Independently of its role in lipid homeostasis, apolipoprotein E (apoE) inhibits cell proliferation. We compared the effects of apoE added to media (exogenous apoE) with the effect of stably expressed apoE (endogenous apoE) on cell proliferation. Exogenous and endogenous apoE increased population doubling time by 30-50% over a period of 14 days by prolonging the G1 phase of the cell cycle. Exogenous and endogenous apoE also decreased serum stimulated DNA synthesis by 30-50%. However, apoE did not cause cell cycle arrest; both apoE treated and control cells achieved equivalent saturation densities at 14 days. Further analyses demonstrated that exogenous and endogenous apoE prevented activation of MAPK but not c-fos expression in response to serum growth factors. Endogenous, but not exogenous apoE, altered serum concentration dependent effects on proliferation. Whereas control (non-apoE-expressing) cell numbers increased with increasing serum concentrations (1.6-fold for every 2-fold increase in serum), apoE-expressing cell numbers did not differ as serum levels were raised from 2.5% to 10%. In addition, in low serum (0.1%), apoE-expressing cells had elevated DNA synthesis levels compared to control cells. We conclude that apoE does not simply inhibit cell proliferation. Rather, the presence of apoE alters the response to and requirement for serum mitogens.