Colony-Stimulating Factor 1 Promotes Progression of Mammary Tumors to Malignancy

TLDR

In human breast carcinomas, overexpression of the macrophage colony–stimulating factor (CSF‑1) and its receptor (CSF‑1R) correlates with poor prognosis. To determine whether CSF‑1 causally drives breast cancer progression, the authors crossed a mammary cancer–susceptible transgenic mouse with a CSF‑1 null mutant and monitored tumor progression in wild‑type versus null mice. The study used this CSF‑1 null model and found that CSF‑1 promotes metastasis by regulating tumor‑associated macrophage infiltration and function, with CSF‑1R expression confined to macrophages. CSF‑1 is not required for primary tumor incidence or growth but accelerates progression to invasive, metastatic carcinoma, increases pulmonary metastasis, and is linked to enhanced macrophage infiltration, indicating that CSF‑1 selectively promotes malignancy and that targeting CSF‑1/CSF‑1R could have therapeutic benefit.

Abstract

In human breast carcinomas, overexpression of the macrophage colony–stimulating factor (CSF-1) and its receptor (CSF-1R) correlates with poor prognosis. To establish if there is a causal relationship between CSF-1 and breast cancer progression, we crossed a transgenic mouse susceptible to mammary cancer with mice containing a recessive null mutation in the CSF-1 gene (Csf1op) and followed tumor progression in wild-type and null mutant mice. The absence of CSF-1 affects neither the incidence nor the growth of the primary tumors but delayed their development to invasive, metastatic carcinomas. Transgenic expression of CSF-1 in the mammary epithelium of both Csf1op/Csf1op and wild-type tumor-prone mice led to an acceleration to the late stages of carcinoma and to a significant increase in pulmonary metastasis. This was associated with an enhanced infiltration of macrophages into the primary tumor. These studies demonstrate that the growth of mammary tumors and the development to malignancy are separate processes and that CSF-1 selectively promotes the latter process. CSF-1 may promote metastatic potential by regulating the infiltration and function of tumor-associated macrophages as, at the tumor site, CSF-1R expression was restricted to macrophages. Our data suggest that agents directed at CSF-1/CSF-1R activity could have important therapeutic effects.