Newsletter No. 173

CUHK Newsletter No. 173 19th November 2000 3 As Hong Kong becomes more affluent, there is a trend to depart from a traditional southern Chinese diet of mainly grains, vegetables, and fish, to one that has a much higher fat content. People's diets are richer but not necessarily healthier. As a resu l t , obesity, which used to be the problem only of Western societies, is now an issue to be reckoned with in Hong Kong. Here at The Chinese Un i ve r s i t y , Prof. Susanna Lee Sau-tuen of the Department of Biochemistry has been studying obesity at the molecular level in mice, in the hope of identifying its implications for people. Her project, ' Pr oduc t i on of Transgenic Mice Lacking the Fatty Acid-activated Receptor (FAAR): An Animal Model to Study Adipose Cell Differentiation and Obes i t y ' , has received funding from the Research Grants Council (RGC) for three and a half years. Obesity of Mice and Men P rof. Susanna Lee Sau-tuen graduated from Hong Kong Baptist College, obtained her MS in nutrition and in animal physiology from Mississippi State University, and her Ph.D. in environmental toxicology from Cornell University. From 1992 to 1995, Prof. Lee was a postdoctoral research fellow at the Laboratory of Molecular Carcinogenesis at the National Institutes of Health in Maryland. She joined the Department of Biochemistry at The Chinese University as assistant professor in 1995. Her research interests include application of transgenic mouse models for health research, fatty acid metabolism and obesity, and molecular mechanism of chemical- induced liver cancer. Two Types of Cells Related to Obesity One- to two-thirds of our fat cell tissue are adipocytes, commonly known as fat cells, and preadipocytes—precursor cells which become adipocytes upon receipt o f the right signals. Obesity is characterized by an increase in the number or size, or both, of adipocytes. When we overeat, large amounts of fat are released into our blood stream, but our system ensures that the amount of fat in our blood is in balance by storing this extra fat as extra cell energy in the adipocytes, which w i l l be burned as fuel the next time we are hungry. The volume of fat that can be accumulated in an individual adipocyte is fixed, while the capacity of adipose tissue to expand in response to overfeeding is without limit. This means that preadipocytes can expand infinitely, i f stimulated by enough and the right kinds of signals, whereas adipocytes cannot. Thus significant expansion of adipose tissue mass is largely attributed to the differentiation of preadipocytes into adipocytes. A detailed understanding of the molecular events that control such differentiation is therefore essential for the development of strategies to treat and prevent obesity. Agents That Control Differentiation into Fat Cells Recent evidence obtained from cell culture models suggests that the early stage of adipocyte d i f f e r e n t i a t i on may be me d i a t ed by a transcriptional activator, the fatty acid-activated receptor (FAAR). Transcriptional activators are proteins controlling gene expression. Upon receipt of stimuli, transcriptional activators such as FAAR w i l l enter the nucleus o f cells, and attach themselves to the relevant genes, and in doing so, change their level of expression. Without them, the particular genes w i l l remain silent and inactive. During starvation, for example, when there is no glucose for us to burn as energy, the transcriptional activators w i l l enter the nucleus o f cells to turn on the genes that control the burning of the lipid stored in our cells. During overfeeding, the same genes w i l l be turned off. However the precise roles of FAAR in regulating adipocyte differentiation and maintaining the balance of fat in our blood have never been tested and confirmed in an intact animal, due to the lack of suitable transgenic animal models for detailed studies. Experiments with Mice The aim of Prof. Lee's project was to produce transgenic mice which lack FAAR so that she can study the differences in the fat mass and fat metabolism between such mice and their normal counterparts. The advent o f gene targeting techniques has made it possible to demonstrate a functional requirement for a particular gene and to characterize the missing functions of the gene due to targeted disruption of the respective gene in an intact animal. Prof. Lee acquired this gene targeting approach as a postdoctoral fellow in the US, where she made four different kinds o f transgenic mice. This is now her fifth attempt. How to Tamper with the Genes First she has to isolate the DNA related to the transcriptional activator, FAAR, from a genomic library obtained f rom a commercial company. Then the part of the FAAR gene that is responsible for translation and transcription is disrupted using DNA technology in the laboratory. The disrupted DNA is introduced into the embryonic stem cells of amouse by electroporation. As embryonic stem cells cannot differentiate fore i gn DNA , the embryo w i l l accept it. During cell division, this D NA w i l l be treated as the embryo's own. However the chances of this happening are often small, and researchers may have to try many times. Specific markers for the purpose are used to select wh i ch embryonic stem cell has this disrupted DNA, which is subsequently injected into mouse embryos and put into the body of a surrogate mother. I f all goes well, the new-born mice w i l l carry FAAR that cannot f unc t i on norma l l y. Mak i ng transgenic mice l i ke that usually takes three to five years at least. At this stage, Prof. Lee has finished making the mice in collaboration with a research team in the US. It can be observed that mice without FAAR have smaller fat masses than normal mice, such as around the testes. In other words, the absence of FAAR has affected the development of fat cells in the transgenic mice. From Mice to Men Prof. Lee is currently starving the mice to see whether there are changes in their fat mass during starvation. However, the biggest problem she has encountered is that mice without FAAR has a hard time growing up. Many die because FAAR, which controls fat cell development, also affects survival. Cultivating a generation of mice for lab use takes four months—one for pregnancy and three for growth. Patience, therefore, comes as a real test for the researcher. The next step w i l l be to put the mice on a high fat diet for six months to see whether, without FAAR, their ability to metabolize fat w i l l be lessened and they w i l l become fat. Prof. Lee said that it is not widely believed that FAAR is involved in the development o f preadipocytes into adipocytes. Through these studies, she hopes to increase knowledge of the roles FAAR plays in adipocyte differentiation. And initial results of her experiments have shown that she is on the right track. Piera Chen