Mechanism of acid signaling in bone organ metastases on tumor and myeloid cell evolution towards immune suppression and tumor progression - Project Summary/Abstract Bone is the most common organ site of metastasis in breast cancer (BC), with occurrence rates of 70% in ER+, 46% in TNBC, and 50% in HER2-enriched cases. In bone metastases, breast tumor cells secrete factors to enhance osteoclast (OC) mediated bone destruction which can result in severe bone pain, fractures, and spinal cord compression. Immune therapy provides long-lasting remissions and even cures many cancers; however, it is significantly less effective in breast cancer (BC)- only 5-6% of metastatic breast cancer patients are eligible for immune checkpoint blockade (ICB). It is recognized that the presence of bone metastases confers resistance to ICB. Previous studies show BC can have high levels of infiltrating myeloid cells, which promote immune suppression and resistance to ICB. However, we recently uncovered a novel mechanism that requires acid and cytokine production with GM-CSF through which breast cancers induce the evolution of myeloid cells to a pro- tumor immune suppressive phenotype. We found a vicious cycle driven by acid and cytokines that fuels cross talk between myeloid cells and tumor cells to promote immune suppression. We showed that blockade of tumoral GM-CSF genetically and pharmacologically restored ICB efficacy in ER+ and TNBC preclinical bone metastases. However, GM-CSF is important for dendritic cell biology and is not an ideal therapeutic target. This led us to investigate targeting of acid and acid signaling to disrupt tumor evolution towards immune therapy resistance. It is established that the bone marrow environment is significantly more acidic than other organ tissues and that BC in this setting secretes factors that modulate myeloid cell heterogeneity towards osteoclasts, which contribute to bone pain and bone destruction. While neutralizing acid in the TME has been tried, the results are limited in part by compensatory increases in tumor acid production. We identified that acid signals in both myeloid cells and in tumor cells through cAMP via the heterotrimeric G protein subunit G-alpha S (GNAS). Our preliminary data showed that disruption of GNAS in either tumor cells or myeloid cells decreased levels of acid-induced cytokines and the progression of bone metastasis and the evolution to an immune suppressive phenotype. Collectively, our data provides a strong rationale to pursue the hypothesis that disruption of acid-induced GNAS- dependent signaling will lead to molecular and phenotypic diversity across the tumor and myeloid cells throughout the bone organ resulting in sensitivity to immune therapy, decreased bone tumor burden and progression to bone destruction. We will use genetic and pharmacologic approaches to delineate the mechanisms of acid-induced GNAS signaling during tumor progression in bone. We will integrate our single cell RNA-Seq (scRNA-Seq) analyses of genetically modified myeloid and tumor cells, with myeloid and tumor cells from breast cancer patients bone metastases to understand the molecular diversity that leads to metastasis. Given our track record of translating preclinical mechanistic studies into clinical trials, this proposal could have significant translational impact by developing a novel approach to inhibit metastasis and recurrence.
