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Stuart H. Yuspa, M.D.
The Pathogenesis of Squamous Cell Cancer
A remarkable insight emerging from the molecular era of cancer research is the universality of certain molecular pathways in the pathogenesis of a diverse array of tumor types in multiple species. The elucidation of specific genetic changes associated with cancer pathogenesis has focused efforts to relate these changes to specific characteristics of the neoplastic phenotype. The mouse skin carcinogenesis model lends itself well to these analyses, particularly for dissecting the development of squamous cell cancer. In the mouse model, cancer evolves through predictable premalignant stages, and each stage exhibits reproducible genetic and epigenetic changes characteristic of a specific premalignant or malignant phenotype. Mutations of the ras gene family and p53, alterations in TGF beta and EGF receptor signaling, and changes in NFkappaB and AP-1 transcriptional control are frequently detected in human and mouse squamous cell carcinomas from skin and other internal organs. We have used a combined in vivo-in vitro approach, culturing wild-type keratinocytes or those derived from genetically modified mice, producing additional genetic or biochemical modifications in vitro, and often grafting modified cells as skin grafts in nude mice to assess the consequences of the alteration in vivo. We have also engineered inducible gene alterations in mice to examine the requirement for specific gene products at particular stages of tumor induction in vivo. In keratinocytes activation of the ras gene initiates an autocrine loop through the epidermal growth factor receptor (EGFR). Genetic ablation of the EGFR inhibits the growth of Ras transformed keratinocytes by allowing the neoplastic cells to enter their differentiation program and undergo premature cell cycle arrest. Thus the EGFR pathway functions to maintain squamous cells in the proliferative pool in tissues rather than as a proliferative stimulus. Oncogenic Ras also modifies AP-1 and NFkappaB transcriptional activity, producing aberrant expression of keratinocyte genes characteristic of tumors and inducing an inflammatory microenvironment. Both transcriptional regulators are downstream from Ras- activated protein kinase C (PKC) alpha, a crucial pathway for the upregulation of specific chemokines. These pro-inflammatory chemokine ligands recruit inflammatory cells and also feedback onto their receptor that is expressed on keratinocytes, producing a second autocrine loop important for tumor development. Ras activation also inactivates PKC delta through tyrosine phosphorylation mediated by src family kinases. PKC delta is a component of the keratinocyte death pathway. These studies suggest that PKC family members could be novel targets to modify both the inflammatory and survival pathways critical for tumor formation in squamous tissues.
Inactivation of p53 or impairment of TGFbeta signaling in conjunction with ras gene activation results in rapid premalignant progression and undifferentiated cancers. TGFbeta and p53 regulate the expression of a chloride channel protein of mitochondria and cytoplasm (CLIC 4), and the level of this protein determines cell viability. In normal keratinocytes, upregulation of p53, TGFbeta signaling and a variety of stress inducers cause cytoplasmic CLIC 4 to translocate to the nucleus where it contributes to cell cycle arrest or apoptosis. In human and mouse tumor cells and tumor tissue, CLIC4 nuclear translocation is restricted, and CLIC4 protein expression is diminished as tumors progress. In xenograph models, targeting CLIC4 overexpression in cancer cells by adenoviral vectors or inducible constructs prevents tumor growth. Paradoxically, CLIC4 is highly upregulated in tumor stroma in association with myofibroblast conversion. In fact, overexpression of CLIC4 in fibroblasts induces alpha smooth muscle actin, a marker of myofibroblast conversion and poor prognosis. In xenograft models where CLIC4 is constructed for overexpression in stromal fibroblasts, tumor growth is enhanced. Thus CLIC4 loss in tumor epithelium or increase in tumor stroma constitutes a powerful marker for tumor progression and poor prognosis. Current studies are directed to illuminate the mechanisms controlling nuclear translocation of CLIC4 and the downstream pathways through which CLIC4 mediates growth arrest and apoptosis. One of the compelling reasons for focusing studies in a model where genetic and epigenetic changes characterize multiple stages in cancer pathogenesis is to identify tumor markers associated with premalignant lesions at high risk for progression and to define the pathways involved. Such information is essential to address intervention strategies designed to prevent premalignant progression and malignant conversion.
This page was last updated on 11/25/2013.