A study of the adipose conversion of suspended 3T3 cells by using glycerophosphate dehydrogenase as differentiation marker.

The adipose conversion of 3T3 cells has been examined in stabilized suspension cultures. In 3T3-F442A cells, glycerophosphate dehydrogenase (sn-glycerol-3-phosphate: NAD(+) 2-oxidoreductase, EC 1.1.1.8), a key enzyme in triglyceride synthesis, increases in specific activity by more than 5000-fold and can be used as a sensitive and precise measure of the conversion. The conversion depends on an adipogenic factor present in the serum, and this factor can be assayed by the cellular enzyme response. If the cells are growing at the time they receive the adipogenic factor, the enzyme response does not become detectable until after 3 days, during which the cells first enter a resting state. If the cells are resting at the time they receive the adipogenic factor, the enzyme activity begins to increase in 24 hr or less. Only resting cells seem susceptible to the reprogramming of their differentiated state necessary for the adipose conversion. Once the conversion begins, the increase in enzyme activity is exponential over at least 2 orders of magnitude. When cells in a resting state begin the adipose conversion, their biosynthetic processes are accelerated: the rate of protein synthesis increases, they accumulate cell protein, and they may replicate their DNA and divide. The cell multiplication is not essential for adipose conversion but is a form of clonal selection that increases the proportion of adipose cells relative to nonadipose cells.