The Role of Runx2 in Microtubule Acetylation in Bone Metastatic Breast Cancer Cells

Simple Summary Breast cancer is the most commonly diagnosed cancer type, making up a quarter of all cases among women. While modern-day research has shown tremendous progress in the development of treatment options, complications related to bone metastasis remain an obstacle that demands further attention. Cancer cells that migrate into the bone microenvironment show significant alterations in their metabolism and gene expression profiles. In this study, we set out to better understand the changes induced by Runx2 in bone-derived breast cancer cells. We identified key changes in the stability of the microtubule cytoskeleton of bone-derived cells expressing Runx2. These changes have implications on a variety of processes related to metabolism, cellular stress response, and response to common chemotherapies. These studies help to shed light on the changes that take place as a tumor begins to metastasize and to better predict which therapies will benefit patients with bone metastasis. Bone metastasis of breast cancer results in severe bone loss, fractures, and death. Crosstalk between breast cancer cells and bone resident cells promotes osteoclast activity and the release of growth factors from the bone matrix resulting in aggressive tumor growth and bone loss. We and others have shown that Runt-related transcription factor-2 (Runx2) promotes metastatic tumor growth-associated bone loss. Breast cancer cells also induce autophagy to survive metabolic stress at the metastatic site. Recently, we reported a Runx2-dependent increase in autophagy. In this study, to examine the underlying mechanisms of metastasis and tumor resistance to stress, we used a bone metastatic isogenic variant of breast cancer MDA-MB-231 cells isolated from a xenograft tumor mouse model of metastasis. Our results with immunofluorescence and biochemical approaches revealed that Runx2 promotes microtubule (MT) stability to facilitate autophagy. Stable MTs are critical for autophagosome trafficking and display increased acetylation at Lysine 40 of alpha-tubulin. Runx2 silencing decreases acetylated alpha-tubulin levels. The expression levels of HDAC6 and alpha TAT1, which serve to regulate the acetylation of alpha-tubulin, were not altered with Runx2 silencing. We found that HDAC6 interaction with alpha-tubulin is inhibited by Runt-related factor-2 (Runx2). We show that the expression of wild-type Runx2 can restore the acetylated polymer of MTs in Runx2 knockdown cells, while the C-terminal deletion mutant fails to rescue the polymer of MTs. Importantly, cellular stress, such as glucose starvation also increases the acetylation of alpha-tubulin. We found that the loss of Runx2 increases the sensitivity of breast cancer cells to MT-targeting agents. Overall, our results indicate a novel regulatory mechanism of microtubule acetylation and suggest that Runx2 and acetylated microtubules may serve as therapeutic targets for bone metastatic tumors.