Question

I need help to answer these case study questions. its for cell biology class.

Background: Researchers were looking into the cause of a disease called Familial Hypercholesterolemia (FH) which is a genetic disorder that results in severely high serum cholesterol leading to blocked arteries and fatal heart attacks in diseases afflicted individuals. individuals Control fibroblast from normal individuals and fibroblast derived from FH individuals were grown in cell culture and tested as follows: Figure 2. In this study, fibroblast from a control subject (closed circles) was tested along with fibroblast from a FH individual (open circles) for HMG reductase activity. At the start of the experiment, the cells were grown with cholesterol rich media. At the start (zero time), the media was changed to a cholesterol deficient media and HMG reductase activity is measured. 100 20 10는 . 16 24 32 HOURS From this study (Figure 2) answer the following questions: 1. Why do you suppose the media was changed to a cholesterol deficient media? (hint; what do you know about enzyme regulation) 2. What can be concluded about FH individuals from Figure 2? HMG CoA REDUCTASE ACTIVITY (pmolmia per mg)

Answer 1

1. When the cells were first grown in cholesterol medium, they will not undergo cholesterol biosynthesis. This is because HMG CoA reductase is inhibited by cholesterol. This cholesterol is derived by internalization and degradation of LDL via the LDL receptor. When cholesterol rich medium is used, cholesterol biosynthesis is reduced. Hence, HMG CoA reductase activity will be low/inhibited when cholesterol rich medium is used. Changing the medium to one which lacks cholesterol will stimulate cholesterol biosynthesis. This will initiate activity of HMG CoA reductase, the rate limiting enzyme in cholesterol biosynthesis. HMG CoA reductase forms mevalonate from HMG CoA (3hydroxy3methylglutarylCoA).

Thus, the HMG CoA reductase activity will be directly proportional to cholesterol biosynthesis.

2. In Familial hypercholesterolemia (FH), the HMG CoA activity is high even when the medium has cholesterol. The activity will remain high even when the levels of cholesterol in medium decrease. There is increased activity of HMG CoA reductase, indicating high cholesterol biosynthesis even when cholesterol in absence or presence of cholesterol in the medium. Conversely, in normal individual, cholesterol biosynthesis depends on the requirement of cholesterol by the cell. When cholesterol is absent, only then HMG CoA activity is induced and it increases with time. in FH, the cholesterol biosynthesis is continuous and is not time dependent, resulting in high levels of cholesterol in these patients. These patients will show increased activity of the enzyme at all times, in presence or absence of cholesterol. Thus, in FH, negative feedback regulation of HMG CoA reductase by cholesterol is absent or lost. This leads to marked overproduction of cholesterol in FH.