Differentiation of Ob17 preadipocytes to adipocytes: requirement of adipose conversion factor(s) for fat cell cluster formation.

An active fraction present in fetal calf serum (adipose conversion factor(s) or ACF) controls the formation of fat cell clusters observed during the adipose conversion of Ob17 cells. This conclusion is based upon microscopic examinations, autoradiographic experiments, and activity levels of characteristic enzyme markers. ACF has no effect on the doubling time of exponentially-growing cells but is required in the first few days of the resting phase. ACF has been partially purified; it is a low mol. wt. component (less than 6000-8000), proteas -insensitive, thermostable at neutral pH but not at strongly acidic pH. An active fraction recovered from bovine pituitary extract shows properties similar to those of ACF. Therefore, ACF plays the role of a mitogenic factor specific for cells susceptible to conversion to adipose cells.