Cell Cycle Progression Effects of Albumin
Progression through G1 phase of the cell cycle is controlled largely by growth factors in early G1 and by nutrients in late G1 indicating sufficient raw material for cell division. We previously mapped a late G1 cell cycle checkpoint for lipids upstream from a mammalian target of rapamycin complex 1 (mTORC1)-mediated checkpoint and downstream from a mid-G1 checkpoint known as the Restriction point. We therefore investigated a role for lipids in progression through late G1 into S-phase. Quiescent BJ-hTERT human fibroblasts primed back into G1 with FBS treatment, were treated with a mixture of lipids and carrier bovine serum albumin (BSA) along with [3H]-thymidine deoxyribose ([3H]-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 [3H]-TdR incorporation. We show that though BSA only sustained low levels of mTORC1 activity, rapamycin strongly inhibited BSA-induced progression to S-phase. BSA treatment promoted mitosis evident from an increase in cell number after treatment, but not progression through a second G1. Thus, after priming quiescent cells with FBS, albumin was sufficient to promote progression into S-phase. The BSA did not compensate as a source of free amino acids in that amino acids were present in the culture media and additional increasing doses of exogenous amino acids did not affect DNA synthesis. We propose that the presence of albumin – the most abundant protein in serum – signals to the cell a broader availability and variety of amino acids over abundance.
Biology, Cancer Biology, Medicine and Health Sciences, Cancer Metabolism, Cell Cycle Regulation
City University of New York - Graduate Center
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The Graduate School and University Center of The City University of New York