Opposing Roles of Mitogenic and Stress Signaling Pathways in the Induction of Cancer Dormancy

Cancer dormancy is a poorly understood stage of cancer progression. However, the ability to control this step of the disease offers novel therapeutic opportunities. Here we summarize recent findings that implicate the extracellular matrix and adhesion receptor signaling in the escape or induction of tumor dormancy. We further review evidence suggesting that imbalances in the activity ratio of ERK to p38 signaling may determine the fate (i.e. tumorigenicity vs. dormancy) of different carcinoma cells. Special attention is placed on the mechanisms that p38 signaling regulates during the induction of dormancy and how modulation of these pathways may offer a therapeutic opportunity. We also review evidence for a novel drug-resistance mechanism in dormant tumor cells that when blocked may enable killing of dormant tumor cells. Finally, we explore the notion that dormancy of tumor cells may be the result of a selective adaptive response that allows disseminated tumor cells to pause their growth and cope with stress signaling imposed by dissemination and/or treatment until growth can be restored.     

Engineered <Emphasis Type="Italic">In Vitro</Emphasis> Models of Tumor Dormancy and Reactivation

Metastatic recurrence is a major hurdle to overcome for successful control of cancer-associated death. Residual tumor cells in the primary site, or disseminated tumor cells in secondary sites, can lie in a dormant state for long time periods, years to decades, before being reactivated into a proliferative growth state. The microenvironmental signals and biological mechanisms that mediate the fate of disseminated cancer cells with respect to cell death, single cell dormancy, tumor mass dormancy and metastatic growth, as well as the factors that induce reactivation, are discussed in this review. Emphasis is placed on engineered, in vitro, biomaterial-based approaches to model tumor dormancy and subsequent reactivation, with a focus on the roles of extracellular matrix, secondary cell types, biochemical signaling and drug treatment. A brief perspective of molecular targets and treatment approaches for dormant tumors is also presented. Advances in tissue-engineered platforms to induce, model, and monitor tumor dormancy and reactivation may provide much needed insight into the regulation of these processes and serve as drug discovery and testing platforms.     

SMAD signaling and redox imbalance cooperate to induce prostate cancer cell dormancy

Metastasis involves the dissemination of single or small clumps of cancer cells through blood or lymphatic vessels and their extravasation into distant organs. Despite the strong regulation of metastases development by a cell dormancy phenomenon, the dormant state of cancer cells remains poorly characterized due to the difficulty of in vivo studies. We have recently shown in vitro that clonogenicity of prostate cancer cells is regulated by a dormancy phenomenon that is strongly induced when cells are cultured both at low cell density and in a slightly hypertonic medium. Here, we characterized by RT-qPCR a genetic expression signature of this dormant state which combines the presence of both stemness and differentiation markers. We showed that both TFGβ/BMP signaling and redox imbalance are required for the full induction of this dormancy signature and cell quiescence. Moreover, reconstruction experiments showed that TFGβ/BMP signaling and redox imbalance are sufficient to generate a pattern of genetic expression displaying all characteristic features of the dormancy signature. Finally, we observed that low cell density was sufficient to activate TGFβ/BMP signaling and to generate a slight redox imbalance thus priming cells for dormancy that can be attained with a co-stimulus like hypertonicity, most likely through an increased redox imbalance. The identification of a dual regulation of dormancy provides a framework for the interpretation of previous reports showing a restricted ability of BMP signaling to regulate cancer cell dormancy in vivo and draws attention on the role of oxidative stress in the metastatic process.     

High-throughput screening of cellular redox sensors using modern redox proteomics approaches

Cancer cells are characterized by higher levels of intracellular reactive oxygen species (ROS) due to metabolic aberrations. ROS are widely accepted as second messengers triggering pivotal signaling pathways involved in the process of cell metabolism, cell cycle, apoptosis, and autophagy. However, the underlying cellular mechanisms remain largely unknown. Recently, accumulating evidence has demonstrated that ROS initiate redox signaling through direct oxidative modification of the cysteines of key redox-sensitive proteins (termed redox sensors). Uncovering the functional changes underlying redox regulation of redox sensors is urgently required, and the role of different redox sensors in distinct disease states still remains to be identified. To assist this, redox proteomics has been developed for the high-throughput screening of redox sensors, which will benefit the development of novel therapeutic strategies for cancer treatment. Highlighted here are recent advances in redox proteomics approaches and their applications in identifying redox sensors involved in tumor development.     

Model of Tumor Dormancy/Recurrence after Short-Term Chemotherapy

Although many tumors regress in response to neoadjuvant chemotherapy, residual tumor cells are detected in most cancer patients post-treatment. These residual tumor cells are thought to remain dormant for years before resuming growth, resulting in tumor recurrence. Considering that recurrent tumors are most often responsible for patient mortality, there exists an urgent need to study signaling pathways that drive tumor dormancy/recurrence. We have developed an in vitro model of tumor dormancy/recurrence. Short-term exposure of tumor cells (breast or prostate) to chemotherapy at clinically relevant doses enriches for a dormant tumor cell population. Several days after removing chemotherapy, dormant tumor cells regain proliferative ability and establish colonies, resembling tumor recurrence. Tumor cells from “recurrent” colonies exhibit increased chemotherapy resistance, similar to the therapy resistance of recurrent tumors in cancer patients. Previous studies using long-term chemotherapy selection models identified acquired mutations that drive tumor resistance. In contrast, our short term chemotherapy exposure model enriches for a slow-cycling, dormant, chemo-resistant tumor cell sub-population that can resume growth after drug removal. Studying unique signaling pathways in dormant tumor cells enriched by short-term chemotherapy treatment is expected to identify novel therapeutic targets for preventing tumor recurrence.     

Osteoblastic protein kinase D1 contributes to the prostate cancer cells dormancy via GAS6-circadian clock signaling

Disseminated prostate cancer (PCa) is known to have a strong propensity for bone marrow. These disseminated tumor cells (DTCs) can survive in bone marrow for years without obvious proliferation, while maintaining the ability to develop into metastatic lesions. However, how DTCs kept dormant and recur is still uncertain. Here, we focus on the role of osteoblastic protein kinase D1 (PKD1) in PCa (PC-3 and DU145) dormancy using co-culture experiments. Using flow cytometry, western blotting, and immunofluorescence, we observed that in co-cultures osteoblasts could induce a dormant state in PCa cells, which is manifested by a fewer cell divisions, a decrease Ki-67-positive populations and a lower ERK/p38 ratio. In contrast, silencing of PKD1 gene in osteoblasts impedes co-cultured prostate cancer cell's dormancy ability. Mechanismly, protein kinase D1 (PKD1) in osteoblasts induces PCa dormancy via activating CREB1, which promoting the expression and secretion of growth arrest specific 6 (GAS6). Furthermore, GAS6-induced dormancy signaling significantly increased the expression of core circadian clock molecules in PCa cells, and a negative correlation of circadian clock proteins (BMAL1, CLOCK and DEC2) with recurrence-free survival is observed in metastatic prostate cancer patients. Interestingly, the expression of cell cycle factors (p21, p27, CDK1 and PCNA) which regulated by circadian clock also upregulated in response to GAS6 stimulation. Taken together, we provide evidence that osteoblastic PKD1/CREB1/GAS6 signaling regulates cellular dormancy of PCa cells, and highlights the importance of circadian clock in PCa cells dormancy.     

Immunogenic,cellular, and angiogenic drivers of tumor dormancy‐a melanoma view

In tumor cells, the ability to maintain viability over long time periods without proliferation is referred to as a state of dormancy. Maintenance of dormancy is controlled by numerous cellular and environmental factors, from immune surveillance and tumor–stroma interaction to intracellular signaling. Interference of dormancy (to an ‘awaken’ state) is associated with reduced response to therapy, resulting in relapse or in metastatic burst. Thus, maintaining a dormant state should prolong therapeutic responses and delay metastasis. Technical obstacles in studying tumor dormancy have limited our understanding of underlying mechanisms and hampered our ability to target dormant cells. In this review, we summarize the progress of research in the field of immunogenic, angiogenic, and cellular dormancy in diverse malignancies with particular attention to our current understanding in melanoma.     

细胞休眠在肿瘤耐药和复发中的作用（英文）

Despite progresses achieved in the therapy of tumors, the prognosis of patients is still limited by reccurence of residual tumor cells. Cancer cell dormancy plays a pivotal role in cancer relapse and drug resistance. In recent years, tumor cells undergoing EMT(epithelial-mesenchymal transition), CSCs(cancer stem cells) and CTCs(circulating tumor cells) are proved to share some common characteristics and show a cell cycle arrest phenotype. Thus, understanding the dormant stage of tumor cells could facilitate us in discovering ways to accelerate the development of tumor therapy and prevent its reccurence. In this review, we summarize the specific process of tumor cell dormancy induced by pharmacotherapy, and consider that dormancy is an initiative response rather than a passive defense to cytotoxicity. Besides, we probe into the mechanisms of tumor cell dormancy-mediated drug resistance, anticipating paving a way to target dormant tumor cells and result in better clinical outcomes.     

The BMP Inhibitor Coco Reactivates Breast Cancer Cells at Lung Metastatic Sites

The mechanistic underpinnings of metastatic dormancy and reactivation are poorly understood. A gain-of-function cDNA screen reveals that Coco, a secreted antagonist of TGF-β ligands, induces dormant breast cancer cells to undergo reactivation in the lung. Mechanistic studies indicate that Coco exerts this effect by blocking lung-derived BMP ligands. Whereas Coco enhances the manifestation of traits associated with cancer stem cells, BMP signaling suppresses it. Coco induces a discrete gene expression signature, which is strongly associated with metastatic relapse to the lung, but not to the bone or brain in patients. Experiments in mouse models suggest that these latter organs contain niches devoid of bioactive BMP. These findings reveal that metastasis-initiating cells need to overcome organ-specific antimetastatic signals in order to undergo reactivation.     

细胞休眠在肿瘤耐药和复发中的作用

目前对于肿瘤的药物治疗已经取得了一定的进展,然而,治疗后残存肿瘤细胞的复发仍然是导致肿瘤治疗失败的主要原因．肿瘤细胞休眠在肿瘤复发及耐药中发挥着重要的作用．近年来,研究发现上皮间质转化(epithelial-mesenchymal transition,EMT)的肿瘤细胞,肿瘤干细胞(cancer stem cells,CSCs)以及循环肿瘤细胞(circulating tumor cells,CTCs)都显示出细胞周期阻滞的状态．因此,对于休眠阶段肿瘤细胞的研究将可能促进肿瘤的治疗及预防肿瘤的复发．本文总结了药物治疗诱导肿瘤细胞发生休眠的具体过程,同时认为休眠是肿瘤细胞面对药物治疗所采取的主动防御措施,而非被动逃避过程．探究药物诱导性肿瘤细胞的休眠机制对于靶向休眠肿瘤细胞治疗及提高临床治疗效果都具有非常重要的意义．     

细胞分裂素调控种子发育、休眠与萌发的研究进展

种子萌发是子代植株建立、生长和繁育的重要阶段,在种子植物生命周期中起重要作用。种子休眠是在发育过程中形成的,在生理成熟期达到峰值。种子休眠与萌发的植物激素调控可能是种子植物中一种高度保守的机制。细胞分裂素(CK)是植物体内的一种重要信号分子,调控植物生长发育的许多方面。生物活性CK的水平由其生物合成、活化、失活、再活化...     

Control of the eukaryotic cell cycle by MAP kinase signaling pathways.

In an often rapidly changing environment, cells must adapt by monitoring and reacting quickly to extracellular stimuli detected by membrane-bound receptors and proteins. Reversible phosphorylation of intracellular regulatory proteins has emerged as a crucial mechanism effecting the transmission and modulation of such signals and is determined by the relative activities of protein kinases and phosphatases within the cell. These are often arranged into complex signaling networks that may function independently or be subject to cross-regulation. Recently, genetic and biochemical analyses have identified the universally conserved mitogen-activated protein (MAP) kinase cascade as one of the most ubiquitous signal transduction systems. This pathway is activated after a variety of cellular stimuli and regulates numerous physiological processes, particularly the cell division cycle. Progression through the cell cycle is critically dependent on the presence of environmental growth factors and stress stimuli, and failure to correctly integrate such signals into the cell cycle machinery can lead to the accumulation of genetic damage and genomic instability characteristic of cancer cells. Here we focus on the MAP kinase cascade and discuss the molecular mechanisms by which these extensively studied signaling pathways influence cell growth and proliferation.     

GAS6 Receptor Status Is Associated with Dormancy and Bone Metastatic Tumor Formation

Disseminated tumor cells (DTCs) are believed to lie dormant in the marrow before they can be activated to form metastases. How DTCs become dormant in the marrow and how dormant DTCs escape dormancy remains unclear. Recent work has shown that prostate cancer (PCa) cell lines express the growth-arrest specific 6 (GAS6) receptors Axl, Tyro3, and Mer, and become growth arrested in response to GAS6. We therefore hypothesized that GAS6 signaling regulates the proliferative activity of DTCs in the marrow. To explore this possibility, in vivo studies were performed where it was observed that when Tyro3 expression levels exceed Axl expression, the PCa cells exhibit rapid growth. When when Axl levels predominate, PCa cells remain largely quiescent. These findings suggest that a balance between the expression of Axl and Tyro3 is associated with a molecular switch between a dormant and a proliferative phenotype in PCa metastases.     

A dormant state modulated by osmotic pressure controls clonogenicity of prostate cancer cells

Cell dormancy constitutes a limiting step of the metastatic process by preventing the proliferation of isolated cancer cells disseminated at distant sites from the primary tumor. The study of cancer cell dormancy is severely hampered by the lack of biological samples so that the mechanisms that regulate cell dormancy have not been extensively explored. In this work, we describe the rapid induction in vitro of a dormant state in prostate cancer cells by exposure to a slightly hypertonic growth medium. This quiescence is observed only when cells are seeded at low density and, once established, requires additional stimuli besides osmotic pressure to be reversed. Media conditioned by cells grown at high density can partially prevent or reverse dormancy, a phenomenon which can be reproduced with citric acid. In addition to this role of small metabolites, inactivation of the p53 and smad pathways also counters the entry into dormancy, whereas exposure to activin A induces it to some extent. Thus, this easily inducible dormancy reproduces several features associated with the dormancy of stem cells and cancer cells in vivo.     

Dormant Wnt-initiated mammary cancer can participate in reconstituting functional mammary glands

The minimal residual disease foci that beget breast cancer relapse after a period of disease dormancy remain uncharacterized despite their enormous clinical importance. To model dormant breast cancer in vivo, we employed a transgenic mouse model in which Wnt1-initiated mammary cancer is doxycycline dependent. After regression of Wnt-dependent cancers, subclinical disease lesions were propagated in vivo using classical tissue recombination techniques. Surprisingly, outgrowths derived from dormant malignant tissue reconstituted morphologically normal ductal trees in wild-type mammary fat pads. Whereas hyperplasia-derived outgrowths remained benign, outgrowths derived from dormant malignancy underwent a morphological transition suggesting single-step transformation following reactivation of Wnt signaling and rapidly yielded invasive mammary tumors. Remarkably, outgrowths derived from dormant malignancy could be serially propagated in vivo and retained the potential to undergo lobuloalveolar differentiation in response to hormones of pregnancy. Matching somatic H-Ras mutations shared by antecedent tumors and descendant mammary ductal outgrowths confirmed their clonal relatedness. Thus, propagation of epithelium that possesses a latent malignant growth program reveals impressive regenerative and developmental potential, supporting the notion that dormant mammary cancers harbor transformed mammary progenitor cells. Our results define an experimental paradigm for elucidating biological properties of dormant malignancy.     

Dormancy of Solitary Metastatic Cells

After arriving in a secondary site metastatic cells either begin proliferating, undergo apoptosis or remain as solitary dormant cells. The process of metastasis, although dangerous, is extremely inefficient with the majority of the cells undergoing apoptosis and thus becoming clinically irrelevant. Of the cells that begin proliferating, the few that make it past the micrometastasis stage may be of immediate clinical relevance. Dormant cells, while not of immediate clinical concern, are believed to be at least in part responsible for cancer recurrence that can occur decades after apparently successful initial treatment. Dormant solitary cells are different from “dormant” micrometastases, in which active proliferation is balanced by apoptosis. The mechanisms of cell cycle regulation and the function of the molecules regulating this process are well understood. However, there is relatively little known about the mechanisms controlling cell cycle regulation and dormancy of solitary metastatic cells. There are several inherent difficulties impeding the study of solitary cells. This review paper will examine the models used in the study of dormant solitary metastatic cells, methods of imaging and studying these cells, the molecular mechanisms believed to be responsible for solitary cell dormancy, and finally the unique treatment challenges posed by these cells.