Posted September 8, 2021

Ori Maller, Ph.D. and Valerie Weaver, Ph.D., University of California, San Francisco (UCSF)

The extracellular matrix (ECM), composed primarily of collagen fibers that are regularly remodeled through tightly regulated processes, provides physical and structural support for cells and initiates important cues needed for tissue function and survival.¹ An excess buildup of ECM with increased collagen stiffness and crosslinking is often seen in the development of invasive breast carcinomas, and those with the stiffest stroma have been shown to be the most aggressive. Furthermore, breast cancer tumors are typically inflamed, and tumors that have a high degree of stromal stiffening have increased amounts of tumor-associated macrophages (TAMs).² With a fiscal year 2013 Breast Cancer Research Program Postdoctoral Fellowship Award, Dr. Ori Maller, a former member of Dr. Valerie Weaver’s team at UCSF and a co-lead author in a recent Nature Materials publication, investigated whether changes in ECM mechanical properties cooperate with inflammatory signaling to suppress immune detection and increase metastatic disease.

As described in their recent publication, the research team demonstrated that inflammatory stromal cell-mediated collagen crosslinking and stiffening promote breast cancer tumor aggression. In close collaboration with Dr. Kirk Hansen’s lab from the University of Colorado Anschutz Medical Campus, the team established a method for characterizing collagen crosslinks mediated by lysyl oxidase (LOX) and lysyl hydroxylase (LH), two families of enzymes responsible for regulating collagen crosslinking. Through this work, they identified a subgroup of LH2-dependent hydroxylysine aldehyde-derived collagen crosslinks (HLCCs) that are vital for tissue mechanical strength. Using human biopsies of early-stage and invasive breast cancers (IBCs), the researchers found distinctive differences in fibrosis and HLCC levels between breast tumor molecular subtypes. The most aggressive triple-negative (TN) tumors demonstrated the highest level of fibrosis and revealed a strong positive correlation between tumor-associated ECM stiffness and the level of HLCCs.

Using publicly available human breast cancer gene expression array data, the researchers investigated the correlation between breast tumor subtypes and the levels of enzymes involved in the regulation of collagen crosslinking. These analyses revealed increased expression of the key HLCC accumulation regulators, LOX and PLOD2 (the gene encoding protein LH2), within TN tumor subtypes. In addition, analyses of microdissected IBCs revealed that LOX and PLOD2 were predominately expressed by stromal rather than epithelial cells. Thus, Dr. Weaver and her research team investigated the involvement of stromal LOX and PLOD2 expression in overall breast cancer patient survival. Through analysis of published gene expression data from breast cancer patients, they found that while neither stromal cell nor epithelial LOX predicted overall patient survival in this cohort, overexpression of stromal PLOD2—not epithelial—was associated with poor patient prognosis. Moreover, increased PLOD2 expression in TN tumors predicted decreased distant metastasis-free survival as well as an increased risk of relapse in TN tumors. Finally, studies using breast cancer patient biopsies showed that the more aggressive breast cancer subtypes have moderate to high stromal LH2 expression, and there was an association between shorter breast cancer-specific survival and high expression of stromal cell LH2, especially in lymph node positive patients. Collectively, these results implicate stromal LH2 expression as a strong indicator of tumor aggression. 

To investigate whether TAMs play a role in breast cancer aggression and metastasis through stimulation of collagen crosslinking and stromal stiffness, Dr. Weaver and her team performed studies using a mouse model of breast cancer metastasis. In this model, a population of TAMs that secrete high amounts of transforming growth factor-β (TGF-β), a growth factor involved in tissue fibrosis, was identified within the tumors. Studies also demonstrated that depletion of TAMs in this model reduced metastasis, stromal LOX and PLOD2, as well as collagen crosslinks and HLCCs. These findings indicate a role for TAMs in promoting stromal LOX/PLOD2 expression thus increasing ECM collagen stiffness and tumor aggression.

This work demonstrated the involvement of TAMs in stromal cell-mediated collagen crosslinking, stiffening, and breast cancer tumor aggression. Notably, the research team’s research also pinpoints stromal LH2 as a novel biomarker and potential early prognostic indicator of metastatic breast cancer disease aggression and poor patient survival, particularly for the highest risk TN subtype. Results from this important research support future preclinical investigations to confirm whether a causal relationship exists between stromal LH2, tumor aggression, and metastasis.

Publication:

Maller O, Drain AP, Barrett AS, et al. 2021. Tumour-associated macrophages drive stromal cell-dependent collagen crosslinking and stiffening to promote breast cancer aggression. Nature Materials 20(4):548-559.

References:

¹ Frantz C, Stewart KM, and Weaver VM. 2010. The extracellular matrix at a glance. Journal of Cell Science 123(24):4195-4200.

² Acerbi I, Cassereau L, Dean I, et al. 2015. Human breast cancer invasion and aggression correlates with ECM stiffening and immune cell infiltration. Integrative Biology 7(10):1120-1134.

Link:

Public and Technical Abstracts:  Tumor Tension Induces Persistent Inflammation and Promotes Breast Cancer Aggression

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