The viral Ki-ras gene must be expressed in the G2 phase if ts Kirsten sarcoma virus-infected NRK cells are to proliferate in serum-free medium

Oncogenic H-ras induces cyclin B1 expression in a p53-independent manner

The role of p53 in controlling the G2 checkpoint, in part by repressing cyclin B1 transcription, has been well established. However, accumulating evidence indicate that p53-independent pathways may also play an important role. Ras proteins have been shown to regulate G1/S, but also G2/M transitions. Since cyclin B1/cdc2 complex is the key regulator controlling the G2/M checkpoint, we were interested in addressing if the H-ras oncogene could regulate cyclin B1 expression in a p53-independent manner. We observed an induction of cyclin B1 promoter activity in the presence of H-ras oncogene in SW480 cells, which contain null p53 alleles. In addition, HeLa cells known to express the HPV18 E6 oncogene that inactivates p53, exhibited increased levels of cyclin B1 mRNA and protein when transfected with the H-ras oncogene. Higher expression of cyclin B1 correlated with higher levels of cyclin B1/cdc2 complex and kinase activity that interestingly, showed no inhibition at G2/M after DNA damage. These data suggest that H-ras participates in pathways that regulate cyclin B1 expression and therefore controls the G2/M checkpoint in a p53-independent manner.

Induced mitotic death of HeLa cells by abnormal expression of c-H-ras

When HeLa cells were selected for stable expression of a neo gene, linked either to mutated or wt c-H-ras genes, morphological examination of selected clones from several experiments revealed formation of giant multinucleated cells. These morphological alterations culminate in cell death, as a consequence of mitotic catastrophe (or mitotic death). Although clones expressing the mutated gene produced significantly larger numbers of these giant cells, those transfected with the normal allele were also found to produce significantly more giant multinucleated cells than non-transfected HeLa cells. Northern blot analysis of mRNA revealed overexpression of the normal H-ras gene in these clones. Chromatin structure analysis of these clones showed gross alterations, including the presence of micronuclei and heteroploid nuclei. Interestingly, odd numbers of nuclei were found in colonies of these giant cells. In addition, alterations in cell cycle parameters were observed, including the appearance of a subpopulation of cells with an abnormal content of DNA, probably representing dying cells. Our data support the notion that abnormal expression of H-ras contributes to mitotic catastrophe and death of a subpopulation of HeLa cells.

Src and the control of cell division

The finely tuned mechanisms that control cell cycle progression go awry in cancer, pointing to proto-oncogene products as important players in cell-cycle regulation. One such proto-oncoprotein, c-Src, has previously been directly implicated, based on its requirement for growth factor-stimulated DNA synthesis. Roche et al. have now shown that c-Src or its close relatives are also required for cell division to occur. The demonstration of essential functions for the Src family at multiple points in the cell cycle raises important questions about the normal and transforming activities of these and other proto-oncoproteins.

tsLA23-NRK cells need pp60v-src protein-tyrosine kinase activity in G2 phase to initiate mitosis in serum-free medium

Lowering the temperature from 41 to 36 degrees C stimulates quiescent tsLA23-NRK rat cells (infected with the tsLA23 mutant of the Rous sarcoma virus) in serum-free medium to resume cycling and initiate DNA replication by reactivating the tsLA23-RSV's abnormally thermolabile pp60v-src protein-tyrosine kinase. Inactivating the enzyme in these pp60v-src-stimulated cells by again raising the temperature to 41 degrees C after the cells had initiated DNA replication did not prevent the completion of DNA replication and entry into the G2 phase, but it stopped the initiation of mitosis. Adding serum at the time of the temperature increase replaced the lost pp60v-src activity and the cells were able to continue to mitosis. The G2-arrested cells at 41 degrees C were able to initiate mitosis when pp60v-src was reactivated again by lowering the temperature to 36 degrees C. These observations suggest that protein-tyrosine kinase activity is needed to initiate mitosis and that the tsLA23-NRK cell is a good model for studying the function of this kinase activity in the initiation of mitosis.

Some ras-transformed cells have increased radiosensitivity and decreased repair of sublethal radiation damage

We examined the effect of 60Co irradiation on the clonogenic survival of rat NRK cells, NRK cells carrying a temperature-sensitive viral K-ras oncogene (tsK-NRK), mouse NIH 3T3 cells, and NIH 3T3 cells transformed with the human bladder cancer (T24) H-ras oncogene (PAP2). We tested the hypothesis that ras oncogene expression renders cells more resistant to radiation, but found in both systems that ras-transformed cells were more, not less, sensitive to radiation. We also found indications of altered repair of sublethal radiation damage. PAP2 cells were more sensitive to radiation than NIH 3T3 cells. Increased sensitivity was reflected in a decreased shoulder region of the survival curve with little effect on its slope (D0). TsK-NRK cells were also slightly more sensitive to radiation than NRK and exhibited decreased repair of sublethal damage at both the permissive and nonpermissive temperatures. Thus, we found that expression of ras oncogenes is not always associated with increased radiation resistance. In summary, our results suggest that (1) ras oncogene expression in some cells may be associated with increased, rather than decreased, radiation sensitivity, and (2) ras oncogene expression may alter the shoulder region of the dose response curve, suggesting changes in the repair of sublethal radiation damage.

Membrane protein kinase C activity rapidly increases in quiescent tsRSV-infected NRK cells upon reactivation of the mitogenic v-src protein kinase

The viral src protein kinase, pp60v-src, is a powerful intracellular mitogen which can initiate and maintain the proliferation of quiescent cells in the absence of any exogenous growth factors. In an attempt to understand how pp60v-src induces proliferation, we examined the early events in the G0 to G1 transition caused by the activation of a thermolabile v-src protein in quiescent, serum-starved tsRSV-transformed NRK cells. The reactivation of pp60v-src, in the absence of exogenous growth factors, triggered a rapid biphasic surge of membrane-associated protein kinase C (PKC) activity. Unlike TPA-stimulated PKC activity, the pp60v-src-induced increase in PKC was readily extracted from membranes by EGTA. The down-regulation of PKC activity in these quiescent cells by prolonged exposure to TPA strongly inhibited the ability of the reactivated v-src protein to stimulate DNA replication in serum-deficient medium, suggesting that PKC plays a role in the initial signal by which the viral enzyme induces the G0 to G1 transition in NRK cells.

The role of signal-transducing events in the proliferative response of cells to a mitogenic viral K-ras protein

Activated oncogenic ras proteins are powerful mitogenic agents which by themselves can initiate and maintain the proliferation of quiescent cells in the absence of any exogenous growth factors. In an attempt to understand how ras proteins induce proliferation we examined the early events in the G0 to G1 transition caused by the activation of a thermolabile K-ras protein in quiescent, serum-starved tsKSV-transformed NRK cells. We show that ras reactivation, in the absence of exogenous growth factors, triggered a rapid surge in free cytosolic Ca2+ and diacylglycerol production, which led to a transient increase in membrane-associated protein kinase C (PKC) activity which was necessary for G1 transit. Unlike TPA-stimulated PKC activity, the ras-induced increase in PKC was readily extracted from membranes by EGTA. These signal transducing events occurred despite the fact that ras activation did not induce the tyrosine phosphorylation of any known surface receptor. The results indicate that the K-ras protein triggers the G0 to G1 transition by an intracellular mechanism and not indirectly via autocrine stimulation.

Cell cycle dependence of inositol phosphate levels in neuroblastoma cells

To investigate the timing of inositol lipid turnover in relation to the cell cycle, inositol phosphates and lipids were measured in neuroblastoma (Neuro-2A) cells that were prelabeled with [3H]inositol and synchronized by a mitotic shakeoff technique. Distinct early and late phases of inositol phosphate production were identified. The early peak occurs between the 2nd and 4th hour after mitosis near the G1/S transition. A later peak occurs around the peak of S phase (DNA synthesis) at 7-8 h after mitosis. These findings suggest that activation of phosphatidylinositol turnover generates signals that play a role in cell cycle progression.

Evidence that the viral Ki-ras protein, but not the pp60v-src protein of ASV, stimulates proliferation through the PDGF receptor

Protamine sulfate (PS), a specific blocker of PDGF action, inhibited the proliferative response of tsKSV-NRK cells to a reactivated, temperature-sensitive viral Ki-RAS protein, but it did not affect the proliferative response of tsASV-NRK cells to a reactivated pp60v-src protein kinase. The inhibition by PS of the proliferation response of tsKSV-NRK cells to reactivated Ki-RAS protein was overcome by serum growth factors, notably EGF, and concentrated serum-free conditioned medium from cultured NRK cells infected with wild-type KSV, but not by a combination of PDGF and insulin. These observations suggest that the viral Ki-RAS protein, but not pp60v-src, stimulates proliferation exclusively by inducing the host cells to produce PDGF or PDGF-like mitogenic factors.

Calcium, cyclic AMP and protein kinase C--partners in mitogenesis

Evidence is steadily mounting that the proto-oncogenes, whose products organize and start the programs that drive normal eukaryotic cells through their chromosome replication/mitosis cycles, are transiently stimulated by sequential signals from a multi-purpose, receptor-operated mechanism (consisting of internal surges of Ca2+ and bursts of protein kinase C activity resulting from phosphatidylinositol 4,5-bisphosphate breakdown and the opening of membrane Ca2+ channels induced by receptor-associated tyrosine-protein kinase activity) and bursts of cyclic AMP-dependent kinase activity. The bypassing or subversion of the receptor-operated Ca2+/phospholipid breakdown/protein kinase C signalling mechanism is probably the basis of the freeing of cell proliferation from external controls that characterizes all neoplastic transformations.