Functions of Paracrine PDGF Signaling in the Proangiogenic Tumor Stroma Revealed by Pharmacological Targeting

Abstract

Background

Important support functions, including promotion of tumor growth, angiogenesis, and invasion, have been attributed to the different cell types populating the tumor stroma, i.e., endothelial cells, cancer-associated fibroblasts, pericytes, and infiltrating inflammatory cells. Fibroblasts have long been recognized inside carcinomas and are increasingly implicated as functional participants. The stroma is prominent in cervical carcinoma, and distinguishable from nonmalignant tissue, suggestive of altered (tumor-promoting) functions. We postulated that pharmacological targeting of putative stromal support functions, in particular those of cancer-associated fibroblasts, could have therapeutic utility, and sought to assess the possibility in a pre-clinical setting.

Methods and Findings

We used a genetically engineered mouse model of cervical carcinogenesis to investigate platelet-derived growth factor (PDGF) receptor signaling in cancer-associated fibroblasts and pericytes. Pharmacological blockade of PDGF receptor signaling with the clinically approved kinase inhibitor imatinib slowed progression of premalignant cervical lesions in this model, and impaired the growth of preexisting invasive carcinomas. Inhibition of stromal PDGF receptors reduced proliferation and angiogenesis in cervical lesions through a mechanism involving suppression of expression of the angiogenic factor fibroblast growth factor 2 (FGF-2) and the epithelial cell growth factor FGF-7 by cancer-associated fibroblasts. Treatment with neutralizing antibodies to the PDGF receptors recapitulated these effects. A ligand trap for the FGFs impaired the angiogenic phenotype similarly to imatinib. Thus PDGF ligands expressed by cancerous epithelia evidently stimulate PDGFR-expressing stroma to up-regulate FGFs, promoting angiogenesis and epithelial proliferation, elements of a multicellular signaling network that elicits functional capabilities in the tumor microenvironment.

Conclusions

This study illustrates the therapeutic benefits in a mouse model of human cervical cancer of mechanism-based targeting of the stroma, in particular cancer-associated fibroblasts. Drugs aimed at stromal fibroblast signals and effector functions may prove complementary to conventional treatments targeting the overt cancer cells for a range of solid tumors, possibly including cervical carcinoma, the second most common lethal malignancy in women worldwide, for which management remains poor.

Abstract

Douglas Hanahan and colleagues investigate a paracrine regulatory circuit centered upon PDGF receptor signaling in cancer-associated fibroblasts and pericytes of a mouse model of cervical carcinogenesis.

Editors' Summary

Background.

Cancers—disorganized, life-threatening masses of cells—develop when cells acquire genetic changes that allow them to divide uncontrollably and to move into (invade) other tissues. Interactions with ostensibly normal cells in the tissue surrounding the tumor (the stroma) support the growth of these abnormal cells. The stroma contains endothelial cells and pericytes (which line the inside and coat the outside, respectively, of blood vessels), cancer-associated fibroblasts, and some immune system cells. Together, these cells support angiogenesis (the formation of a blood supply, which feeds the tumor), produce factors that stimulate tumor cell growth, and facilitate tumor cell invasion into surrounding tissues. One type of tumor with a prominent stromal compartment is cervical cancer. Precancerous changes in the epithelial cells lining the cervix (the structure that connects the womb to the vagina) are usually triggered by infection with human papillomavirus. Some of these early lesions, which are known as cervical intraepithelial neoplasias (CINs), develop into invasive cervical cancer, which is treated by surgery followed by chemotherapy or radiotherapy.

Why Was This Study Done?

The outlook for women whose cervical cancer is detected early is good but only 15%–30% of women whose cancer has spread out of the cervix survive for five years. If, as researchers believe, the stromal compartment is important in the development and growth (neoplastic progression) of cervical cancer, it might be possible to help these women by specifically targeting the cells in the stroma. However, relatively little is known about the role that the stroma plays in the neoplastic progression of cervical cancer or how it is regulated other than that a protein called platelet-derived growth factor (PDGF), which is made by the tumor cells, might be involved in its formation. In this study, the researchers have used a mouse model of cervical cancer (HPV/E ₂ mice) to investigate PDGF signaling in the tumor stroma. HPV/E ₂ mice develop CINs before they are three months old; by five months of age, 90% of them have invasive cervical cancer.

What Did the Researchers Do and Find?

The researchers report that PDGF was expressed in the cervixes of normal and HPV/E ₂ mice, mainly by epithelial cells, and that PDGF receptors (cell-surface proteins that bind PDGF and send a message into the cell that alters the expression of other proteins) were expressed on cells within normal stroma and in fibroblasts and pericytes in the stroma surrounding CINs and tumors (but not on the cancer cells). The expression of PDGF and its receptors increased slightly during tumor progression. Treatment of the HPV/E ₂ mice with imatinib, an inhibitor of PDGF signaling, slowed the progression of precancerous lesions, impaired the growth of invasive cancers, and reduced the number of blood vessels formed in the tumors and the coverage of these vessels with pericytes. Other experiments indicate that imatinib had these effects because its inhibition of stromal PDGF receptors suppressed the expression of FGF-7 (a factor that encourages epithelial cell division) and FGF-2 (a proangiogenic factor) by cancer-associated fibroblasts. Finally, as in HPV/E ₂ mice, FGF-2 and PDGF receptors were expressed in the stroma of human cervical cancers whereas PDGF was expressed in the cancer cells.

What Do These Findings Mean?

These findings suggest that PDGF receptor signaling in the stromal cells associated with cervical tumors in mice has a functional role during tumor progression. More specifically, they suggest that PDGF released by the tumor cells triggers PDGF signaling in the stromal cells, which increases the expression of factors that both directly and indirectly stimulate the growth of the tumor cells. Confirmation of this scheme will require additional experiments in mouse models of cervical cancer and the careful examination of more human material. Importantly, although approaches that work in mice do not always work in people, the current findings suggest that targeted therapeutics that prevent the stromal support of tumor growth (such as inhibitors of PDGF receptor signaling) might provide a complementary approach to conventional treatments that target the cancer cells themselves.

Additional Information.

Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.0050019.

• The US National Cancer Institute provides information on all aspects of cancer, including information about cervical cancer (in English and Spanish)

• The UK charity Cancerbackup also provides information on all aspects of cancer, including information on cervical cancer and on imatinib

• Wikipedia has pages on platelet-derived growth factor, on PDGF receptors, and on imatinib (note that Wikipedia is a free online encyclopedia that anyone can edit; available in several languages)