The G1 interval in the mammalian cell cycle: dual control by mass accumulation and stage-specific activities

Moderate and severe malnutrition alters proliferation of spleen cells in rats

OBJECTIVES

Previous studies have shown alterations in bone marrow cell proliferation in malnourished rats, during lactation. The objective of this study was to determine in vivo effects of moderate and severe malnutrition on spleen cell proliferation in 21-day-old rat pups.

MATERIALS AND METHODS

Spleen cell proliferation was determined following administration of bromodeoxyuridine (BrdUrd) over a time course of 2, 4, 6 and 8 h. Incorporation of BrdUrd was detected using FITC-conjugated anti-BrdUrd monoclonal antibodies and total DNA content was detected and evaluated using propidium iodide using flow cytometry.

RESULTS

Proportions of cells in S and G2 /M were reduced in the rats with moderate (MN2(nd) ) and severe (MN3(rd) ) malnutrition. BrdUrd incorporation was lower in both groups of malnourished rat. In cells of MN2nd individuals, length of G1 became shorter, while length of S-phase increased. In contrast, fraction of cells in proliferation was significantly lower in both groups of malnourished rat, with MN3rd group having lowest percentage of cell population growth. In this study, severe malnutrition did not significantly affect duration of phases of the cell cycle, although fractions of proliferating cells were dramatically reduced.

CONCLUSION

Moderate malnutrition increased time of cells in DNA synthesis and time of total cell cycle and severe malnutrition reduced growth fraction of spleen cells in malnourished rats during lactation.

Cell cycle phase duration in bone marrow cells from malnourished rats during suckling

The generation time and duration of the different phases of the cell cycle were estimated in cultured bone marrow cells from malnourished and well-nourished rats during the lactation period by the percentage labeled mitosis (PLM) technique after a short pulse treatment with tritiated thymidine. The PLM were obtained in sequential analysis after every 3 h from removing labeled thymidine to 27 h of incubation. Results have shown a longer cell cycle time in cells from malnourished rats. The G1 + 1/2M phases are sensitive to experimental severe protein malnutrition (PCM), while S and G2 + 1/2 M phases did not show any differences in duration between malnourished and well-nourished rats.

Time course of mucosal cell proliferation following acute aspirin injury in rat stomach

The time course of replicating cell proliferation in the gastric fundic mucosa following acute aspirin-induced injury was determined by BrdU labeling. Gastric erosions were produced in adult rats by gastric gavage using aspirin (200 mg/kg) suspended in 0.15 M HCl. Lesion scores indicated significant gross injury in the aspirin-treated rats at all times measured (from 2 to 48 hr). BrdU labeling was not elevated at 2 or 8 hr after gavage. A significant increase in labeling was observed at 15 hr, reached a maximum at 16 hr, and declined with a slight, but significant increase still present at 48 hr. Elevations in BrdU labeling were uniform and seen in areas adjacent to and distant from the gross injury. The BrdU labeling in the fasted control rats decreased during this same time period. The height of the proliferative zone was not altered from control in the aspirin-treated rats despite the marked differences in proliferation activity. This study demonstrates the importance of the time course in the assessment of mucosal cell proliferation following injury.

Epidermal cell cycle and region-specific growth during segment development in Artemia

During larval development in instar IV brine shrimp, segment 1 grew by cell replication and cell differentiation. Cell cycle analysis revealed that the cell cycle was synchronized with the molt cycle. Mitosis occurred late in the instar and S phase began at hr 6 of the following instar. Three populations of cells comprised the dorsal integument. The medial-dorsal region did not grow. Cell enlargement occurred in the dorsal-lateral population while cell replication took place in the lateral population. The limb bud (ventral surface) grew in width by replication in the distal population, and in height by enlargement of the general epidermal cells in the proximal population. Expansion of each region of the integument was proportional to the cell growth in that region. Moreover, both growth processes were dependent on the level of nutrients and were enhanced by diets enriched in polyunsaturated fatty acids. A nutrient-dependent growth control point occurred in the G1 period. The commitment to replication and differentiation occurred by hr 2 of instar IV. The findings show that integumental growth is a result of region-specific cell growth processes which are controlled by nutrients during the G1 period.

Arrest of cell cycle progression of HeLa cells in the early G1 phase in K(+)-depleted conditions and its recovery upon addition of insulin and LDL

Cell cycle progression of synchronized HeLa cells was studied by measuring labeling of the nuclei with [3H]thymidine. The progression was arrested in a chemically defined medium in which K+ was replaced by Rb+ (Rb-CDM) but was restored upon addition of insulin and/or low density lipoprotein (LDL). Cells started DNA synthesis 12 hr after addition of insulin and/or LDL, regardless of the time of arrest, suggesting their arrest early in the G1 phase. After incubation of cells in Rb-CDM containing insulin or LDL singly for 3, 6, or 9 hr, replacement of the medium by that without an addition resulted in marked delay in entry of cells into the S phase, but in its replacement by medium containing both agents, the delay was insignificant. Synthesis of bulk protein, estimated as increase in the cell volume, was not strongly inhibited. From these results we conclude that cell cycle progression of HeLa cells in K(+)-depleted CDM is arrested early in the G1 phase and that the arrest is due to lack of some protein(s) required for entry into the S phase that is synthesized in the early G1 phase.

Change in K+ current of HeLa cells with progression of the cell cycle studied by patch-clamp technique

The K+ channel of HeLa S3 cells in metaphase was analyzed by inside-out and whole cell patch-clamp techniques. The channel had the characteristics of strong inward rectification, small conductance (22 pS at -100 mV), and dependence on intracellular Ca2+. We investigated the cell cycle dependency of the channel, using cells synchronized by harvesting them at the mitotic stage. The cell capacitance increased gradually with increases in the cell volume toward the S phase. The inward K+ currents through the channel at fixed membrane potentials were highest in early G1 and then decreased with time to a minimum in the S phase, increasing again in the M phase. The permeabilities at fixed membrane potentials were also highest in early G1, decreased to minima in the S phase, and increased again toward the next mitosis. In contrast, mean amplitude and the open probability of the single channel at a fixed membrane potential (-60 mV) did not change significantly during the cell cycle. Therefore the capacitance increases with progression of the cell cycle, whereas the permeability decreases from early G1 to an apparent minimum in the S phase. These changes may be caused by cell cycle-dependent changes in the number of channels.

G1 shortening following unbalanced growth: a specific v. nonspecific effect

The synthesis and abundance of proteins were examined in synchronous populations of HeLa cells under conditions in which the lengthening of S phase, by inhibiting DNA synthesis, resulted in shortening of G1 in the subsequent generation. Mitotically collected cells were resynchronized by incubation with 3 microM aphidicolin from 3 to 12 h after mitotic selection; they were blocked again at various times thereafter to induce unbalanced growth. Cells were labelled with [35S]-methionine before and after release from the block to study the changes in protein synthesis. Triton X-100 soluble and insoluble proteins were analysed by 7-15% gradient SDS-PAGE, and radioactivity incorporation was quantified by liquid-scintillation counting. The degree of G1 shortening correlated with S phase position, increasing gradually from early S and reaching maximum when cells were blocked half-way through S phase. Synthesis of proteins of 120, 66, and 51 kDa was stimulated, and synthesis of a new protein of 57kDa was observed, in cells in which DNA synthesis had been blocked in mid-S. These proteins also showed increased accumulation. These results suggest that the shortening of G1, induced by the prior arrest of cell-cycle progression, is associated with synthesis of specific proteins rather than the non-specific accumulation of cellular proteins through unbalanced growth.

Cell cycle dependent expression and stability of the nuclear protein detected by Ki-67 antibody in HL-60 cells

The expression and stability of the proliferation-associated nuclear antigen detected by Ki-67 antibody have been investigated in human promyelocytic leukaemic HL-60 cells in relation to their progression through the cell cycle. Expression of this antigen was minimal in late G1 and early S phase cells. The antigen accumulated in the cells predominantly during S phase, and its rate of increase per cell accelerated during the second half of this phase. The accumulation of Ki-67 antigen during S exceeded the increase in DNA content, and thus the Ki-67/DNA ratio rose 80% from late G1 to G2 + M. This antigen rapidly disappeared from post-mitotic cells. The half-life of this protein estimated in post-mitotic cells during stathmokinesis induced by vinblastine appeared to be shorter than 1 h. This rapid turnover should be compared with the relatively long (6-8 h) duration of G1 of the studied cells. In cells in which de novo protein synthesis was inhibited by 0.1 microgram/ml cycloheximide, the half-life of the Ki-67 antigen was also found to be about 1 h regardless of the cell position in the cell cycle. Thus, the data suggest that variations in the level of this protein during the cell cycle are a consequence of its different synthesis rate rather than phase-specific changes in the rate of its degradation. Because the late G1 and very early S phase cells express the antigen at levels only slightly above background, it is possible that, when using Ki-67 antibody as a marker of the cell growth fraction, some late G1 cells can be erroneously classified as non-cycling cells.