Cancer cells in solid tumors interact with their microenvironment to grow, migrate, invade, and spread. The tumor microenvironment consists of stromal cells such as cancer-associated fibroblasts, cancer-associated macrophages, and endothelial cells that form abnormal tumor blood vessels, as well as the extracellular matrix (ECM), which contains a diverse meshwork of fiber-network forming proteins, in which Collagen-1 (Col1) is the major component. High mammary Col1 fiber density is a known risk factor for tumor initiation, progression, and metastasis. Cancer cells actively participate in remodeling Col1 fibers through signaling pathways. Cancer-associated fibroblasts together with cancer cells remodel the Col1 fiber network of breast cancers and other tumors with the help of lysyl oxidases (LOXs), Collagenases, and other matrix metalloproteinases (MMPs), as well as additional proteases such as Cathepsins, which eventually leads to the formation of radially aligned Col1 fibers along which cancer cells migrate. Cancer cells travel along these radially aligned Col1 fibers into surrounding stroma to invade, extravasate, and metastasize. Such radially aligned Col1 signatures were able to predict survival in human breast cancer patients. We showed in a pilot study that Col1 fiber density and its texture features in primary human breast tumors were associated with lymph node status. This study was done with second harmonic generation (SHG) microscopy, a label-free multiphoton microscopy approach that detects intrinsic signal from the noncentrosymmetric properties of Col1 fibers.

Col1 fibers also facilitate molecular transport through the tumor interstitium. Hypoxic tumor regions were shown to contain fewer Col1 fibers as compared to normoxic tumor regions. By characterizing extravascular transport of the macromolecular MRI contrast agent albumin-Gadolinium-diethylenetriamine penta-acetic acid (DTPA) with dynamic contrast enhanced (DCE) MRI focused on the late phase of contrast agent dynamics, we observed that hypoxic regions with sparse Col1 fibers had reduced transport parameters as compared with denser Col1 fiber containing normoxic regions. These studies identified hypoxic tumor regions as silent areas with little macromolecular transport, which likely resulted in poor drug delivery to these regions and, consequently, tumor recurrence from these areas. A dense mesh of Col1 fibers was observed around hypoxic pockets, which may be used by aggressive hypoxic cancer cells to travel along these Col1 fiber avenues and metastasize. Late-phase macromolecular contrast agent kinetics were able to measure stromal tumor features such as Col1 fiber and overall ECM density, and hence may allow us to draw conclusions about the likelihood of metastasis to occur based on the connection between Col1 fibers and metastasis. We have recently also shown that water diffusion directionality as detected by noninvasive diffusion tensor imaging (DTI) was able to detect Col1 fibers in breast lesions. In these studies, we observed that water diffusion and anisotropy followed the Col1 fiber distribution in human breast cancer specimens. Tumor regions with low Col1 fiber content contained decreased apparent diffusion coefficients (ADC) and fractional anisotropies (FA) compared to regions with high Col1 fiber content, suggesting that Col1 fibers facilitate molecular transport through the ECM and can be detected by noninvasive DTI.

This presentation will cover the importance of the tumor stroma, and in particular the Col1 fiber meshwork, in cancer migration and metastasis, along with novel MRI approaches such as macromolecular contrast agent based DCE MRI and DTI to noninvasively detect critical features of the Col1 fiber network in tumors.