Molecular Regulators of Embryonic Diapause and Cancer Diapause-like State

New insights into how to induce and maintain embryonic diapause in the blastocyst

Embryonic diapause in mammals is a period of developmental pause of the embryo at the blastocyst stage. During diapause, the blastocyst has minimal cell proliferation, metabolic activity and gene expression. At reactivation, blastocyst development resumes, characterised by increases in cell number, biosynthesis and metabolism. Until recently, it has been unknown how diapause is maintained without any loss of blastocyst viability. This review focuses on recent progress in the identification of molecular pathways occurring in the blastocyst that can both cause and maintain the diapause state. A switch to lipid metabolism now appears essential to maintaining the diapause state and is induced by forkhead box protein O1. The forkhead box protein O transcription family is important for diapause in insects, nematodes and fish, but this is the first time a conclusive role has been established in mammals. Multiple epigenetic modifications are also essential to inducing and maintaining the diapause state, including both DNA and RNA methylation mechanisms. Finally, it now appears that diapause embryos, dormant stem cells and chemotherapeutic-resistant cancer cells may all share a universal system of quiescence.

Unveiling the role of cellular dormancy in cancer progression and recurrence

PURPOSE OF REVIEW

Cellular dormancy is a major contributor to cancer progression and recurrence. This review explores recent findings on the molecular mechanisms implicated in cancer dormancy and investigates potential strategies to improve therapeutic interventions.

RECENT FINDINGS

Research on cancer dormancy reveals a complex and multifaceted phenomenon. Providing a latent reservoir of tumor cells with reduced proliferation and enhanced drug-tolerance, dormant cancer cells emerge from a clonally diverse population after therapy or at metastatic sites. These cells exhibit distinct transcriptional and epigenetic profiles, involving the downregulation of Myc and mechanistic target of rapamycin (mTOR) pathways, and the induction of autophagy. Senescence traits, under the control of factors such as p53, also contribute significantly. The tumor microenvironment can either promote or prevent dormancy establishment, notably through the involvement of T and NK cells within the dormant tumor niche. Strategies to combat dormancy-related relapse include direct elimination of dormant tumor cells, sustaining dormancy to prolong survival, or awakening dormant cells to re-sensitize them to antiproliferative drugs.

SUMMARY

Improving our understanding of cancer dormancy at primary and secondary sites provides valuable insights into patient care and relapse prevention.

The molecular mechanisms of diapause and diapause-like reversible arrest

Diapause is a protective mechanism that many organisms deploy to overcome environmental adversities. Diapause extends lifespan and fertility to enhance the reproductive success and survival of the species. Although diapause states have been known and employed for commercial purposes, for example in the silk industry, detailed molecular and cell biological studies are an exciting frontier. Understanding diapause-like protective mechanisms will shed light on pathways that steer organisms through adverse conditions. One hope is that an understanding of the mechanisms that support diapause might be leveraged to extend the lifespan and/or health span of humans as well as species threatened by climate change. In addition, recent findings suggest that cancer cells that persist after treatment mimic diapause-like states, implying that these programs may facilitate cancer cell survival from chemotherapy and cause relapse. Here, we review the molecular mechanisms underlying diapause programs in a variety of organisms, and we discuss pathways supporting diapause-like states in tumor persister cells.

Fight, flight, and freeze!

Just like time and tide, embryonic development waits for no man but progresses forcefully to its completion, with just one exception. Diapause is an enigmatic, reversible, dormant halt that protects the developing embryo. Cancer cells have evolved to hijack many useful stem cell capabilities, and diapause is no exception. Recent work has revealed a diapause-like cancer cell state, prompting the quest for its key molecular regulators useful for cancer therapies. The present paper by Sun et al.1 addresses this knowledge gap by revealing a key player in regulating the diapause-like cancer cell stage, the condensin protein SMC4.

Always stressed but never exhausted: how stem cells in myeloid neoplasms avoid extinction in inflammatory conditions

Chronic or recurrent episodes of acute inflammation cause attrition of normal hematopoietic stem cells (HSCs) that can lead to hematopoietic failure but they drive progression in myeloid malignancies and their precursor clonal hematopoiesis. Mechanistic parallels exist between hematopoiesis in chronic inflammation and the continuously increased proliferation of myeloid malignancies, particularly myeloproliferative neoplasms (MPNs). The ability to enter dormancy, a state of deep quiescence characterized by low oxidative phosphorylation, low glycolysis, reduced protein synthesis, and increased autophagy is central to the preservation of long-term HSCs and likely MPN SCs. The metabolic features of dormancy resemble those of diapause, a state of arrested embryonic development triggered by adverse environmental conditions. To outcompete their normal counterparts in the inflammatory MPN environment, MPN SCs co-opt mechanisms used by HSCs to avoid exhaustion, including signal attenuation by negative regulators, insulation from activating cytokine signals, anti-inflammatory signaling, and epigenetic reprogramming. We propose that new therapeutic strategies may be derived from conceptualizing myeloid malignancies as an ecosystem out of balance, in which residual normal and malignant hematopoietic cells interact in multiple ways, only few of which have been characterized in detail. Disrupting MPN SC insulation to overcome dormancy, interfering with aberrant cytokine circuits that favor MPN cells, and directly boosting residual normal HSCs are potential strategies to tip the balance in favor of normal hematopoiesis. Although eradicating the malignant cell clones remains the goal of therapy, rebalancing the ecosystem may be a more attainable objective in the short term.

KDM5D Histone Demethylase Identifies Platinum-Tolerant Head and Neck Cancer Cells Vulnerable to Mitotic Catastrophe

Head and neck squamous cell carcinoma (HNSCC) is a major contributor to cancer incidence globally and is currently managed by surgical resection followed by adjuvant chemoradiotherapy. However, local recurrence is the major cause of mortality, indicating the emergence of drug-tolerant persister cells. A specific histone demethylase, namely lysine-specific demethylase 5D (KDM5D), is overexpressed in diverse types of cancers and involved in cancer cell cycle regulation. However, the role of KDM5D in the development of cisplatin-tolerant persister cells remains unexplored. Here, we demonstrated that KDM5D contributes to the development of persister cells. Aurora Kinase B (AURKB) disruption affected the vulnerability of persister cells in a mitotic catastrophe–dependent manner. Comprehensive in silico, in vitro, and in vivo experiments were performed. KDM5D expression was upregulated in HNSCC tumor cells, cancer stem cells, and cisplatin-resistant cells with biologically distinct signaling alterations. In an HNSCC cohort, high KDM5D expression was associated with a poor response to platinum treatment and early disease recurrence. KDM5D knockdown reduced the tolerance of persister cells to platinum agents and caused marked cell cycle deregulation, including the loss of DNA damage prevention, and abnormal mitosis-enhanced cell cycle arrest. By modulating mRNA levels of AURKB, KDM5D promoted the generation of platinum-tolerant persister cells in vitro, leading to the identification of the KDM5D/AURKB axis, which regulates cancer stemness and drug tolerance of HNSCC. Treatment with an AURKB inhibitor, namely barasertib, resulted in a lethal consequence of mitotic catastrophe in HNSCC persister cells. The cotreatment of cisplatin and barasertib suppressed tumor growth in the tumor mouse model. Thus, KDM5D might be involved in the development of persister cells, and AURKB disruption can overcome tolerance to platinum treatment in HNSCC.

Stress-induced reversible cell-cycle arrest requires PRC2/PRC1-mediated control of mitophagy in Drosophila germline stem cells and human iPSCs

Following acute genotoxic stress, both normal and tumorous stem cells can undergo cell-cycle arrest to avoid apoptosis and later re-enter the cell cycle to regenerate daughter cells. However, the mechanism of protective, reversible proliferative arrest, "quiescence," remains unresolved. Here, we show that mitophagy is a prerequisite for reversible quiescence in both irradiated Drosophila germline stem cells (GSCs) and human induced pluripotent stem cells (hiPSCs). In GSCs, mitofission (Drp1) or mitophagy (Pink1/Parkin) genes are essential to enter quiescence, whereas mitochondrial biogenesis (PGC1α) or fusion (Mfn2) genes are crucial for exiting quiescence. Furthermore, mitophagy-dependent quiescence lies downstream of mTOR- and PRC2-mediated repression and relies on the mitochondrial pool of cyclin E. Mitophagy-dependent reduction of cyclin E in GSCs and in hiPSCs during mTOR inhibition prevents the usual G1/S transition, pushing the cells toward reversible quiescence (G0). This alternative method of G1/S control may present new opportunities for therapeutic purposes.