The Daily Sign-Out

Field cancerization though precancerous stromal changes

An important question in cancer research is the causes and mechanisms for the development of multifocal carcinomas, especially for non-small cell lung carcinomas (as I recently blogged on).  While a “field cancerization” effect has been postulated as a model for multifocal carcinomas for a long time, it remains enigmatic why cancers develop at one site and not another given that the entire “field” is presumably exposed to the same carcinogenic agent or stimulus.  More specifically, it is also obscure what the role is of the peritumoral stromal changes observed around multifocal carcinomas.  Are these changes a cause or consequence (or, more likely, both) of these tumors?  Under normal circumstances, the epithelial tissue stromal architecture imposes growth restraints upon epithelial structures and exerts a tumor-suppressive effect on “transformed” neoplastic cells.  A similar effect is observed in the metastatic process in which the vast majority of metastatic tumor cells fail to form tumors at distant metastatic sites.  The conventional view has been that the tumor stromal microenvironment is reactive in nature; that is, the tumor stroma develops as a result of molecular cues from the tumor itself.  A recent article in Cell (8 June 2012) by Bing Ho and colleagues presents data suggesting that the loss of mesenchymal Notch/CSL signaling causes stromal changes that are potent stimuli for the development of carcinomas (abstract).

Mice carrying a mesenchymal-specific deletion of CSL-RBP-Jk, a key effector in the Notch signaling pathway, develop spontaneous multifocal squamous cell carcinomas after dermal atrophy and chronic inflammation.  The authors found that CSL-deficient dermal fibroblasts promote increased tumor cell proliferation through upregulation of c-Jun and c-Fos expression.  This, in turn, was associated with higher levels of diffusible growth factors, inflammatory cytokines, and matrix-remodeling enzymes.

The authors then tested these findings with human skin biopsy specimens of multifocal actinic keratosis, a premalignant skin lesion for cutaneous squamous cell carcinoma strongly associated with sun exposure.  Stromal fields adjacent to multifocal actinic keratosis showed decreased Notch/CSL signaling, albeit only with exposure to UVA and not UVB exposure, through DNA methylation and epigenetic silencing of Notch signaling in dermal fibroblasts.  These findings imply that the phenomenon of field cancerization can be mediated through stromal alterations preceding the development of neoplastic epithelial clones—in other words, the stroma is precancerous due to mutagen exposure.

This model would be a fascinating one to pursue for future lung cancer research with both non-small cell and small cell lung cancers since these cancers are all also strongly associated with a known mutagenic exposure, are posited to develop from a field cancerization effect from this exposure, and are frequently associated with multifocal synchronous tumors.  Unfortunately, there isn’t exactly an analogous premalignant “actinic keratosis” lesion for NSCLC, at least one that can be easily recognize and sample, although I think such atypical adenomatous hyperplasia foci may be easier to study in mouse models.  Anyone interested in pursuing this, let me know! ; )

 

Medscape Interview with NIH Director

Medscape recently produced a fascinating interview with Dr. Francis Collins, Director of the National Intitutes of Health that focuses on genomics but ranges broadly over several areas, including medical education and preventative medicine.  It's not that long and there is also a text format if you don't want to watch the interview.  Highly recommended.  This a thought-provoking interview.

I think Dr. Collins is overly optimistic about the impact of genomics on day-to-day patient care.  Pharmacogenomics, for one example he uses, has not made any impact thus far in my experience.  While it is a tool, perhaps, for demonstrating why a patient may not be responding to a particular drug, the use of pharmacogenomic testing before initiating treatment has not been prospectively studied to my knowledge in any trials, for any drug.  This leaves aside the further studies regarding cost-benefit analyses as well as the issue of payment/reimbursement.  Right now, it seems cheaper and tried to put a patient on a standard dose and see what happens.

Another example is the idea of genomic sequencing of cancers.  It sounds good because it provides a "personalized" profile of a cancer.  What about tumor heterogeneity?  Do we sequence the primary tumor or metastasis?  Which part of the primary tumor?  Which metastasis?  Finally, how does one compare "unique" tumor genotypes and develop groups to statistically compare treatment responses?  At some point, patients have to be lumped together (and thus muddying up personalized differences) in order to find appropriate guidelines for treatment and provide some prognostic information.

Lots more to think about!

Is Notch signaling involved in lung carcinogenesis?

This is the question asked by Joyce Osanyingbemi-Obidi and colleagues from Johns Hopkins in an interesting recent paper published in Molecular Cancer Research.  The cancer stem cell hypothesis proposes that there is a small proportion of tumor cells with stem cell-like characteristics, including self-renewal capability, as well as a much larger proportion of "differentiated" cells with limited proliferative capability.  While stem cell-like cells have been characterized in a variety of malignancies, such a population for NSCLC has not been reported.  I previously blogged on a murine model for evidence of lung stem/progenitor cells. The authors cite the emerging body of data (perhaps not surprisingly) suggesting that the same key signaling pathways in embryonic and fetal development (e.g. Notch, Wnt, and Hedgehog) are also involved in the regulation of cancer stem cells.  The intracellular domain of Notch receptors (NICD) is released upon ligand binding and translocates to the nucleus where it converts the CSL complex from a transciptional repressor into an activator.  However, the NICD-CSL complex requires mastermind-like (MAML) proteins to recruit and bind coactivators that drive expression of downstream targets that promote cell growth and proliferation.  Aberrant Notch signaling has been described in a variety of malignancies (albeit with both activating and inactivating Notch mutations) and, indeed, elevated expression of Notch family members and a key downstream target, "hairy and enhancer of split 1" (Hes1), have been reported in NSCLC.

Using NSCLC cell lines showing overexpression of Notch3 and Hes1, the authors demonstrated that pharmacologic inhibition of Notch was associated with suppression of Hes1 expression and inhibition of clonogenic growth.  The authors used a clever LSL-KRAS^G12D mouse model in which an oncogenic KRAS allele is silenced by a transcription stop site flanked by lox P sites; inhalation of an adenovirus vector containing the Cre recombinase promotes rearrangment of the lox sites and deletion of the transcriptional stop and selective overexpression of mutant KRAS in bronchial and alveolar cells.  The authors showed a transient induction of Hes1 expression by immunohistochemistry in airway epithelial cells 1 week postinfection with adenovirus and gradually diminshing Hes1-positive cells in tumors over time.  They then used an novel inducible double-negative MAML knock-in murine model to show that inhibition of Notch signaling in vivo was not sufficient to suppress carcinogenesis or tumor progression in the context of mutant KRAS-driven carcinogenesis.

So--these data suggest that Notch signaling may have a role in early lung adenocarcinogenesis through the transformation of a tumorigenic stem cell-like cell but is not likely required for adenocarcinogenesis.  The authors provide several notable caveats and qualifications of this conclusion but, I think the key consideration is that Notch signaling is considerably more complex that we are given to understand.  Notch signaling in development can maintain stem cell characteristics in certain cell populations or drive cell differentiation in other.  Likewise, Notch pathway activation in neoplasia appears to have tumor-promoting and tumor-suppressing effects.  Also, not all Notch family members appear to equally contribute to lung neoplasia.  Finally, individual Notch isoforms may have opposing effects on carcinogenesis.

This paper may not be of general interest, especially for the busy practicing pathologist, but is worthwhile to reflect upon and ruminate over.  Intriguing stuff.

Systems biology for studying cancer

This month's issue of Future Oncology has a well-written editorial from Dr. John McDonald from Georgia Tech regarding systems biology in cancer research.  This is a free download and I highly recommend it as a nice overview contrasting the traditional "reductionist" approach to cancer cell biology research versus the emerging application of a systems biology approach.

First, it is appreciated that the traditional approach has grudgingly yielded some valuable insight into cancer cell biology which has led to the development of "rationally designed" targeted therapies.  But while many of these therapies have had some initial impact for patients enrolled in clinical trials (and even some dramatic responses), I think it is fair to say that they also have not radically shifted overall survival or long-term outlook of the most common lethal cancers.

However, we are beginning to realize the potential for combining high-throughput molecular technologies with computational strategies and capabilities for combining multidimensional datasets to find inferred network relationships.  Dr. McDonald refers to this as the "top-down" approach.  He contrasts this with the traditional reductionist "bottom-up" approach which links human oncogenes or tumor suppressor genes with known interacting genes previously characterized in experimental models using cancer cell lines or animal models to identify signaling pathways implicated with cancer.  He stresses that these approaches are by no means mutally exclusive but indeed are complementary.

The systems approach starts with establishing a list of component parts.  Using new technologies, we can now speak of the genome, transcriptome, proteome, metabolome, etc.  The revolutionary step is using sophisticated computer models to detemine how these various components interact and which of these interactions are relevant for cancer initiation and progression and which represent potential druggable targets.

Again, this is a nice appetizer for additional reading on this exciting area!

Hallmarks of cancer v. 2.0

In 2000, Douglas Hanahan and Robert Weinberg initially proposed six hallmarks of cancer that form an organizing principle for understanding the remarkable diversity of "cancer."  Their update, "Hallmarks of Cancer: The Next Generation," was published in the March 4, 2011 issue of Cell.

The six hallmarks of cancer are distinctive and complementary capabilities enabling tumor growth and metastatic dissemination and are a solid foundation for understanding the biology of cancer . These six hallmark capabilities of cancer, as originally defined, include the ability to sustain chronic proliferation, the ability to evade growth suppressors, the ability to resist cell death, the ability to achieve replicative immortalization, the ability to induce angiogenesis, and, finally, the ability to activate mechanisms for invasion and metastasis. These characteristics, common to all types of cancer, form a fundamental an understanding of the biology of cancer.

The authors use this review article, however, to update the massive progress that is been made in the last decade from their original article. They identify to enabling characteristics that allow cancer cells to acquire these functional capabilities. The most common is the development of genomic instability in cancer cells. This not only generates random mutations including chromosomal rearrangements but also rare genetic changes that can drive some of these hallmark capabilities.  A second enabling characteristic involves the inflammatory state of premalignant and frankly malignant lesions which serve to promote tumor progression through various mechanisms.

The authors highlight other distinct attributes of cancer cells which have been proposed as also being functionally important for the development of cancer and may also be added to the list of core hallmarks.  Two are especially compelling. The first involves the major reprogramming of cellular energy metabolism in order to support continuous cell growth and proliferation. The second involves active evasion by cancer cells from attack and elimination by immune cells. This latter capability highlights the dichotomous role of the immune system which can both antagonize and enhanced tumor development and progression.

The authors also include a very nice summary of the advances made in our understanding of the tumor microenvironment. They enumerate the various cell types that are known to contribute to the biology of the tumor microenvironment and also discuss some of the regulatory signaling that controls their individual and collective functions. Also included in their discussion is recent research showing the connections between the acquisition of cancer stem-like cells and the epithelial-mesenchymal transition differentiation program.

One interesting topic they discuss is the role of pericytes which is a cell type that I haven't thought of as being an important part of the tumor microenvironment. Pericytes represent a very specialized mesenchymal cell type that have fingerlike projections that wrap around the endothelial tubing of blood vessels. In normal cells or normal tissues, pericytes are known to provide paracrine support signals to the normally quiesent endothelium. The authors discuss research in which genetic and pharmacological disruption of the recruitment and association of pericytes which has demonstrated the functional importance of the cells in supporting the tumor endothelium. The authors conclude with a brief summary of possible therapeutic targeting of the hallmarks of cancer. They propose drugs that could interfere with each of the acquired capabilities necessary for tumor growth and progression and summarize some of which are involved in clinical trials or in some cases approved for clinical use in treating certain forms of cancer.

This is a long review article (hence the long post!) but well worth your effort and it is the most up-to-date and accessible review of our current understanding of the biology of cancer.