ß-heavy-spectrin stabilizes the constricting contractile ring during cytokinesis.

Cytokinesis requires the constriction of an actomyosin-based contractile ring and involves multiple F-actin crosslinkers. We show that partial depletion of the C. elegans cytokinetic formin generates contractile rings with low F-actin levels that constrict but are structurally fragile, and we use this background to investigate the roles of the crosslinkers plastin/PLST-1 and ß-heavy-spectrin/SMA-1 during ring constriction. We show that the removal of PLST-1 or SMA-1 has opposite effects on the structural integrity of fragile rings. PLST-1 loss reduces cortical tension that resists ring constriction and makes fragile rings less prone to ruptures and regressions, whereas SMA-1 loss exacerbates structural defects, leading to frequent ruptures and cytokinesis failure. Fragile rings without SMA-1 or containing a shorter SMA-1, repeatedly rupture at the same site, and SMA-1::GFP accumulates at repair sites in fragile rings and in rings cut by laser microsurgery. These results establish that ß-heavy-spectrin stabilizes the constricting ring and reveals the importance of ß-heavy-spectrin size for network connectivity at low F-actin density.

Plastin and spectrin cooperate to stabilize the actomyosin cortex during cytokinesis.

Cytokinesis, the process that partitions the mother cell into two daughter cells, requires the assembly and constriction of an equatorial actomyosin network. Different types of non-motor F-actin crosslinkers localize to the network, but their functional contribution remains poorly understood. Here, we describe a synergy between the small rigid crosslinker plastin and the large flexible crosslinker spectrin in the C. elegans one-cell embryo. In contrast to single inhibitions, co-inhibition of plastin and the ßH-spectrin (SMA-1) results in cytokinesis failure due to progressive disorganization and eventual collapse of the equatorial actomyosin network. Cortical localization dynamics of non-muscle myosin II in co-inhibited embryos mimic those observed after drug-induced F-actin depolymerization, suggesting that the combined action of plastin and spectrin stabilizes F-actin in the contractile ring. An in silico model predicts that spectrin is more efficient than plastin at stabilizing the ring and that ring formation is relatively insensitive to ßH-spectrin length, which is confirmed in vivo with a sma-1 mutant that lacks 11 of its 29 spectrin repeats. Our findings provide the first evidence that spectrin contributes to cytokinesis and highlight the importance of crosslinker interplay for actomyosin network integrity.

Network Contractility During Cytokinesis-from Molecular to Global Views.

Cytokinesis is the last stage of cell division, which partitions the mother cell into two daughter cells. It requires the assembly and constriction of a contractile ring that consists of a filamentous contractile network of actin and myosin. Network contractility depends on network architecture, level of connectivity and myosin motor activity, but how exactly is the contractile ring network organized or interconnected and how much it depends on motor activity remains unclear. Moreover, the contractile ring is not an isolated entity; rather, it is integrated into the surrounding cortex. Therefore, the mechanical properties of the cell cortex and cortical behaviors are expected to impact contractile ring functioning. Due to the complexity of the process, experimental approaches have been coupled to theoretical modeling in order to advance its global understanding. While earlier coarse-grained descriptions attempted to provide an integrated view of the process, recent models have mostly focused on understanding the behavior of an isolated contractile ring. Here we provide an overview of the organization and dynamics of the actomyosin network during cytokinesis and discuss existing theoretical models in light of cortical behaviors and experimental evidence from several systems. Our view on what is missing in current models and should be tested in the future is provided.

Anti-tumor efficacy of new 7α-substituted androstanes as aromatase inhibitors in hormone-sensitive and resistant breast cancer cells.

The majority of breast cancer cases are estrogen receptor positive (ER+). Although, third-generation aromatase inhibitors (AIs) are used as first-line treatment in post-menopausal women, they cause endocrine resistance and bone loss, which limits their success. Therefore, there is a demand to discover new potent molecules, with less toxicity that can circumvent these drawbacks. Our group has previously demonstrated that new 7α-substituted steroidal molecules, 7α-(2ξ,3ξ-epoxypropyl)androsta-1,4-diene-3,17-dione (3), 7α-allylandrost-4-ene-3,17-dione (6), 7α-allylandrost-4-en-17-one (9), 7α-allyl-3-oxoandrosta-1,4-dien-17ß-ol (10) and 7α-allylandrosta-1,4-diene-3,17-dione (12) are potent AIs in placental microsomes. In this work, it was investigated their anti-aromatase activity and in vitro effects in sensitive and resistant breast cancer cells. All the steroids efficiently inhibit aromatase in breast cancer cells, allowing to establish new structure-activity relationships for this class of compounds. Moreover, the new AIs can inhibit breast cancer cell growth, by causing cell cycle arrest and apoptosis. The effects of AIs 3 and 12 on sensitive cells were dependent on aromatase inhibition and androgen receptor (AR), while for AI 9 and AI 10 were AR- and ER-dependent, respectively. In addition, it was shown that all the AIs can sensitize resistant cancer cells being their behavior similar to the sensitive cells. In summary, this study contributes to the understanding of the structural modifications in steroidal scaffold that are translated into better aromatase inhibition and anti-tumor properties, providing important information for the rational design/synthesis of more effective AIs. In addition, allowed the discovery of new potent 7α-substituted androstane molecules to inhibit tumor growth and prevent endocrine resistance.

Unravelling exemestane: From biology to clinical prospects.

Aromatase inhibitors (AIs) are anti-tumor agents used in clinic to treat hormone-dependent breast cancer. AIs block estrogens biosynthesis by inhibiting the enzyme aromatase, preventing tumor progression. Exemestane, a third-generation steroidal AI, belongs to this class of drugs and is currently used in clinic to treat postmenopausal women, due to its high efficacy and good tolerability. Here, its pharmacological and biological aspects as well as its clinical applications and comparison to other endocrine therapeutic agents, are reviewed. It is also focused the benefits and risks of exemestane, drawbacks to be overcome and aspects to be explored.