Peptides from the amino terminal mdm-2-binding domain of p53, designed from conformational analysis, are selectively cytotoxic to transformed cells

We have synthesized three peptides from the mdm-2 binding domain of human p53, residues 12-26 (PPLSQETFSDLWKLL), residues 12-20, and 17-26. To enable transport of the peptides across the cell membrane and at the same time to maximize the active mdm-2 binding alpha-helical conformation for these peptides, each was attached at its carboxyl terminus to the penetratin sequence, KKWKMRRNQFWVKVQRG, that contains many positively charged residues that stabilize an alpha-helix when present on its carboxyl terminal end. All three peptides were cytotoxic to human cancer cells in culture, whereas a control, unrelated peptide attached to the same penetratin sequence had no effect on these cell lines. The same three cytotoxic peptides had no effect on the growth of normal cells, including human cord blood-derived stem cells. These peptides were as effective in causing cell death in p53-null cancer cells as in those having mutant or normal p53. Peptide-induced cell death is not accompanied by expression of apoptosis-associated proteins such as Bax and waf(p21). Based on these findings, we conclude that the antiproliferative effects of these p53-derived peptides are not completely dependent on p53 activity and may prove useful as general anticancer agents.

Differences in patterns of activation of MAP kinases induced by oncogenic ras-p21 and insulin in oocytes

Oncogenic ras (Val 12-containing)-p21 protein induces oocyte maturation by a pathway that is blocked by peptides from effector domains of ras-p21, i.e., residues 35-47 (that block Val 12-p21-activated raf) and 96-110 and 115-126, which do not affect the ability of insulin-activated cellular p21 to induce maturation. Oncogenic p21 binds directly to jun-N-terminal kinase (JNK), which is blocked by the p21 96-110 and 115-126 peptides. This finding predicts that oncogenic p21, but not insulin, induces maturation by early and sustained activation of JNK. We now directly confirm this prediction by showing that oncogenic p21 induces activating phosphorylation of JNK (JNK-P) and of ERK (MAP kinase) (MAPK-P), whose levels correlate with oocyte maturation. p21 peptides 35-47 and 96-110 block formation of JNK-P and MAPK-P, further confirming this correlation and suggesting, unexpectedly, that raf-MEK-MAPK and JNK-jun pathways strongly interact on the oncogenic p21 pathway. In contrast, insulin activates only low levels of JNK-P, and, surprisingly, we find that insulin induces only low levels of MAPK-P, indicating that insulin and activated normal p21 utilize MAP kinase-independent signal transduction pathways.

Plasmid expression of a peptide that selectively blocks oncogenic ras-p21-induced oocyte maturation

PURPOSE

We have previously found that a synthetic peptide corresponding to ras-p21 residues 96 110 (PNC2) selectively blocks oncogenic (Val 12-containing) ras-p21 protein-induced oocyte maturation. With a view to introducing this peptide into ras-transformed human cells to inhibit their proliferation, we synthesized an inducible plasmid that expressed this peptide sequence. Our purpose was to test this expression system in oocytes to determine if it was capable of causing selective inhibition of oncogenic ras-p21.

METHODS

We injected this plasmid and a plasmid expressing a control peptide into oocytes either together with oncogenic p21 or in the presence of insulin (that induces maturation that is dependent on normal cellular ras-p21) in the presence and absence of the inducer isopropylthioglucose (IPTG).

RESULTS

Microinjection of this plasmid into oocytes together with Val 12-p21 resulted in complete inhibition of maturation in the presence of inducer. Another plasmid encoding the sequence for the unrelated control peptide, X13, was unable to inhibit Val 12-p21-induced maturation. In contrast, PNC2 plasmid had no effect on the ability of insulin-activated normal cellular or wild-type ras-p21 to induce oocyte maturation, suggesting that it is selective for blocking the mitogenic effects of oncogenic (Val 12) ras p21.

CONCLUSION

We conclude that the PNC2 plasmid selectively inhibits oncogenic ras-p21 and may therefore be highly effective in blocking proliferation of ras-induced cancer cells. Also, from the patterns of inhibition, by PNC2 and other ras- and raf-related peptides, of raf- and constitutively activated MEK-induced maturation, we conclude that PNC2 peptide inhibits oncogenic ras p21 downstream of raf.

Cell cycle arrest and morphological alterations following microinjection of NIH3T3 cells with Pur alpha

Levels of Pur alpha, a protein implicated in control of both DNA replication and gene transcription, fluctuate during the cell cycle, being lowest in early S phase and highest just after mitosis. Here we have employed a new video time-lapse technique enabling us to determine the cell cycle position of each cell in an asynchronous culture at a given time and to ask whether introduction of Pur alpha protein at specific times can affect cell cycle progression. Approximately 80% of all NIH3T3 cells injected with Pur alpha were inhibited from passing through mitosis. Cells injected with Pur alpha during S or G2 phases were efficiently blocked with a 4N (G2 phase) DNA level, as determined by quantitative DNA photometry of individual cells. Of the cells injected with Pur alpha during G1 phase, 40% experienced a rapid cell death characterized by extreme cellular fragmentation. Of those G1 injected cells which remained viable, approximately equal numbers were arrested with either 2N or 4N DNA levels. Cells arrested by Pur alpha in G2 phase grew to cover a large surface area. These results link fluctuations in Pur alpha levels to aspects of cell cycle control.

Alterations in Pur(alpha) levels and intracellular localization in the CV-1 cell cycle

Levels of the single-stranded DNA-binding protein Pur(alpha), previously implicated in control of both DNA replication and gene transcription, are altered during the CV-1 cell cycle. Just prior to the onset of S phase, Pur(alpha) levels drop precipitously, after which they recover nearly 8-fold throughout S and G2 to peak just after mitosis. As observed previously, Pur(alpha) binds the hypophosphorylated form of the retinoblastoma protein, Rb. Coimmunoprecipitation of Pur(alpha) and Rb reveals that the complex declines as cells enter S phase and does not reform as Pur(alpha) levels recover in S and G2. As Pur(alpha) levels recover, the protein is localized to nuclear foci containing newly replicated DNA, as determined by immunoelectron microscopy using different sized gold beads and antibodies against Pur(alpha) and bromodeoxyuridine-labeled DNA. These foci also contain cyclin A, and Pur(alpha) coimmunoprecipitates with cyclin A from extracts of cells in S and G2 phases. Pur(alpha) remains with these foci throughout G2, after the bulk of DNA synthesis has ceased. Changing levels of Pur(alpha) may affect Pur(alpha) functions at the onset of S phase and during progression to mitosis.

Association of human Pur alpha with the retinoblastoma protein, Rb, regulates binding to the single-stranded DNA Pur alpha recognition element

The retinoblastoma protein, Rb, is detected in extracts of monkey CV-1 cells complexed with Pur alpha, a sequence-specific single-stranded DNA-binding protein implicated in control of gene transcription and DNA replication. These complexes can be immunoextracted from cell lysates using monoclonal antibodies to either Pur alpha or Rb. The Pur alpha-Rb complexes contain a form of Pur alpha with extensive post-synthetic modification, as demonstrated following expression of Pur alpha cDNA fused to a 9-amino acid epitope tag. Human Pur alpha, expressed as a glutathione S-transferase fusion protein, specifically binds to the hypophosphorylated form of Rb with an affinity as high as that of SV40 large T-antigen. In the absence of DNA, glutathione S-transferase-Pur alpha binds to p56RB, an NH2-terminal-truncated Rb protein purified from Escherichia coli, containing the T-antigen binding domain, to form multimeric complexes. The single-stranded DNA Pur alpha recognition element disrupts these complexes. Conversely, high concentrations of p56RB prevent Pur alpha binding to DNA. Through use of a series of deletion mutants, the DNA binding activity of Pur alpha is localized to a series of modular amino acid repeats. Rb binding involves a Pur alpha region with limited homology to the Rb-binding region of SV40 large T-antigen. Binding of Pur alpha to p56RB, the COOH-terminal portion of Rb, is inhibited by a synthetic peptide containing the T-antigen Rb-binding motif.