A Glimpse into the Vital Core of Tumor Cell Development

By John Leavitt, Ph.D. Nerac Analyst

Originally published August 12, 2014

In 1987, my colleagues at Stanford and I published a paper that clearly demonstrated that expression of a charge-altered mutant human beta-actin (glycine to aspartic acid substitution at amino acid 245; G245D) caused non-tumorigenic, immortalized human fibroblasts to form aggressive tumors in nude mice (Leavitt et al, 1987a). When these tumor- derived cells were examined, we discovered that they exhibited further elevated expression of the mutant beta-actin and these tumor-derived cells formed tumors even more rapidly – observations that were consistent with the role of this mutation in the tumorigenic phenotype. Furthermore, over-expression of mutant beta-actin was associated with down-regulation of three abundant tropomyosin isoforms in a well-documented transformation-sensitive manner (Leavitt et al, 1986; Leavitt et al, 1987a and Ng et al, 1988).

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Comments

  1. Carlos Longo says

    Dear Mr. Leavitt,
    I have read with much interest your Nerac White Paper. “A Glimpse into the Vital Core of Tumor Cell Development”.

    I would like to make some comments:
    – Much work has been done on the biochemical, chemical and genetic basis of Cancer. But it has been more neglected other aspects apparently unrelated, as Mechanical/Physical, for example of the only-mechanical-and-structural, uninteresting protein Actin. However, small changes in the behavior of Actin can lead successively, as you found, to:
    o changes in the cytoskeleton
    o protrusion of ruffles in lamellipodia
    o more invasive cells
    o metastasis
    o tumorigenicity
    – So, an apparently trivial mutation in a structural, irrelevant protein lead to tumorigenesis! That is why they is so well evolutionaryly preserved.
    – The importance of Actin: Actin is a molecule that polymerizes, head-to-tail, to form filaments in the Cell that define the scaffold of the cell.
    – Thus, the relevance of Mechanical cues: forces, rigidity, etc. that can shape the development and behavior of cells. In opposition to Chemical cues: growth factors, proteins, genes…
    o Also to remark the book “On Growth and Form”, classical work from Thomas d’Arcy Thompson where he underlines the mathematical, geometrical, shape, and mechanics that model the apparently unrelated realm of living things.
    o More recent reviews:
    o Hoffman BD, Crocker JC. Cell mechanics: dissecting the physical responses of cells to force. Annu Rev Biomed Eng 2009; 11: 259-88. Discusses the Tensegrity model of Actin filaments.
    o Schoen I, Pruitt BL, Vigel V. The Yin-Yang of rigidity sensing: How forces and mechanical properties regulate the cellular response to materials. Annu Rev Mater Res 2013; 43: 589–618.
    o Engler AJ, Sen S, Sweeney HL, Discher DE. 2006. Matrix elasticity directs stem cell lineage specification. Cell 126:677–89. Uses hydrogels to mimic the native rigidities of different tissues and shows that culturing mesenchymal stem cells on these substrates leads to neurogenic, myogenic, or osteogenic differentiation, revealing that environmental rigidity is a major (co)determinant of stem cell fate.

    Thank you, with best regards,
    Carlos Longo
    Patents & Regulatory Affairs Division

    longoc@viscofan.com
    Mobile: +34 630 764 188
    Tel: +34 948 198 444

  2. says

    Thank you, Carlos, for your interesting comment to my article.

    No doubt the protein “actin” was thought of by most molecular and cellular biologists back then as an uninteresting protein largely because of its abundance. In your area, collagen is thought of primarily as an uninteresting space filling protein of connective tissues in spite of the fact that its excessive deposition causes deadly fibrotic diseases of the lungs, liver, and cardiovascular system. In fact at the time most researchers thought of the actin protein as a molecular weight marker in a one-dimensional polyarcylamide gel at 42 kilodaltons (375 amino acids); and the beta-actin messenger RNA (about 1.8 kilobases) is routinely used as a size marker for other messenger RNAs in northern blots because its steady-state levels are constant and it’s always there.

    When I discovered the beta-actin mutation in 1978 many biologists were surprised that actin, known primarily as a contractile muscle protein with myosin, was present in non-muscle cells. In the same year that we published the discovery of the mutant human beta-actin, I published another paper on abnormal expression of beta-actin in a well-known human T-cell leukemia called Molt-4. Thus, we had no difficulty finding actin anomalies in other human tumor types. As I described, it took Klaus Weber (Max-Planck Institute) two weeks by regular mail to tell me that his lab would be willing to commit to sequencing the mutant human actin protein. Klaus Weber understood the importance of the two cytoplasmic actins because his lab had studied and sequenced the four muscle forms of actin. That he had chosen to study the muscle actins over the cytoplasmic non-muscle actins demonstrated the bias of cell biologists at the time.

    Since I was working in the field of cancer, I was more interested in the dynamics of uncontrolled cell division and how actin and this mutation factored into that. We now know that there are profound changes in actin organization and dynamics through the proteins that regulate actin organization.

    After I made this discovery and with Klaus’ help, I presented my work to Linus Pauling (two-time Nobel Prize winner) at his Institute in Palo Alto. He had predicted the molecular nature of sickle cell disease (a structural mutation in hemoglobin) before the genetic code and structure of DNA was known. I can remember Pauling sitting in the front row of the seminar room with a large smile on his face. What I was describing was a lot like his discovery of the hemoglobin mutation. When I joined the Pauling Institute leaving my tenured position at NIH, I quickly persuaded NIH and the American Cancer Society to fund the research which would further examine the relationship of this mutation and an actin bundling protein which I named “plastin” in cancer. The head of the NIH study section that funded this research, Tom Maniatis (Harvard), championed the grant overcoming the doubt of Richard Axel (Columbia) who believed that only “oncogenes” could contribute to development of cancer. Robert Weinberg (MIT) who discovered the first oncogene, ras, had the same point-of-view. Now the term “oncogene” is rarely used because of its diluted meaning.

    So the architectural principle “structure determines function” applies to human cellular architecture as well whether we are talking about sickle cell anemia or cancer.