Alpha-Tubulin Acetylation: A Key Player in Breast Cancer Metastasis
Introduction
Breast cancer (BC) is the most frequently diagnosed cancer and is associated with the most cancer-related deaths in women globally1. BC occurs in women of all ages after puberty1. In 2022, 2.3 million women were diagnosed with BC causing approximately 670,000 deaths worldwide1. Despite the progress made in the early detection and treatment of BC, metastasis, however, significantly complicates treatment and remains the major cause of cancer-related deaths2,3. Metastasis refers to the process in which cancer cells spread from the primary tumor site to establish in different anatomical sites2. These spreading cells are hard to treat, grow rapidly, and can lead to organ failure at the metastasized site4. Thus, understanding the detailed molecular mechanisms that drive BC metastasis will be crucial in developing more effective therapeutic interventions.
Acetylation of α-tubulin is one such mechanism that has been linked with BC progression and metastasis3,5–7. It is a posttranslational modification (PTM) that typically occurs on lysine 40 of α-tubulin—a critical protein that dimerizes with β-tubulin. These heterodimers are the building blocks that polymerize to form microtubules (MTs) within cells5,8. PTMs such as acetylation and detyrosination have been associated with cell transformation in cancer9. For example, the acetylation of α-tubulin has been shown to enhance cell attachment, migration, and reattachment providing a selective advantage for metastatic potential7. These modifications often correlate with poor cancer outcome and enhanced metastatic ability, providing a rationale for targeting them as potential therapeutics7,9. This newsletter will explore the role of α-tubulin acetylation in BC metastasis, its biological significance, and its therapeutic potential.
α-Tubulin Acetylation regulates MT stability and cellular function
Acetylation of α-tubulin is one of the conserved PTMs found in tubulin proteins and is associated with stabilized MTs8. It typically occurs in the inner luminal surface of the MT, which affects cellular functions such as MT dynamics, cell migration, autophagy, intracellular trafficking, and cell adhesion8,10,11. Acetylation of the conserved lysine 40 (K40) residue of α-tubulin is catalyzed by a conserved α-tubulin acetyltransferase (Mec-17/ATAT1) by transferring an acetyl group from acetyl-coenzyme A to the side chain of lysine 4012. De-acetylation of α-tubulin is regulated by histone deacetylase 6 (HDAC6) and NAD+-dependent deacetylase SIRT213. These enzymes regulate the level of α-tubulin acetylation in the cells12.
Early studies investigating the role of tubulin acetylation in MT stability reached conflicting findings suggesting that tubulin acetylation was likely a result of MT stability, not a factor that caused it12. Hubbert et al showed that HDAC6 overexpression led to increased cell motility, which they suggested may be due to decreased MT stability caused by reduced tubulin acetylation14. Pallazzo and colleagues disputed this finding by showing that inhibiting HDAC6 to increase tubulin acetylation did not increase the level of stable MTs in vitro and suggested that stability may be caused by other mechanisms15. In support of Hubbert and colleagues, recent studies showed that α-tubulin acetylation protected MTs from mechanical aging and breakage16,17, reduced interactions between protofilaments, and enhanced protofilament sliding which led to increased MT flexibility8,13. Similarly, reduced levels of K40 acetylation are linked to many cellular defects associated with axonal transport defects in many human diseases13. Based on these studies, it is apparent that acetylation of α-tubulin enhances the stability of MTs, making them more resistant to mechanical stress and less prone to depolymerization (see figure 1)8. This increased stability is crucial for various cellular processes, including cell migration and invasion—key steps in the metastatic spread of cancer cells12,18. Thus, regulation of α-tubulin acetylation level could be a key target to maintain MT dynamics and cellular functions, particularly in metastasis.
Above: Acetylation of α-Tubulin promotes MT stability and reduces flexural rigidity making them more resistant to mechanical bending and disassembly. Adapted from Janke and Magiera, 202019.
α-Tubulin Acetylation promotes metastasis in breast cancer
MTs are important components of the cytoskeleton and are critical for an array of cellular functions such as mitotic chromosome segregation, cell shape and motility, and vesicle and cargo transport20. Post-translational alteration of MTs has been implicated in metastatic dissemination21. Specifically, α-tubulin acetylation— a hallmark of stabilized MTs— has been closely associated with tumor migration and reattachment that mediate metastasis7. In BC, high levels of α-tubulin acetylation have been implicated in metastatic processes3,7,22–25. Invasive BC cells are characterized by high frequencies of long and dynamic MT-based membrane protrusions known as microtentacles (McTN)26. These McTN promote cell–cell aggregation and facilitate the reattachment of tumor cells necessary for cancer dissemination26. Boggs and colleagues reported that an increased level of α-tubulin acetylation can lead to BC metastasis7. The authors also found high levels of α-tubulin acetylation extending along the McTN protrusions in suspended metastatic BC cells which enhanced adhesion and invasion7. Moreover, comparative analysis of primary and metastatic tumors in patients found an increased level of α-tubulin acetylation in lymph node metastases compared with primary tumors7. It has also been demonstrated that α-tubulin acetylation promotes BC progression by preventing excessive endoplasmic reticulum stress via downregulation of gene expression related to cancer-related pathways24. A recent study showed that elevated levels of hydrogen peroxide in the BC cell lines caused acetylation of α-tubulin that mediated metastasis and poor BC prognosis3. Mechanistically, a transcription factor RUNX2 has been shown to enhance autophagy by increasing acetylation of α-tubulin; thus, promoting metastasis in BC cells23. Low expression of MT-associated protein, tektin4, enhanced TNBC metastasis by destabilizing MTs via HDAC6-mediated deacetylation27. Inhibition of α-tubulin acetylation by novel MT acetylation-specific inhibitors (GM-90257 and GM-90631) in vitro and in vivo led to suppression of tumor activities and metastasis via JNK/AP-1 pathways28. Altogether, these studies suggest that elevated levels of acetylated α-tubulin are associated with aggressive and metastatic behavior in BC implying that high acetylation levels could serve as a prognostic marker, indicating a worse outcome for BC patients.
Therapeutic Potential and Future Prospects
Acetylation of α-tubulin plays a crucial role in BC dissemination and metastasis7. Thus, regulation of α-tubulin acetylation and/or its associated regulatory proteins may be a promising therapeutic approach for treating BC metastasis. Specifically, small molecule inhibitors that can reduce α-tubulin acetylation level and ultimately impair the metastatic spread of BC cells could be a potential therapeutic option. Knock-in and knock-out gain of functions studies have uncovered key proteins, enzymes and molecules that can potentially inhibit α-tubulin acetylation6,22,27,28. Similarly, noncoding RNAs, such as long noncoding RNA (lncRNA) and microRNA (or miRNA) have been shown to modulate the expression of acetylated tubulin and could potentially be therapeutic targets for BC progression5. In addition, α-tubulin acetyltransferase (Mec-17/ATAT1) and its deacetylase counterparts HDAC6 and SIRT2 enzyme modulators are gaining attention for their anti-cancer properties29.
However, despite evidence supporting the use of these molecules as therapeutic targets for BC, several challenges still exist. Notably, information on the underlying mechanisms of these molecule inhibitors which can impair cancer progression is limited30. Moreover, the specificity of such therapies evokes high concern due to unintended off-target effects that may impair normal cellular functions. Thus, further research is warranted to fully elucidate the complex regulatory networks involving α-tubulin acetylation and to identify the most effective therapeutic targets within these pathways.
References
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