Modulation of the extracellular matrix by Streptococcus gallolyticus subsp. gallolyticus and importance in cell proliferation

Streptococcus gallolyticus subspecies gallolyticus ( Sgg ) has a strong clinical association with colorectal cancer (CRC) and actively promotes the development of colon tumors. Previous work showed that this organism stimulates CRC cells proliferation and tumor growth. However, the molecular mechanisms underlying these activities are not well understood. Here, we found that Sgg upregulates the expression of several types of collagens in HT29 and HCT116 cells, with type VI collagen (ColVI) being the highest upregulated collagen type. Knockdown of ColVI abolished the ability of Sgg to induce cell proliferation and reduced the adherence of Sgg to CRC cells. The extracellular matrix (ECM) is an important regulator of cell proliferation. Therefore, we further examined the role of decellularized matrix (dc-matrix), which is free of live bacteria or cells, in Sgg -induced cell proliferation. Dc-matrix prepared from Sgg -treated cells showed a significantly higher pro-proliferative activity than that from untreated cells or cells treated with the control bacteria. On the other hand, dc-matrix from Sgg -treated ColVI knockdown cells showed no difference in the capacity to support cell proliferation compared to that from untreated ColVI knockdown cells, suggesting that the ECM by itself is a mediator of Sgg -induced cell proliferation. Furthermore, Sgg -treated CRC cells formed significantly larger tumors in vivo , whereas Sgg treatment had no effect on ColVI knockdown cells, suggesting that ColVI is important for Sgg to promote tumor growth in vivo . These results highlight a dynamic bidirectional interplay between Sgg and the ECM, where Sgg upregulates collagen expression. The Sgg -modified ECM in turn affects the ability of Sgg to adhere to host cells and more importantly, acts as a mediator for Sgg -induced CRC cell proliferation. Taken together, our results reveal a novel mechanism in which Sgg stimulates CRC proliferation through modulation of the ECM. Author Summary Colorectal cancer (CRC) is a leading cause of cancer-related death. The development of CRC can be strongly influenced by specific gut microbes. Understanding how gut microbes modulate CRC is critical to developing novel strategies to improve clinical diagnosis and treatment of this disease. S. gallolyticus subsp. gallolyticus ( Sgg ) has a strong clinical association with CRC and actively promotes the development of colon tumors. However, the mechanisms Sgg utilizes to promote tumors are not well understood. Our results showed for the first time a dynamic interplay between Sgg and the extracellular matrix. We found that Sgg upregulates the expression of collagens which in turn affects the interaction between Sgg and CRC cells and mediates CRC cell proliferation. These findings draw attention to a previously unrecognized dynamic bidirectional interplay between a CRC-associated microbe and the extracellular matrix (ECM). Given the importance of the ECM in normal homeostasis and in tumor microenvironment, these findings have important implications in the context of microbial contribution to cancer.