Renu Mohan

Microtubule Assembly Dynamics: An Attractive Target for Anticancer Drugs

Microtubules, composed of αβ tubulin dimers, are dynamic polymers of eukaryotic cells. They play important roles in various cellular functions including mitosis. Microtubules exhibit differential dynamic behaviors during different phases of the cell cycle. Inhibition of the microtubule assembly dynamics causes cell cycle arrest leading to apoptosis; thus, qualifying them as important drug targets for treating several diseases including cancer, neuronal, fungal, and parasitic diseases. Although several microtubule‐targeted drugs are successfully being used in cancer chemotherapy, the development of resistance against these drugs and their inherent toxicities warrant the development of new agents with improved efficacy. Several antimicrotubule agents are currently being evaluated for their possible uses in cancer chemotherapy. Benomyl, griseofulvin, and sulfonamides have been used as antifungal and antibacterial drugs. Recent reports have shown that these drugs have potent antitumor potential. These agents are shown to inhibit proliferation of different types of tumor cells and induce apoptosis by targeting microtubule assembly dynamics. However, unlike vincas and taxanes, which inhibit cancer cell proliferation in nanomolar concentration range, these agents act in micromolar range and are considered to have limited toxicities. Here, we suggest that these drugs may have a significant use in cancer chemotherapy when used in combination with other anticancer drugs.

Kinetic Stabilization of Microtubule Dynamics by Estramustine Is Associated with Tubulin Acetylation, Spindle Abnormalities, and Mitotic Arrest

Estramustine (EM) alone or in combination with other anticancer agents is clinically used for the treatment of hormone refractory prostate cancer. Furthermore, EM has been shown to potently inhibit the proliferation of different types of cancer cells in culture apparently by targeting microtubules; however, the antiproliferative mechanism of action of EM is not clear. In this work, we have shown that EM strongly suppressed the dynamic instability of individual microtubules in MCF-7 cells by reducing the rates of growing and shortening excursions and increasing the time microtubule spent in the pause state. At its half maximal proliferation inhibitory concentration (IC(50)), EM exerted strong suppressive effects on the dynamics of microtubules in MCF-7 cells without detectably affecting either the organization or the polymerized mass of microtubules. At relatively high concentrations (5 x IC(50)), EM significantly depolymerized microtubules in the cells. Furthermore, the microtubules were found highly acetylated, supporting the conclusion that they were stabilized by the drug. EM treatment induced spindle abnormalities in MCF-7 cells, and a major population of the arrested mitotic cells was multipolar. EM also perturbed the microtubule-kinetochore interaction, thereby activating the spindle assembly checkpoint and leading to apoptotic cell death.

The Morita–Baylis–Hillman adducts of β-aryl nitroethylenes with other activated alkenes: synthesis and anticancer activity studies

The Morita-Baylis-Hillman (MBH) adducts of beta-aryl nitroethylenes with methyl vinyl ketone (MVK) and acrylate, formed in moderate to good yield when mediated by imidazole/LiCl in THF at room temperature, inhibit HeLa cell proliferation by binding to tubulin.

Inhibition of HDAC6 Deacetylase Activity Increases Its Binding with Microtubules and Suppresses Microtubule Dynamic Instability in MCF-7 Cells

Background: The causal relationship between tubulin acetylation and microtubule stability has remained poorly understood. Results: Pharmacological inhibition of HDAC6 increased HDAC6 binding to microtubules, enhanced microtubule stability, and suppressed dynamics. Conclusion: Increased binding of HDAC6, rather than acetylation per se, causes microtubule stability. Significance: The study indicates a MAP-like function of HDAC6 that extends beyond its tubulin deacetylase function. The post-translational modification of tubulin appears to be a highly controlled mechanism that regulates microtubule functioning. Acetylation of the ϵ-amino group of Lys-40 of α-tubulin marks stable microtubules, although the causal relationship between tubulin acetylation and microtubule stability has remained poorly understood. HDAC6, the tubulin deacetylase, plays a key role in maintaining typical distribution of acetylated microtubules in cells. Here, by using tubastatin A, an HDAC6-specific inhibitor, and siRNA-mediated depletion of HDAC6, we have explored whether tubulin acetylation has a role in regulating microtubule stability. We found that whereas both pharmacological inhibition of HDAC6 as well as its depletion enhance microtubule acetylation, only pharmacological inhibition of HDAC6 activity leads to an increase in microtubule stability against cold and nocodazole-induced depolymerizing conditions. Tubastatin A treatment suppressed the dynamics of individual microtubules in MCF-7 cells and delayed the reassembly of depolymerized microtubules. Interestingly, both the localization of HDAC6 on microtubules and the amount of HDAC6 associated with polymeric fraction of tubulin were found to increase in the tubastatin A-treated cells compared with the control cells, suggesting that the pharmacological inhibition of HDAC6 enhances the binding of HDAC6 to microtubules. The evidence presented in this study indicated that the increased binding of HDAC6, rather than the acetylation per se, causes microtubule stability. The results are in support of a hypothesis that in addition to its deacetylase function, HDAC6 might function as a MAP that regulates microtubule dynamics under certain conditions.

Synthesis and anticancer activity studies of α-aminoalkylated conjugated nitroalkenes

Novel alpha-aminoalkylated conjugated nitroalkenes which inhibit human cervical cancer (HeLa) cell proliferation by binding to tubulin were synthesized by imidazole/LiCl-mediated reaction of conjugated nitroalkenes with N-tosylimines.

Microtubule-associated proteins as direct crosslinkers of actin filaments and microtubules.

The cytoskeletal polymers—actin, microtubules, and intermediate filaments—are interlinked by coordinated protein interactions to form a complex three‐dimensional cytoskeletal network. Association of actin filaments with microtubules is important for various cellular processes such as cell division, migration, vesicle and organelle transport, and axonal growth. Several proteins including signaling molecules, motor proteins, and proteins directly or indirectly associated with microtubules and actin are involved in bridging the cytoskeletal components. Microtubule‐associated proteins (MAPs) belonging to the MAP1, 2, 4 family and Tau proteins have been identified as key players that directly crosslink the two cytoskeletons. This review summarizes the current understanding of the interactions of these MAPs with actin filaments and their role in forming the actin–microtubule network and further discusses how the in vitro reconstitution assays can be used to study the dynamics of coordinated networks. Understanding the mechanisms by which actin and microtubules interact is key to decipher cancer, wound healing, and neuronal regeneration.