Understanding the Role of Elastic Modulus in the Relationship Between Second Harmonic Generation Scattering and Tumor Cell Motility

Abstract

Background Second harmonic generation (SHG) is an intrinsic optical property of fibrillar collagen. SHG directionality is quantified by F/B, the ratio of forward- to backward-propagating signal, which is affected by collagen fiber internal structure, specifically the diameter, spacing, and disorder of fibrils within a collagen fiber. We have previously shown that F/B of primary invasive ductal carcinoma sections is prognostic of metastatic outcome. One possible cause of this relationship was revealed by our observation that tumor cells’ motility on collagen I gels varied when collagen fiber internal structure in those gels was manipulated. The mechanism by which tumor cells sense changes in collagen fiber internal structure remains unknown: here we evaluate the role of elastic modulus in the relationship between collagen fiber internal structure (as reported by F/B) and tumor cell motility. Methods The 4T1 murine mammary adenocarcinoma, a model of metastatic triple negative breast cancer, (TNBC) and the 67NR line, a non-metastatic luminal phenotype, were introduced to a series of collagen-polyacrylamide mixed gels wherein collagen fiber internal structure and gel elastic modulus were independently controlled. F/B was measured with SHG, while elastic modulus was measured globally via rheometry and locally via atomic force microscopy (AFM). Tumor cell motility was quantified over three hours. Results The motility of both cell lines varied with F/B while elastic modulus was held constant, and did so at two physiological modulus values. Interestingly, 4T1 cell motility increased as F/B increased, while 67NR cell motility decreased. Conclusions Our results suggest that elastic modulus does not play a significant role in the observed relationship between collagen fiber internal structure (as reported by F/B) and tumor cell motility, for two cell lines that are models of TNBC and luminal-like breast cancer. Our observation that the two lines exhibit opposite motility trends as F/B increases is consistent with the trends in metastatic outcome versus F/B observed in our published clinical data for luminal versus basal cohorts, and suggests that a tumor’s subtype may play a role in their response to collagen fiber internal structure.