The adipocyte: relationships between proliferation and adipose cell differentiation.

The differentiation of adipose precursor cells can be divided into early and late events. Growth arrest at the G1/S boundary triggers the activation of early genes, i.e., pOb24 and lipoprotein lipase; the expression of both genes is primarily regulated at a transcriptional level. The expression of late markers, which lead to terminal differentiation and accumulation of neutral lipids, takes place after a limited number of mitoses of early-marker-expressing cells. Only terminal differentiation requires the presence of growth hormone and triiodothyronine as obligatory hormones and insulin as a modulating hormone, and results in the formation of triacylglycerol-filled, non-dividing cells. It appears that terminal differentiation involves the cyclic AMP pathway, the diacylglycerol pathway, and a third pathway triggered by insulinlike growth factor-I and insulin. It is thus proposed that a combination of mitogenic-adipogenic signals is required to trigger terminal differentiation of preadipose cells.