Download Carcinogenesis : Are the toxicity models correct

Carcinogenesis: Are the toxicity models correct?
Comments by Kenneth L. Campbell, Department of Biology, University of
Massachusetts Boston
The Traditional Carcinogenesis Model:
The definitions given by Casarett & Doull (Pitot III & Dragan, Chapter 8: Chemical
Carcinogenesis, In: Klaassen, Ed., Casarett & Doull’s Toxicology: The Basic Science of
Poisons, 6th Ed., McGraw-Hill: New York, NY, 2001, 241-319) tend to rule out
endogenous hormones as being classed as carcinogens unless they are exogenously
administered. But what about endogenous overproduction of trophic or growth
factors? Or, incidental exposures through food and the environment?
The toxicology text provides a disappointingly naive discussion of endocrine systems
given the similarities of toxicology, pharmacology, and endocrinology with respect to
methods and many of the central concepts concerning mechanisms of action,
chemical kinetics, clearance, and general physiological function.
The current carcinogenesis model looks at three stages in the "natural history of
neoplasia:"
1. Initiation (primary molecular lesion formation, adducts, mutation; reversible)
2. Promotion (enhanced ability to escape normal controls for removing adducts or
mutations and to allow abnormal cell proliferation; may still be reversible)
3. Progression (overt cell transformation, nuclear instability coupled with loss of
mitotic controls and metastatic behavior; irreversible)
A chemical may play a role in the actual induction of mutation or the initial stages of
neoplastic transformation in the first cell involved. It may play a role in preserving that
cell and making certain it can continue to divide. Or, it may play a role in allowing the
initial cell mass to invade other tissue and/or to metastasize to other sites. If the
model is correct there should be examples of chemicals among the thousands
examined that can be identified with each of these steps. Yet there are few, if any, that
can unequivocally be assigned to initiation and progression. This would suggest there
may be a problem with the specification or design of the current model. Or, it may
simply be a reflection of our current ignorance about the mechanisms by which many
of these chemicals exert their toxic effects.
Why screen breast cancers for estrogen sensitivity if hormones are not directly
involved in initiation of neoplastic growth as suggested by Pitot III & Dragan?
Breast tumors are screened for the presence of estrogen receptors because estrogen
acts through such receptors to promote expression of those genes that lead to cellular
replication via mitosis. Estrogen is pro-mitotic in many of the tissues in which it
normally acts. Thus, knowing if a tumor is sensitive to estrogen can provide a means
to suppress tumor growth. Aromatase inhibitors can be given to suppress ovarian (if a
pre-menopausal woman is involved) or peripheral production of estrogens. This will
often slow the tumor growth or its production and response to paracrine growth
factors like EGF, or TGF. In turn, this provides more opportunity to apply radiation,
surgery, and/or directed chemotherapy agents to ablate the tumor. Obviously, if the
tumor does not have estrogen receptors, it will not respond to such treatments.
Likewise, testosterone or dihydrotestosterone are pro-mitotic in male secondary
structures like the prostate. Here anti-androgens can be applied to good effect so
screening could be used for some tumors of the prostate. (Normally slow progression
of these growths and the lack of ready anatomic access to the prostate tends to make
this a less frequently used approach.)
Pitot III & Dragan point to the actions of growth hormone and other growth factors and
suggest these may be considered chemical carcinogens. This is contrary to the
definition of toxicants (toxic substances produced by anthropogenic actions or natural
forces, e.g., fire) and really only fits the notion of toxins (toxic substances produced by
biological systems) except that both definitions refer to substances administered or
derived from exogenous sources. Certainly hormones are not exogenous. Their
actions at the high levels characteristic of endocrinopathies may eventuate in
neoplastic conditions. Whether they are truly the initiators of these neoplasias can
probably be questioned (environmental factors, exogenous radiation, etc.). Model
systems such as knockout or overexpression models do, however, suggest that
hormones can, if not initiating cancers, at least act as sensitizing or predisposing
agents. Such is probably the case for the elevation of dimethylbenz[a]anthracene
(DMBA) tumor formation seen in rats expressing high levels of prolactin. Both
estrogens and prolactin are mitogens in rat breast tissue. Their actions will increase
the number of cells undergoing division in any given time. If DMBA is applied
simultaneously, or subsequent to this action, the intercalating agent DMBA will have
additional opportunities to disrupt the crucial machinery involved in DNA synthesis
and repair associated with mitosis. That does not make estrogen or prolactin cocarcinogens, but it does mean that they will predispose responsive tissues to agents
that interact with DNA. The same situation will also apply to dimethylnitrosourea
application to prostate tissue simultaneous with or subsequent to promitotic
androgens such as testosterone. In fact, it should apply in any situation where a
hormone alone stimulates a tissue to increase mitotic activity.
Note the dates of the references in Pitot III & Dragan’s Table 8-2 concerning the
development of tumors subsequent to disruption of normal hormonal feedback cycles
(publications from 1944, 1975, 1980, 1989, 1991, and 1994). Certainly there has been
work done in these areas over the past decade that may be pertinent to these
discussions. Hormones and their control circuits are natural, homeostatic
mechanisms. They are only carcinogenic if the stimulation of cell division processes,
especially mitosis, becomes disrupted due to high rates of division coupled with
failures of checkpoints or apoptotic controls and immune surveillance that normally
remove aberrant cells. We think of carcinogens as inducers of transformation/
mistakes/ mutations. Hormones don't do that, thus, they are probably best classed as
promoters, or possibly supporters of progression (including suppression of apoptosis,
stimulation of angiogenesis, and/or suppression of immune surveillance).
Alternative Views of Carcinogenesis:
The text authors Pitot III & Dragan also seem to hold a somewhat dated understanding
of how hormones and genes play roles in neoplastic and cancer development and
growth. I would definitely refer you to the publications of Drs. Robert Weinberg of the
Whitehead Institute at MIT and Polly Matzinger of NIH with respect to critical factors
involved in the processes of cellular transformation and immune surveillance
associated with cellular transformation, respectively (see below).
Weinberg, in a lecture to the Endocrine Society in Denver in 2001, mentioned at least
four cellular features that had to be altered before a cell could undergo transformation
from a normal to a cancerous cell. First, mutations or alterations had to occur in one
or both the the cellular mitotic checkpoint proteins, pRb, and p53. Second, a change
in expression of proteins involved in the apoptotic pathways had to occur so that this
process was either not initiated or was actively blocked. Third, telomerase had to be
expressed so that the cell could overcome the limitation on division normally
encountered due to shortening of the telomeres in each round of cellular division.
Normal differentiated cells express little or no telomerase. Transformed cells express
significant levels. Finally, to actually form a tumor, the transformed cell has to avoid
immune surveillance.
The latter point is probably best addressed by Matzinger's work which posits that the
immune system is not active as normally thought, but rather, reacts only to the
products of cell death. Either released cytokines or chemicals that ultimately cause
production of immunoattractants like leukotrienes or interleukins (or possibly released
cytochrome c?).
Note that the complex of steps required for expression of full-blown
carcinogenesis means that the frequency of truncated events that fail to
reach this level is probably considerable under normal circumstances.
The elevation in this basal rate is what can be associated with
carcinogenic toxicants.
Review the description of proto-oncogenes, oncogenes, and oncogene products that
occurs in an endocrinology or cell biology text. Note that proto-oncogenes are normal
genes, they are not strictly associated with transformation. Oncogenes are normal
genes that are inappropriately expressed with respect to developmental timing or
level. They may be, but do not have to be, altered structurally. In fact, many are
structurally normal, but are altered in genetic position or in their regulatory domains
so that they are inappropriately expressed. Those that have been described often
include elements of transmembrane signaling pathways associated with cellular
growth and division. Note the proto-oncogenes or the oncogenes themselves may be
quite neutral in cellular transformation; what is important is the inappropriate
expression and subsequent actions of the oncogene products. If chemicals cause
alterations in DNA synthesis or repair that result in mutations to proto-oncogenes or
their regulatory regions, the result is the oncogenic expression of the altered protooncogene (now referred to as an oncogene). The only other route to this condition is a
toxicant alteration of the expression of a particular proto-oncogene (or a set of such
genes) so that it acts as an oncogene. This might be via a targeted mutation or a
specific alteration in the biochemical pathways leading to expression (or suppression)
of that particular gene product.
Note that the above view of the integration of proto-oncogenes and oncogenes into
cellular physiology seems to differ from the picture of the "foreign invader" provided
by Pitot III & Dragan. It views cellular transformation as a natural process exacerbated
by the presence of toxicants, not as a novel process imposed on the system by the
presentation of a toxicant.
Several pertinent files or articles are linked to these lecture notes:
Tyler Jacks, Robert A. Weinberg, Cell-cycle control and its watchman, Nature 381: 643-644, 20
June 1996.
Kathleen Collins, Tyler Jacks, Nikola P. Pavletich, The cell cycle and cancer, Proc. Natl. Acad. Sci. USA
Vol. 94, pp. 2776–2778, April 1997
Ante S. Lundberg, Robert A. Weinberg, Functional inactivation of the retinoblastoma protein requires
sequential modification by at least two distinct cyclin-cdk complexes, Mol Cell Biol, Vol. 18, No. 2:753761, February 1998.
Terry L. Orr-Weaver, Robert A. Weinberg, A checkpoint on the road to cancer, Nature Vol 392:223-224,
19 March 1998.
Tyler Jacks, Robert A. Weinberg, Cell Cycle: The Expanding Role of Cell Cycle Regulators, Science 15
May 1998: Vol. 280. no. 5366, pp. 1035 - 1036 DOI: 10.1126/science.280.5366.1035.
Weinberg, Telomeres: Bumps on the road to immortality, Nature 396:23-24, 5 November 1998.
Christopher M. Counter, William C. Hahn, Wenyi Wei, Stephanie Dickinson Caddle, Roderick L.
Beijersbergen, Peter M. Lansdorp, John M. Sedivy, Robert A. Weinberg, Dissociation among in vitro
telomerase activity, telomere maintenance, and cellular immortalization, Proc. Natl. Acad. Sci. USA, Vol.
95:14723–14728, December 1998.
Jacqueline A. Lees, Robert A. Weinberg, Commentary: Tossing monkey wrenches into the clock: New
ways of treating cancer, Proc. Natl. Acad. Sci. USA,
Vol. 96:4221–4223, April 1999.
Russell E. Vance, Cutting Edge Commentary: A Copernican Revolution? Doubts about the Danger
Theory, The Journal of Immunology, 2000, 165: 1725–1728.
Nevins, The Rb/E2F pathway and cancer, Human Molecular Genetics, 2001, Vol. 10, No. 7: 699703.
William C. Hahn, M.D., PH.D., Robert A. Weinberg, PH.D., Rules for making human tumor cells, N Engl
J Med, Vol. 347, No. 20, November 14, 2002 · www.nejm.org · 1593- 1603 + erratum
Telomeres
William C.Hahn, Robert A.Weinberg, Modelling the molecular circuitry of cancer, Nature Reviews |
Cancer Volume 2 | May 2002 | 331 – 341.
Polly Matzinger, The real function of the immune system or tolerance and the four D's (danger, death,
destruction and distress), http://glamdring.ucsd.edu/others/aai/polly.html
Polly Matzinger, Turned on by Danger, BBC TV Programme Notes, 2002.
Ittai Ben-Porath, Robert A. Weinberg Review: The signals and pathways activating cellular senescence,
The International Journal of Biochemistry & Cell Biology 37 (2005) 961–976.
More on carcinogenesis stages and mechanisms (PowerPoint Slides).