Julie B. Sneddon*, Hanson H. Zhen, Kelli Montgomery, Matt van de Rijn, Aaron D. Tward, Robert West, Hayes Gladstone, Howard Y. Chang, Greg S. Morganroth, Anthony E. Oro, and Patrick O. Brown*,¶,||
Departments of *Biochemistry, Dermatology, and Pathology, and ¶Howard Hughes Medical Institute, Stanford University Medical Center, Stanford, CA 94305; and G. W. Hooper Foundation, University of California, San Francisco, CA 94143
Contributed by Patrick O. Brown, August 9, 2006
Although tissue microenvironments play critical roles in epithelialdevelopment and tumorigenesis, the factors mediating these effectsare poorly understood. In this work, we used a genomic approachto identify factors produced by cells in the microenvironmentof basal cell carcinoma (BCC) of the skin, one of the most commonhuman cancers. The global gene expression programs of stromalcell cultures derived from human BCCs showed consistent, systematicdifferences from those derived from nontumor skin. The genemost consistently expressed at a higher level in BCC tumor stromalcells compared with those from nontumor skin was GREMLIN 1,which encodes a secreted antagonist of the bone morphogeneticprotein (BMP) pathway. BMPs and their antagonists are knownto play a crucial role in stem and progenitor cell biology asregulators of the balance between expansion and differentiation.Consistent with the hypothesis that BMP antagonists might havea similar role in cancer, we found GREMLIN 1 expression in thestroma of human BCC tumors but not in normal skin in vivo. Furthermore,BMP 2 and 4 are expressed by BCC cells. Ex vivo, BMP inhibits,and Gremlin 1 promotes, proliferation of cultured BCC cells.We further found that GREMLIN 1 is expressed by stromal cellsin many carcinomas but not in the corresponding normal tissuecounterparts that we examined. Our data suggest that BMP antagonistsmay be important constituents of tumor stroma, providing a favorablemicroenvironment for cancer cell survival and expansion in manycancers.
cancer biology | stem cell regulation | tissue microenvironment | tumor stroma
PNAS | October 3, 2006 | vol. 103 | no. 40 | 14842-14847. OPEN ACCESS ARTICLE.
Tissue microenvironments play a critical role in specifyingcellular niches in both the developing embryo and adult organisms(1, 2). In development, cell fate decisions are dictated notonly by cell-autonomous signals but also by stimuli from thesurrounding tissue microenvironment (3, 4). Similarly, in adulttissues that continue to renew throughout the lifetime of theorganism, such as the skin, intestinal epithelium, and hematopoieticsystem, the self renewal and maturation of the stem cell populationare regulated by specific molecular cues derived from the correspondingmicroenvironments (5–7). In the skin, hair follicle morphogenesisis regulated by signals coming from the dermal papilla, a specializedmesenchymal structure that signals to matrix stem cells locatedacross the basement membrane (8, 9). Similarly, the modulationof stem cell activity in the intestine is also subject to cuesderived from underlying mesenchymal cells that surround thecrypt (10, 11). Hematopoietic stem cells are regulated in partby osteoblasts, cells that reside in the adjacent bone spicule(12, 13). In all of these cases, a crucial feature of the regulationof stem cell compartment size, location, and timing of selfrenewal is the production of critical factors by a specializedset of mesenchymal cells that create a customized microenvironment.
During carcinogenesis, an analogous system of specialized tissuemicroenvironment cells may also be important in specifying a"tumor cell niche" that supports a self-renewing populationof tumor cells. Paradoxically, although uncontrolled proliferationand survival are the cardinal characteristics of cancer cells,it can be difficult to sustain these cells away from their correspondingmicroenvironment, either in culture or as explants (14). Thereis accumulating evidence that tumor stroma influences tumordevelopment (15, 16). Genetic studies have shown that stromalcells are altered in some inherited cancer-susceptibility syndromes(17). In breast cancer, rearrangements at several loci havebeen noted exclusively in tumor-associated stromal cells (18).In vivo and in vitro experiments demonstrated that human prostaticepithelial cells showed dramatic changes both in histology andgrowth rate when grown with human fibroblast cells derived fromprostatic carcinoma, suggesting that carcinoma-derived fibroblastscan stimulate tumorigenesis (19). Others have shown that coinjectionof fibroblasts with tumor epithelial cells into mice can enhancetumor formation (20).
To identify factors produced by tumor stromal cells that contributeto the initiation or maintenance of the tumor, we used a genomicapproach with basal cell carcinoma (BCC) of the skin, one ofthe most common human neoplasms, as our model system. Previouswork with human autotransplants of BCC lesions has suggestedthat stromal cells in the tumor tissue play a crucial role insustaining the tumor (21). Mouse models of the disease haveshown that sustained activation of the Sonic Hedgehog pathway,a major genetic component of BCC, is maintained only in thecontext of the animal in vivo; when explanted in culture, tumorcells lose pathway activity (22).
We cultured stromal cells from BCC tumor and nontumor humanskin and compared those two cell populations by cDNA microarrayanalysis. Antagonists of the bone morphogenetic protein (BMP)pathway were among the genes most consistently and significantlydifferentially expressed between the two populations. Givenwhat is already known about the role of BMPs and their antagonistsin regulating stem cell compartments in normal development andphysiology, we hypothesized that a similar role could be playedby BMPs and BMP antagonists in the context of the tumor.
BMPs are important regulators of stem cell fate (23). In diversesettings, BMPs promote differentiation of stem cells, thus promotingexit from the stem cell compartment (12, 24). In the skin, conditionalgene targeting of BMPRIA in mice has demonstrated that BMPRIAis required for proper differentiation of progenitor cells inthe hair shaft (25, 26). The BMP inhibitor noggin is expressedby cells in the follicular mesenchyme, and mice lacking noggindisplay defects in hair follicle induction and morphogenesis(9, 27). High levels of GREMLIN 1 transcript have been observedin mouse embryonic fibroblast cells that are capable of maintaininghuman embryonic stem cells in culture (28). These observationsled us to investigate the hypothesis that BMP antagonists secretedby stromal cells in cancer tissues might be an important partof the specialized tumor microenvironment that allows continuedproliferation and self renewal of cancer cells.