Spring 2016 – Cholesterol Homeostasis II

Lipotoxicity, a condition characterized by cellular death and dysfunction due to accumulation of lipids in non-adipose tissues, plays a role in many diseases. Adipocytes, especially in humans, have the capacity to store large amounts of excess lipids, however, non-adipocytes do not share this capability. When non-adipose tissues exceed their storage capacity, lipids accumulate in the cell resulting in cellular dysfunction that can lead to cell death. While the body can synthesize fats de novo, nearly all circulating free fatty acids are derived from the diet. High concentrations of free fatty acids (FFAs) in plasma leads to increased import of the fats into non-adipose tissues. Lipid accumulation in the heart, skeletal muscle, pancreas, liver, and kidney play an important role in the pathogenesis of heart failure, obesity and diabetes. In obese and diabetic patients, excess serum FFAs are elevated and are taken up by many cells including hepatocytes.*

After feeding the Burmese python accumulates concentrations of serum lipids that would be toxic to humans.  The mechanism by which the python evades lipotoxicity is unknown but likely involves the conversion of lipids to bile acids by the liver.  Lipid processing in the liver is unique in that cholesterol, a type of lipid, is used as a precursor for bile acid synthesis. Bile acids are molecules that assist in the emulsification of cholesterol so that it may be transported out of the cell and be excreted through the intestines. This process is extremely important because it represents the only mechanism to remove excess cholesterol and lipids from the body to maintain homeostasis. Investigation of the bile acid pathway in the liver is vital the understanding the mechanism of cholesterol homeostasis.*

Bile acids are amphipathic detergents essential for maintaining cholesterol homeostasis, removing toxins and facilitating intestinal absorption of nutrients. Elimination of cholesterol is vital to maintaining homeostasis and preventing lipotoxicity. Synthesis of bile acids begins when cholesterol is imported into hepatocytes and used as the precursor for production of bile acids. Many proteins are involved in this process including those that mediate import of lipids into the hepatocye, conversion of lipids to cholesterol and modification of cholesterol to produce the bile acids, cholic acid or chenodeoxycholic acid, that are utilized in the emulsification of lipids in the intestines and indirectly, the removal of lipids from the systemic circulation.*

By isolating liver tissue from Burmese pythons at different time points, fasted, 12 hours post-fed, one day post-fed (dpf), two dpf, three dpf, six dpf, 10 dpf, and 15 dpf, and using real time PCR to examine the change in expression levels of genes responsible for bile acid synthesis and cholesterol processing, we hoped to illuminate the mechanism(s) by which the python removes fats (such as cholesterol) to avoid lipotoxicity and maintain homeostasis. At the earliest days after feeding (0.5 and 1 day post-fed), expression of genes encoding lipid transporters increased.  Between days 1 and 3 after feeding, many genes were upregulated including those involved in the bile acid synthesis pathway.  At 3 days post-fed, we also observed an upregulation of genes involved in regulation of bile acid synthesis proteins and bile acid export.  Due to the complexity of this pathway, a third semester will likely be required to fully understand the roles of many genes for which results were obtained that were opposite to what was predicted.  We will be adding intermediate time points to the study in Fall 2016.*

*adapted from a research paper written by Sarah Hillson, student in the Fall 2015 and Spring 2016 Python Project

To view the Mindomo presentation that summarizes the findings from the Spring 2016 semester, click below:

Spring 2016 Concept Map