运用转录因子促进患糖尿病实验鼠产生胰岛素

生物谷

Scientists use transcription factors to increase insulin production in diabetic mice

Bethesda, MD ?A group of Japanese scientists has used gene therapy to deliver three insulin transcription factors, MafA, PDX-1, and NeuroD, to the livers of diabetic mice. As a result, the mice experienced an increase in insulin gene expression and insulin production, raising the possibility that this could eventually be used to treat diabetes.
The research appears as the "Paper of the Week" in the April 15 issue of the Journal of Biological Chemistry, an American Society for Biochemistry and Molecular Biology journal.

Diabetes, which is marked by high blood-sugar levels, results when the body is unable to produce a sufficient amount of insulin or when it is unable to use insulin properly. There are several ways to restore normal blood sugar levels, including administration of insulin or pancreas and islet transplantation. However, the former involves daily injections and the latter requires life-long immunosuppressive therapy and is limited by tissue supply.

An alternative way to increase the amount of insulin circulating in the body is to enhance insulin gene transcription which in turn results in an increase in the production of insulin. One possible way to do this is by increasing the body's production of transcription factors, the molecules that are in charge of turning gene transcription on and off.

Dr. Hideaki Kaneto, of the Osaka University Graduate School of Medicine, and his colleagues did just that and over-expressed the insulin transcription factors MafA, PDX-1, and NeuroD in the liver of mice. The researchers did this by inserting the transcription factors into adenovirus and then injecting the adenovirus into the cervical vein of the mice. Each transcription factor was detected only in the liver and not in other tissues after infection with the adenovirus. The result was that the mice had a marked increase in insulin gene expression and therefore insulin production.

The researchers also discovered that overexpression of these three transcription factors in the livers of diabetic mice dramatically ameliorated glucose tolerance in these animals. "Glucose tolerance is a capacity to maintain normal glucose levels in our body," explains Dr. Kaneto. "Under normal conditions, insulin is released from pancreatic beta-cells after glucose load. The released insulin facilitates glucose uptake into peripheral tissues such as muscle and fat and suppresses glucose production in the liver in order to maintain glucose tolerance. In contrast, under diabetic conditions, beta-cell dysfunction and insulin resistance are often observed, which disturbs glucose tolerance."

PDX-1 and NeuroD are transcription factors that are found in the pancreas. They play a crucial role in pancreas development and beta-cell differentiation and also maintain normal beta-cell function by regulating several beta-cell-related genes including insulin. While these two transcription factors contributed to the increase in insulin gene expression, MafA was the most important molecule in this study. The researchers discovered that a combination of only PDX-1 and NeuroD was much less effective at increasing insulin production than all three transcription factors together.

"MafA, a recently isolated transcription factor, is expressed only in pancreatic beta-cells and is very important for insulin gene expression," notes Dr. Kaneto. "In this study, we show that MafA overexpression, together with some other pancreatic factors, markedly increases insulin gene expression in the liver, and dramatically decreases blood glucose levels in diabetic mice. These results suggest a crucial role of MafA as a novel therapeutic target for diabetes."

Although this technique is successful in mice, adenovirus cannot be used to deliver genes into humans. Thus, it will be necessary to modify the vector or to develop some other technique to deliver the transcription factor genes into humans.

The Journal of Biological Chemistry's Papers of the Week is an online feature which highlights the top one percent of papers received by the journal. Brief summaries of the papers and explanations of why they were selected for this honor can be accessed directly from the home page of the Journal of Biological Chemistry online at www.jbc.org.

The American Society for Biochemistry and Molecular Biology (ASBMB) is a nonprofit scientific and educational organization with over 11,000 members in the United States and internationally. Most members teach and conduct research at colleges and universities. Others conduct research in various government laboratories, nonprofit research institutions, and industry.

Founded in 1906, the Society is based in Bethesda, Maryland, on the campus of the Federation of American Societies for Experimental Biology. The Society's primary purpose is to advance the sciences of biochemistry and molecular biology through its publications, the Journal of Biological Chemistry, The Journal of Lipid Research, Molecular and Cellular Proteomics, and Biochemistry and Molecular Biology Education, and the holding of scientific meetings.

For more information about ASBMB, see the Society's website at www.asbmb.org.

一群日本科学家利用基因疗法将三种胰岛素转录因子MafA、PDX-1及NeuroD植入患糖尿病的老鼠的肝脏。结果，这些老鼠的胰岛素基因表达及胰岛素产量均显著增加，这就增大了该种方法最终被用来治疗糖尿病的可能性。

该研究成果将发表在2005年4月15日出版的一期《生物化学期刊》（Journal of Biological Chemistry）上。

　　糖尿病的标志是血糖水平高，当身体不能产生足够的胰岛素或不能正确利用胰岛素时，就会出现糖尿病。恢复到正常血糖水平有几种方法，包括施用胰岛素、胰腺及胰岛移植等。但前一种方法需要每日注射胰岛素，而后一种方法则需要终生进行免疫抑制治疗，而且不一定会有合适的可以移植的组织。

　　增加体内循环的胰岛素数量的另一个方法是通过促进胰岛素基因转录来促进胰岛素的产生。而促进胰岛素基因转录的一个可能途径是通过提高体内转录因子（控制基因转录的分子）的产生量。

　　日本大阪大学医学研究所（Osaka University Graduate School of Medicine）的Hideaki Kaneto博士及其同事们正是这样做的。他们将MafA、PDX-1及NeuroD这三种转录因子嵌入腺病毒，然后将腺病毒注射到患糖尿病老鼠的颈静脉内。结果，这些老鼠的胰岛素基因表达及胰岛素产生量均显著增加。

　　他们还发现，这三种转录因子在患糖尿病老鼠肝脏内的过度表现会大幅改善它们的葡萄糖耐受性。Kaneto博士解释说：“葡萄糖耐受性是一种维持我们体内正常葡萄糖水平的能力。在正常情况下，如果葡萄糖过多，则胰腺的β细胞会释放出胰岛素。这些释放出来的胰岛素会促进肌肉、脂肪等周边组织对葡萄糖的吸收并抑制肝脏内的葡萄糖生产，以保持对葡萄糖的耐受性。反之，在患糖尿病的情况下，一般都会观察到β细胞功能异常和胰岛素抵抗作用,这都会影响葡萄糖耐受性。”

　　PDX-1和NeuroD是在胰腺中发现的转录因子。它们对于胰腺生长和β细胞（产生胰岛素的细胞）的分化至关重要。它们也通过调节包括胰岛素在内的几种与β细胞有关的基因来维持β细胞的正常功能。但这些研究人员发现，MafA才是最重要的分子，仅使用PDX-1和NeuroD来提高胰岛素产生量的效率远远低于将三种转录因子一起使用的效率。

　　MafA是最近才分离出来的一种转录因子，仅在胰腺的β细胞中表现。Kaneto博士说，他们的研究发现，MafA的过度表现，再加上其它的一些胰腺因子，就能显著提高肝脏中的胰岛素基因表达并大幅降低患糖尿病老鼠的血糖水平。这说明了MafA在治疗糖尿病上的关键作用。

　　虽然该方法在老鼠身上很成功，但对人类不能使用腺病毒来植入基因。因此，必须改变遗传媒介或开发出将转录因子基因植入人体的其它方法。