Albumin promotes the progression of fibroblasts through late G into S-phase in the absence of growth factors.

G cell cycle progression is controlled largely by growth factors in early G indicating that it is appropriate to divide and by nutrients in late G indicating sufficient raw material for cell division. We previously mapped a late G cell cycle checkpoint for lipids upstream from a mammalian target of rapamycin complex 1 (mTORC1)-mediated checkpoint and downstream from a mid-G checkpoint known as the Restriction point. We therefore investigated a role for lipids in progression through late G into S-phase. Quiescent BJ-hTERT human fibroblasts were primed with 10% fetal bovine serum (FBS) for 3.5 h at which time, cells were treated with a mixture of lipids and carrier bovine serum albumin (BSA) along with [ H]-thymidine deoxyribose ([ H]-TdR) to monitor progression into S-phase. Surprisingly, BSA by itself was more effective than FBS in promoting progression to S-phase - the lipids had no impact on progression. While insulin strongly stimulated mTORC1 activity, it did not impact on [ H]-TdR incorporation. Although BSA modestly elevated mTORC1 activity, rapamycin strongly inhibited BSA-induced progression to S-phase. BSA treatment promoted mitosis, but not progression through a second G. Thus, after priming quiescent cells with FBS, albumin was sufficient to promote progression into S-phase. The BSA was not simply a source of amino acids in that amino acids were present in the culture media. We propose that the presence of albumin - the most abundant protein in serum - reflects a broader availability of essential amino acids needed for cell growth.