Network of mutually repressive metastasis regulators can promote cell heterogeneity and metastatic transitions

Abstract

The sources and consequences of nongenetic variability in metastatic progression are largely unknown. To address these questions, we characterized a transcriptional regulatory network for the metastasis suppressor Raf kinase inhibitory protein (RKIP). We previously showed that the transcription factor BACH1 is negatively regulated by RKIP and promotes breast cancer metastasis. Here we demonstrate that BACH1 acts in a double-negative (overall positive) feedback loop to inhibit RKIP transcription in breast cancer cells. BACH1 also negatively regulates its own transcription. Analysis of the BACH1 network reveals the existence of an inverse relationship between BACH1 and RKIP involving both monostable and bistable transitions that can potentially give rise to nongenetic variability. Single-cell analysis confirmed monostable and bistablelike behavior. Treatment with histone deacetylase inhibitors or depletion of the polycomb repressor enhancer of zeste homolog 2 altered relative RKIP and BACH1 levels in a manner consistent with a prometastatic state. Together, our results suggest that the mutually repressive relationship between metastatic regulators such as RKIP and BACH1 can play a key role in determining metastatic progression in cancer.ancer progression is an evolutionary process of variant cells competing to expand first locally and then distally within the human body. Ultimately, tumor evolution generates metastatic lesions that account for over 90% of cancer deaths (1). Whereas the requirement of heritably variant cells for tumor progression is undisputed, how they emerge is much less clear. Traditionally, genetic mutations causing oncogene activation or tumor suppressor loss were considered essential (2). However, accumulating evidence for tumor-promoting epigenetic, microenvironmental, and stochastic forms of heritable variation is challenging the traditional view (3, 4). Genetically identical tumor cells show highly variable responses to apoptosis-inducing ligands (5) and chemotherapeutic drugs (6, 7) attributable to cell-cell differences in gene expression and pathway activity. Tumor cells diversify in vitro (8) and in vivo (9) showing different levels of stem cell marker expression. However, the molecular mechanisms and interactions controlling such nongenetic forms of diversity are not fully known (10, 11).Multiple studies have implicated positive feedback loops in amplifying and maintaining stochastic fluctuations, creating heritable diversity in genetically homogenous cell populations (12, 13). This heritable nongenetic diversity then generates random subpopulations that can survive during various forms of environmental stress (14, 15), enabling subsequent evolutionary adaptation (16). Understanding how feedback loops and other network structures may affect nongenetic heterogeneity and contribute to metastatic cancer progression will be crucial for combating the disease (3, 10). However, the regulation of metastasis-related genes is incompletely understood, and the role of regulatory network-mediated n...

Toward Decoding the Principles of Cancer Metastasis Circuits

“…RKIP activates let-7 indirectly, which inhibits BACH1, and BACH1 inhibits RKIP directly. A similar mutual repression also exists between RKIP and SNAI, again via let-7 (75,76).…”

Noise in gene expression is coupled to growth rate

“…Since cell-to-cell variability in gene expression underlies important phenotypic phenomena such as persistence (Balaban et al 2004), competence (Maamar et al 2007), latency (Dar et al 2014), metastasis (Lee et al 2014), responsiveness to fluctuating environments (Acar et al 2005(Acar et al , 2008Blake et al 2006;Kaufmann et al 2007;Levy et al 2012;Vardi et al 2013), and triggering of meiosis (Nachman et al 2007), our results suggest that the probability and efficiency of these processes will be tightly coupled to environmental conditions, with potential evolutionary implications. Gene expression noise is higher at lower growth rates.…”

“…However, genetically identical cells exposed to the same environment display heterogeneity in gene expression (noise), with important phenotypic consequences (Grossman 1995;Rao et al 2002;Blake et al 2003;Balaban et al 2004;Colman-Lerner et al 2005;Kaern et al 2005;Balázsi et al 2011;Munsky et al 2012; Lee et al 2014). Variability in expression is anti-correlated to population average gene expression, which in turn is tightly coupled to growth rate (Tyson et al 1979;Ingraham et al 1983;Bar-Even et al 2006;Newman et al 2006;Brauer et al 2008;Klumpp et al 2009;Taniguchi et al 2010;Keren et al 2013).…”

Wearing Red for Signaling: The Heme-Bach Axis in Heme Metabolism, Oxidative Stress Response and Iron Immunology

“…These results suggest that the development of some cancers depends on Bach1 to modulate the gene expression pattern in a favorable way for transformation. Bach1 has been characterized as a key metastatic gene located downstream of the RKIP-Let-7 pathway (Yun et al 2011;Lee et al 2014). When the tumor suppressor RKIP is inactivated, the processing of Let-7 microRNA is decreased, resulting in a higher expression of its target gene, Bach1 (Yun et al 2011).…”