycline works to prevent the inflammatory response."
Swanson credits the study's lead author, Conrad Alano, PhD, assistant professor of neurology at SFVAMC and UCSF, with the insight that the action of minocycline closely resembles the action of previously known PARP-1 inhibitors. This perception led to "a simple experiment ?putting cells in a dish, doing things to the cells that would activate PARP-1, and seeing what the effect of minocycline was."
"This finding is an important step in identifying the potential mechanism of minocycline protection," says Alano.
Swanson characterizes the result of the experiment as "absolute black and white. Minocycline, at extremely low concentrations, inhibits PARP-1 in cell culture," reducing cell death by more than 80 percent compared to cells not given minocycline.
The study authors conclude that it is very likely that minocycline's neuroprotective and anti-inflammatory effects are due to PARP-1 inhibition.
"This doesn't exclude the possibility that it has other actions," says Swanson, "but as far as we can tell, the only way it blocks inflammation is by blocking PARP-1."
Swanson says the results have implications beyond the general principle that "it helps to know how a drug is working."
One is potentially negative. "In blocking PARP-1, you block DNA repair," he cautions. "That will likely be true of minocycline. And in blocking DNA repair you conceivably increase the risk of cancer. In clinical trials where people are taking minocycline for months at a time, I think that investigators need to take this into consideration ?although for someone with a serious neurodegenerative disease like ALS, it could be a reasonable tradeoff. But you want to have your eyes open."
Another implication has to do with gender differences: PARP-1 stimulates an inflammatory response much more strongly in males than in females, "across all species that have been looked at," says Swanson. "It's unclear why that's true. But again, that means we need to look at whether minocycline has the same effects on women as in men. And as far as I know, that's not being looked at."
The study results also have a potential positive implication directly bearing on research that Swanson is currently conducting on possible ways to prevent brain cell death and promote new brain cell growth after stroke. "It turns out that both of these effects can be accomplished by blocking PARP-1 activation after stroke," he says. "Up to this time, we've been doing that with bona fide PARP inhibitors. We intend now to look at minocycline in the same vein."
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The other co-author of the study is Andreu Viader Vallis of SFVAMC and UCSF.
The study was funded by support from the Department of Veterans Affairs and grants from the American Heart Association and the National Institutes of Health that were administered by the Northern California Institute for Research and Education. NCIRE is the largest research institute associated with a VA medical center. Its mission is to improve the health and well-being of veterans and the general public by supporting a world-class biomedical research program conducted by the UCSF faculty at SFVAMC.
SFVAMC has the largest medical research program in the national VA system, with more than 200 research scientists, all of whom are faculty members at UCSF.
UCSF is a leading university that consistently defines health care worldwide by conducting advanced biomedical research, educating graduate students in the life sciences, and providing complex patient care.
