Bone metastasis is a frequent complication of breast cancer with nearly 70% of metastatic breast cancer patients developing bone metastasis during the course of their disease. The bone represents a dynamic microenvironment which provides a fertile soil for disseminated tumor cells, however, the mechanisms which regulate the interactions between a metastatic tumor and the bone microenvironment remain poorly understood. Recent studies indicate that during the metastatic process a bidirectional relationship between metastatic tumor cells and the bone microenvironment begins to develop. Metastatic cells display aberrant expression of genes typically reserved for skeletal development and alter the activity of resident cells within the bone microenvironment to promote tumor development, resulting in the severe bone loss. While transcriptional regulation of the metastatic process has been well established, recent findings from our and other research groups highlight the role of the autophagy and secretory pathways in interactions between resident and tumor cells during bone metastatic tumor growth. These reports show high levels of autophagy-related markers, regulatory factors of the autophagy pathway, and autophagy-mediated secretion of matrix metalloproteinases (MMP’s), receptor activator of nuclear factor kappa B ligand (RANKL), parathyroid hormone related protein (PTHrP), as well as WNT5A in bone metastatic breast cancer cells. In this review, we discuss the recently elucidated mechanisms and their crosstalk with signaling pathways, and potential therapeutic targets for bone metastatic disease.

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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 α-tubulin. Runx2 silencing decreases acetylated α-tubulin levels. The expression levels of HDAC6 and αTAT1, which serve to regulate the acetylation of α-tubulin, were not altered with Runx2 silencing. We found that HDAC6 interaction with α-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 α-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.

Bone metastasis of breast cancer is a significant cause of patient mortality. Recent studies suggest that metastatic cancer cells induce autophagy to survive metabolic stress. During autophagy, cytoplasmic components and damaged organelles are captured by autophagosomes followed by lysosomal fusion and degradation, releasing metabolites as energy sources to meet metabolic demands. Although the components of the autophagy pathway have been well characterized, the regulatory mechanisms of autophagy in metastatic cancer cells remain unknown. Previously, we have shown that Runt-related transcription factor-2 (Runx2) promotes cell survival, bone metastasis, and osteolysis. Using a bone metastatic isogenic variant of breast cancer MDA-MB-231, we examined levels of the autophagosome specific marker LC3B to define the regulation of autophagy during bone metastasis. Additionally, we examined whether Runx2 regulates autophagy for increased cell survival in the bone microenvironment. Microscopic and biochemical studies showed elevated levels of autophagic flux among bone derived cells relative to parental breast cancer cells. Interestingly, we also observed that Runx2 enhanced the turnover of autophagic vesicles while Runx2 silencing resulted in accumulation of vesicles due to reduced turnover. Interestingly, Runx2 knockdown or inhibition of autophagy in bone derived cells increased AMPK levels suggesting higher levels of cellular stress as a consequence of impaired autophagy. In addition to AMPK activity, MAP kinase mutations have been demonstrated to result in constitutive activation of the autophagy pathway. Treatment with the MEK inhibitor PD184161 resulted in accumulation of LC3B-II in control cells. Furthermore, Runx2 knockdown in bone derived cells display lower levels of ERK activity relative to controls. These results suggest that Runx2/ERK signaling is critical for autophagy in metastatic breast cancer cells. Our mechanistic studies revealed that Runx2 promotes autophagy by increasing acetylation of α-tubulin sub-units of microtubules and enhancing trafficking of autophagic vesicles. Introduction of a mutant α-tubulin construct incapable of being acetylated resulted in the accumulation of autophagic vesicles in control cells, similar to silencing of Runx2. Inhibition of autophagy resulted in decreased adhesion, migration, and survival of Runx2 knockdown cells. Furthermore, analysis of LC3B protein in clinical breast cancer specimens and tumor xenografts revealed significant association between high Runx2 and low LC3B protein levels. Our studies reveal a novel regulatory mechanism of autophagy via Runx2 and provide insights into the role of autophagy in bone metastatic breast cancer cells. Citation Format: Ahmad H. Othman, Manish Tandon, Vivek Ashok, Marcus Winogradzki, Gary Stein, Jitesh Pratap. Regulation of autophagy in bone metastatic breast cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 43.

Metastasis of breast cancer (BC) to bone results in severe bone loss, fractures, and death. The crosstalk between BC cells and bone resident cells dramatically increases osteoclast activity, resulting in the release of growth factors from the bone matrix that causes aggressive tumor growth and bone loss. A potentially important aspect of this process is vesicular trafficking on microtubules (MTs) which can affect the output of signaling pathways and secretory activity of metastatic bone cells. MTs are cytoskeletal filaments composed of heterodimers, α- and β-tubulin. The tubulin isotypes and their variety of post-translational modifications (PTMs) control the properties and functions of MT filaments, a concept known as the ‘tubulin code’. Recent studies show an emerging link between alterations of the tubulin code with poor prognosis of breast cancer. However, the regulation of the code in metastatic bone cells is currently unknown. Tubulin acetylation occurs via α-tubulin N-acetyl transferase-1 (αTAT1) and can be reversed by histone deacetylase-6 (HDAC6). MTs lacking acetylation lose flexural rigidity and are prone to breaks following repetitive bending during vesicular trafficking. We found that HDAC6 interaction with α-tubulin is inhibited by a Runt-related factor (Runx2). Our biochemical, mass spectrometry and IPs analyses of MTs revealed that loss of Runx2 can reduce (i) acetylation and stability of MTs, (ii) interaction of HSP90 with α-tubulin, and (iii) levels of β 2a-tubulin. Our studies with Runx2 mutants indicate that the C-terminal of Runx2 serves a scaffolding function by interacting with MTs and HDAC6. Confocal microscopy revealed reduced puncta and altered distribution of endosomal vesicles and autophagosomes with Runx2 silencing. As MT targeting agents are often used as chemotherapeutics, we found that loss of Runx2 sensitizes breast cancer cells to docetaxel and vinblastine and reduces the secretion of IL-6. We found MDA-MB-231 cells metastasized to the bone show significant differences in microtubule isotype expression, with 10 of 19 studied showing 1.3 to 4.1 fold increases. Further analysis of whether Runx2 knockdown affects isotype expression, we found the majority of the 19 studied remain unchanged. Immunohistochemistry for Ac-α-Tub levels in bone metastatic patient samples shows significantly more Ac-α-Tub positive cells in metastatic bone tumors than primary tumors. These findings suggest a novel control mechanism of MTs stability via Runx2-HDAC6 interactions that can impact trafficking and cellular activity. Our results indicate that inhibition of Runx2 may sensitize metastatic tumors to MT targeting agents, and Runx2/HDAC6/Ac-α-Tub levels may serve as markers for metastatic tumors to help stratify patients for optimal treatment for metastatic bone disease. Citation Format: Shreya Patel, Marcus Winogradzki, Ahmad Othman, Waddell Holmes, Jitesh Pratap. The novel control mechanism of the tubulin code and vesicular trafficking in breast cancer bone metastatic cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 270.