Decoding new mechanisms of stromal-driven inflammation in breast cancer

Cancer-associated fibroblasts (CAFs) are an abundant cell population of the breast tumor microenvironment (TME). Several CAF phenotypes exist in tumors, being the inflammatory type (iCAF) characterized by the high expression of interleukin 6 (IL-6). Hypoxia promotes the iCAF phenotype and is linked to poor prognosis of breast cancer patients. However, the molecular mechanism driving the iCAF program upon hypoxia exposure remains elusive. We analyzed the changes in the proteome and secretome of patient-derived CAFs exposed to hypoxia and identified a Leucine Rich Repeat Containing Protein (LRRC) as one of the most up-regulated proteins. Further analyses revealed that LRRC in induced by hypoxia at transcriptional level in a HIF1alpha-dependent manner. Histological analyses show that LRRC is uniquely expressed in CAFs across species, in both human and murine breast tumors. Stromal specificity of LRRC was further confirmed by single-cell RNA sequencing of breast cancer patients. Through loss-of-function approaches in CAFs we uncovered that LRRC is an upstream regulator of IL-6, a key driver of pathological angiogenesis and inflammation in cancer. Notably, LRRC + CAFs activate STAT signaling in cancer and TME cells. Supporting similar roles in tumors, increased STAT phosphorylation was also observed in tumors of breast cancer patients with high expression of LRRC, in proteomics data from TCGA. Thus, by regulating IL-6, LRRC may function as a driver of pathological angiogenesis and inflammation in breast cancer. Indeed, we observed that LRRC promotes endothelial sprouting angiogenesis and an inflammatory transcriptional program in CAFs and TME cells. Suggesting tumor-promoting roles of LRRC, high levels of LRRC correlate with worsened survival of breast cancer patients in TCGA datasets. To determine the influence of LRRC + CAFs on the surrounding TME, we are utilizing spatial proteomic analysis of breast cancer patient tissues. Together, our findings position LRRC as a central regulator of inflammatory signaling in breast cancer. By functioning upstream of IL-6 and driving activation of STAT, LRRC emerges as a key molecular nexus linking hypoxia to the iCAF phenotype and its tumor-promoting functions. The discovery of microenvironmental factors that fuel breast cancer inflammation is a sought-after milestone in the field since it may lead to the development of novel therapeutic interventions aimed at disrupting stromal-driven inflammation and tumor progression.