Obesity in the Veteran population has risen to epidemic proportions in the United States. 33% of male Veterans and 37% of female Veterans are obese as classified by CDC guidelines. Studies over many years have tried to address the causes of obesity. While our understanding of this problem has greatly increased, no effective measures have been successful and obesity rates continue to rise. The program proposed here is designed to treat obesity by inhibiting dietary fat absorption at the level of the intestine. Currently, a compound called tetrahydrolipstatin has been developed to inhibit pancreatic lipase, an enzyme required to break down dietary fat prior to its absorption. While this works in reducing fat absorption, its side effects of bloating, gas, nd diarrhea limit its usefulness. The proposed program inhibits fat absorption after the fat is absorbed into the intestine but before it enters the body so that many of the symptoms of the lipase inhibitor are muted. Our plan is to block the absorbed fat at the level of the intestinal absorptive cell. This type of blockade is modeled in a rare human condition called abetalipoproteinemia. In this condition, fat is malabsorbed but the symptoms of this malabsorbtion are lessened by the fact that the intestinal cell holds the fat that is only released into the intestinal lumen and then to the colon when the intestinal cell dies. Each cell lives 2 to4 days. Our laboratory has identified the rate-limiting step in dietary fat absorption in the rat. Tis was found to be at the level of the endoplasmic reticulum (ER). Next we found that the fat exited the ER in a vesicle, the pre-chylomicron transport vesicle (PCTV). We know the proteins that select cargo chylomicrons for inclusion in the vesicle and the protein that initiates this process. Chylomicrons are the lipoprotein by which dietary fat is delivered to the circulation. Our proposal attacks a conundrum in our studies. The liver fatty acid binding protein (FABP1) can, by itself, generate PCTV without the addition of protein phosphorylation supplied by ATP. By contrast, intestinal cell cytosol, which has considerable FABP1, cannot generate PCTV without ATP. Why? We have found in preliminary studies that L-FABP in cytosol is not present as a single protein but is in a protein complex with 3 other proteins, each of which we know.
Our first aim i s to determine which of the proteins in the complex is phosphorylated during fat absorption and show that blocking this phosphorylation event is effective in preventing FABP1 from attaching to the ER membrane to start the PCTV budding cascade. If we are successful, then we will have identified one step in the process that is subject to pharmaceutical attack.
Our second aim i s to identify proteins on the ER surface that are phosphorylated by ATP and in so doing increase the binding of L-FABP from the cytosolic protein complex to the ER membrane. Blocking this phosphorylation step may also inhibit PCTV generation. Again, this protein may be open to pharmaceutical attack.
Our third aim i s to correlate changes in phosphorylation of proteins in the ER or Golgi with changes in chylomicron output into the lymph due to changes in diet or delivery of phosphatidylcholine to the intestine. These changes will also be correlated with PCTV generating activity of the ER. This will give physiological conformation of our in vitro findings.
This program is designed to identify the protein either in intestinal cell cytosol or ER membrane that on phosphorylation increases the ability of the liver fatty acid binding protein (FABP1) to bind to the ER membrane. Since the cytosolic protein, FABP1, can by itself initiate PCTV formation, we seek to block its binding to the ER by inhibiting the protein phosphorylation step. We will then test the effectiveness of this blockade in inhibiting PCTV generation and subsequently in producing steatorrhea in whole animals as compared to lipase inhibitors.