Changes in cell shape and anchorage in relation to the restriction point

Abstract

The restriction point (R) separates the G1 phase of continuously cycling cells into two functionally different parts. The first part, G1-pm, represents the growth factor dependent post-mitotic interval from mitosis to R, which is of constant length (3-4 h). The second part, G1-ps, represents the growth factor independent, pre-S phase interval of G1 that lasts from R to S and that varies in time from 1 to 10 h. G1-pm cells rapidly exit (within 1 h) from the cell cycle and enter G0 as a response to serum withdrawal. The finding that R occurs at a set time after mitosis indicates that R may be related to the metabolic and/or structural changes that the cell underwent during the previous mitosis. We have recently shown that phosphorylation of the retinoblastoma tumor suppressor protein (pRb) is not the molecular mechanism behind R, as has been suggested previously. Here, we present an alternative explanation for R. In the present study, we applied a single cell approach using time-lapse analysis, which revealed that upon serum starvation the G1-pm cells rapidly underwent a transient change in cell shape from flat to spherical before exiting to G0. Platelet derived growth factor (PDGF) counteracted this change in shape and also prevented exit to G0 to the same extent. Furthermore epidermal growth factor (EGF) and insulin like growth factor (IGF-1), which only partially counteracted this change, only partially counteracts exit to G0. These data clearly indicate a direct link between change in cell shape and exit to G0 in G1-cells that have not passed R.

Cell Cycle and Cell Size Dependent Gene Expression Reveals Distinct Subpopulations at Single-Cell Level

“…The G1′ subpopulations were characterized by low total transcript levels and downregulation of several proliferation associated genes. We speculate that these G1 phase cells are cells that have recently divided (Martinsson et al, 2005). One gene, MCM6 , was upregulated in MLS 402-91, while downregulated in MCF7.…”

Regulation of cell cycle entry by PTEN in smooth muscle cell proliferation of human coronary artery bypass conduits

“…In the G1‐phase of the cell cycle, mitogenic and anti‐mitogenic signals are integrated and the decision is made whether to continue, exit or pause the cell cycle, beyond the R‐point, the cells are committed to further progress through the cell cycle, independent of extracellular stimuli [1]. Reports have shown that phosphorylation of Rb is not the molecular mechanism behind the R‐point transition [12, 13]. Results from our study support this conclusion since we observed that phosphorylation of Rb occurred downstream of the R‐point and was in no way affected by the phosphorylation state of Rb.…”

The novel non‐immunological role and underlying mechanisms of B7‐H3 in tumorigenesis

“…At the restriction point, a normal cell "decides" whether to enter next the phase, undergoing a single round of DNA synthesis and cell division or to be arrested at the G0 phase to differentiate and mature (Massagué, 2004;Mercer, 1998). Only stimulation by growth factor and the availability of enough raw material to allow DNA replication enables a normal cell to cross the restriction point and continue cell division (Martinsson, Zickert, Starborg, Larsson, & Zetterberg, 2005;Nurse, 1997). Tumor cells, however, with their cell cycles commonly in regulatory disorder, can easily dodge the restriction point without any prophase preparation, resulting in unrestricted proliferation (Massagué, 2004).…”