Posts Tagged ‘anticancer resistance’

Drug-resistance Mechanism in Tumor Cells

Curator: Larry H. Bernstein, MD, FCAP

Targeting the RNA-binding protein that promotes resistance could lead to better cancer therapies.

Anne Trafton | MIT News Office

P53, which helps healthy cells prevent genetic mutations, is missing from about half of all tumors. Researchers have found that a backup system takes over when p53 is disabled and encourages cancer cells to continue dividing. In the background of this illustration are crystal structures of p53 DNA-binding domains.

About half of all tumors are missing a gene called p53, which helps healthy cells prevent genetic mutations. Many of these tumors develop resistance to chemotherapy drugs that kill cells by damaging their DNA.

MIT cancer biologists have now discovered how this happens: A backup system that takes over when p53 is disabled encourages cancer cells to continue dividing even when they have suffered extensive DNA damage. The researchers also discovered that an RNA-binding protein called hnRNPA0 is a key player in this pathway.

“I would argue that this particular RNA-binding protein is really what makes tumor cells resistant to being killed by chemotherapy when p53 is not around,” says Michael Yaffe, the David H. Koch Professor in Science, a member of the Koch Institute for Integrative Cancer Research, and the senior author of the study, which appears in the Oct. 22 issue of Cancer Cell.

The findings suggest that shutting off this backup system could make p53-deficient tumors much more susceptible to chemotherapy. It may also be possible to predict which patients are most likely to benefit from chemotherapy and which will not, by measuring how active this system is in patients’ tumors.

Rewired for resistance

In healthy cells, p53 oversees the cell division process, halting division if necessary to repair damaged DNA. If the damage is too great, p53 induces the cell to undergo programmed cell death.

In many cancer cells, if p53 is lost, cells undergo a rewiring process in which a backup system, known as the MK2 pathway, takes over part of p53’s function. The MK2 pathway allows cells to repair DNA damage and continue dividing, but does not force cells to undergo cell suicide if the damage is too great. This allows cancer cells to continue growing unchecked after chemotherapy treatment.

“It only rescues the bad parts of p53’s function, but it doesn’t rescue the part of p53’s function that you would want, which is killing the tumor cells,” says Yaffe, who first discovered this backup system in 2013.

In the new study, the researchers delved further into the pathway and found that the MK2 protein exerts control by activating the hnRNPA0 RNA-binding protein.

RNA-binding proteins are proteins that bind to RNA and help control many aspects of gene expression. For example, some RNA-binding proteins bind to messenger RNA (mRNA), which carries genetic information copied from DNA. This binding stabilizes the mRNA and helps it stick around longer so the protein it codes for will be produced in larger quantities.

“RNA-binding proteins, as a class, are becoming more appreciated as something that’s important for response to cancer therapy. But the mechanistic details of how those function at the molecular level are not known at all, apart from this one,” says Ian Cannell, a research scientist at the Koch Institute and the lead author of the Cancer Cell paper.

In this paper, Cannell found that hnRNPA0 takes charge at two different checkpoints in the cell division process. In healthy cells, these checkpoints allow the cell to pause to repair genetic abnormalities that may have been introduced during the copying of chromosomes.

One of these checkpoints, known as G2/M, is controlled by a protein called Gadd45, which is normally activated by p53. In lung cancer cells without p53, hnRNPA0 stabilizes mRNA coding for Gadd45. At another checkpoint called G1/S, p53 normally turns on a protein called p21. When p53 is missing, hnRNPA0 stabilizes mRNA for a protein called p27, a backup to p21. Together, Gadd45 and p27 help cancer cells to pause the cell cycle and repair DNA so they can continue dividing.

Personalized medicine

The researchers also found that measuring the levels of mRNA for Gadd45 and p27 could help predict patients’ response to chemotherapy. In a clinical trial of patients with stage 2 lung tumors, they found that patients who responded best had low levels of both of those mRNAs. Those with high levels did not benefit from chemotherapy.

“You could measure the RNAs that this pathway controls, in patient samples, and use that as a surrogate for the presence or absence of this pathway,” Yaffe says. “In this trial, it was very good at predicting which patients responded to chemotherapy and which patients didn’t.”

“The most exciting thing about this study is that it not only fills in gaps in our understanding of how p53-deficient lung cancer cells become resistant to chemotherapy, it also identifies actionable events to target and could help us to identify which patients will respond best to cisplatin, which is a very toxic and harsh drug,” says Daniel Durocher, a senior investigator at the Samuel Lunenfeld Research Institute of Mount Sinai Hospital in Toronto, who was not part of the research team.

The MK2 pathway could also be a good target for new drugs that could make tumors more susceptible to DNA-damaging chemotherapy drugs. Yaffe’s lab is now testing potential drugs in mice, including nanoparticle-based sponges that would soak up all of the RNA binding protein so it could no longer promote cell survival.

Aurelian Udristioiu comment:

A mutation causes the p53 gene to lose any of its functions anfd this will inevitably lead to carcinogenesis by letting the cell grow indefinitely, without any regulation. The p53 gene has been mapped in chromosome 17. In the cell, p53 protein binds DNA, stimulating another gene to produce the protein p21 that interact with cycle cell in division, stimulating a protein of stop division (cdk2).
When p21 forms a complex with cdk2, the cell cannot pass through to the next stage of cell division, and remains arrested in G1.
The p53 protein product of a TP53 mutant gene cannot bind damaged DNA in an effective way, and as a consequence, the p21 protein is not made available to act as the stop signal for the cell cycle/cell division. Therefore, cells divide uncontrollably and form tumors.
Not surprisingly, there is an increased frequency in the amplification of the ubiquitin ligases protein (MDM2) involved in the mechanism for the down regulation of p53 activity through ubiquitin-dependent proteosomal degradation of p53.
P53 has been shown to promote hematopietic stem cells (HSCs) quiescence and self-renewal with recent studies showing that deficiency of p53 likely promotes acute myeloid leukemia (AML) by eliminating its ability to limit aberrant self-renewal in hematopoietic progenitors.
Was demonstrate that the human p53 promoter is trans-activated by high c-Myc expression and repressed by high max-expression. In examining the relative levels of c-Myc and p53 in human Burkitt’s lymphomas and otherB-lymphoid lines, were found that there is a correlation between the levels of c-Myc protein and p53 mRNA expression.
In particular, the cells that express very low levels of c-Myc protein also express low levels of p53mRNA, while the cells that express high levels of c-Myc tend to express high levels of p53 mRNA .
[] Roy B, Beamon J, Balint E, Reisman D. Transactivation of the Human p53 Tumor Suppressor Gene by c-Myc/Max Contributes to Elevated Mutant p53 Expression in Some Tumors. Molecular, 2010 ].
In experimental models, disrupting the MDM2–p53 interaction restored p53 function and sensitized tumors to chemotherapy or radiotherapy.
This strategy could be particularly beneficial in treating cancers that do not harbor TP53 mutations. For example in hematologic malignancies, such as multiple myeloma, chronic lymphocytic leukemia (CLL), acute lymphoblastic leukemia (ALL), acute myeloid leukemia (AML), and Hodgkin’s disease, the induction of p53 – using a small MDM2-inhibitor molecule, nutlin-3 – can induce the apoptosis of malignant cells.
Nutlins are a group of cis-imidazoline analogs, first identified by Vassilev et al [2004], which have a high binding potency and selectivity for MDM2. Crystallization data have shown that nutlin-3 mimics the three residues of the helical region of the trans-activation domain of p53 (Phe19, Trp23 and Leu26), which are conserved across species and critical for binding to MDM2.
Nutlin-3 displaces p53 by competing for MDM2 binding. It has also been found that nutlin-3 potently induces apoptosis in cell lines derived from hematologic malignancies, including AML, myeloma, ALL, and
B-cell CLL. [Secchiero P, Voltan R, Iasio GM, Melloni . The oncogene DEK promotes leukemic cell survival and is down regulated by both Nutlin-3 and chlorambucil in B-chronic lymphocytic leukemic cells. Clin Cancer Res 2010; 16: 1824–1833].
A large cohort study of primary CLL, done on over 100 patients, examined the samples from the patients for a response to MDM2 inhibition. The study found direct correlation between wild-type TP53 status and MDM2 inhibitor-induced (nutlin-3 and MI-219) cytotoxicity across various CLL subtypes.
However, a large number of patients with cancer did produce p53-reactive T cells [Van der Burg SH, Cock K, Menon AG, Franken KL. Long lasting p53-specific T cell memory responses in the absence of anti-p53 antibodies in patients with respected primary colorectal cancer. Eur. J. Immunol 2001; 31: 146–155].
The results from these studies served as a good reason to attempt the vaccination of patients using p53-derived peptides, and a several clinical trials are currently in progress. The most advanced work used a long synthetic peptide mixture derived from p53 (p53-SLP; ISA Pharmaceuticals, Bilthoven, the Netherlands].
The vaccine is delivered in the adjuvant setting and induces T helper type cells. However, the response may not be potent enough to result in clinical benefit as a mono-therapy: This indicated that these p53-specific T-helper responses are not polarized. Therefore, approaches are being investigated to promote a stronger and more correctly polarized response using new clinicla trials.


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