Protein level variability determines phenotypic heterogeneity in proteotoxic stress response

HSP70 mediates a crosstalk between the estrogen and the heat shock response pathways

Cells respond to multiple signals from the environment simultaneously, which often creates crosstalk between pathways affecting the capacity to adapt to the changing environment. Chaperones are an important component in the cellular integration of multiple responses to environmental signals, often implicated in negative feedback and inactivation mechanisms. These mechanisms include the stabilization of steroid hormone nuclear receptors in the cytoplasm in the absence of their ligand. Here, we show using immunofluorescence, chromatin immunoprecipitation, and nascent transcripts production that the heat shock protein 70 (HSP70) chaperone plays a central role in a new crosstalk mechanism between the steroid and heat shock response pathways. HSP70-dependent feedback mechanisms are required to inactivate the heat shock factor 1 (HSF1) after activation. Interestingly, a steroid stimulation leads to faster accumulation of HSF1 in inactive foci following heat shock. Our results further show that in the presence of estrogen, HSP70 accumulates at HSF1-regulated noncoding regions, leading to deactivation of HSF1 and the abrogation of the heat shock transcriptional response. Using an HSP70 inhibitor, we demonstrate that the crosstalk between both pathways is dependent on the chaperone activity. These results suggest that HSP70 availability is a key determinant in the transcriptional integration of multiple external signals. Overall, these results offer a better understanding of the crosstalk between the heat shock and steroid responses, which are salient in neurodegenerative disorders and cancers.

Cellular Environment and Phenotypic Heterogeneity: How Data-Driven Modeling Finds the Smoking Gun

The cellular environment modifies cellular phenotypes, in particular, the stress response phenotype, which easily exhibits high phenotypic heterogeneity due to the common characteristics of its regulatory networks. The aim of this work is to quantify and interpret the impact of collagen type I, a major component of the cellular environment, on the phenotypic heterogeneity of the cellular response. Our approach combines in an original way the monitoring of the response of a single cell and the mathematical modeling of the network. After a detailed statistical description of the phenotypic heterogeneity of the cellular response, the mathematical modeling explains how the observed changes can be explained by an induced increase in the average expression of a central protein of the regulatory network. The predictions of the data-driven model are fully consistent with the biochemical measurements performed. The framework presented here is also a new general methodology to study phenotypic heterogeneity, although we focus here on the response to proteotoxic stress in HeLa cells.

Tracking the Activation of Heat Shock Signaling in Cellular Protection and Damage

Heat Shock (HS) signaling is activated in response to various types of cellular stress. This activation serves to protect cells from immediate threats in the surrounding environment. However, activation of HS signaling occurs in a heterogeneous manner within each cell population and can alter the epigenetic state of the cell, ultimately leading to long-term abnormalities in body function. Here, we summarize recent research findings obtained using molecular and genetic tools to track cells where HS signaling is activated. We then discuss the potential further applications of these tools, their limitations, and the necessary caveats in interpreting data obtained with these tools.

Stochastic and Age-Dependent Proteostasis Decline Underlies Heterogeneity in Heat-Shock Response Dynamics

Significant non-genetic stochastic factors affect aging, causing lifespan differences among individuals, even those sharing the same genetic and environmental background. In Caenorhabditis elegans, differences in heat-shock response (HSR) are predictive of lifespan. However, factors contributing to the heterogeneity of HSR are still not fully elucidated. Here, the authors characterized HSR dynamics in isogenic C. elegans expressing GFP reporter for hsp-16.2 for identifying the key contributors of HSR heterogeneity. Specifically, microfluidic devices that enable cross-sectional and longitudinal measurements of HSR dynamics in C. elegans at different scales are developed: in populations, within individuals, and in embryos. The authors adapted a mathematical model of HSR to single C. elegans and identified model parameters associated with proteostasis-maintenance of protein homeostasis-more specifically, protein turnover, as the major drivers of heterogeneity in HSR dynamics. It is verified that individuals with enhanced proteostasis fidelity in early adulthood live longer. The model-based comparative analysis of protein turnover in day-1 and day-2 adult C. elegans revealed a stochastic-onset of age-related proteostasis decline that increases the heterogeneity of HSR capacity. Finally, the analysis of C. elegans embryos showed higher HSR and proteostasis capacity than young adults and established transgenerational contribution to HSR heterogeneity that depends on maternal age.

Fine-tuned control of stress priming and thermotolerance

A common signature of cell adaptation to stress is the improved resistance upon priming by prior stress exposure. In the context of hyperthermia, priming or preconditioning with sublethal heat shock can be a useful tool to confer thermotolerance and competitive advantage to cells. In the present study, we develop a data-driven modeling framework that is simple and generic enough to capture a broad set of adaptation behaviors to heat stress at both molecular and cellular levels. The model recovers the main features of thermotolerance and clarifies the tradeoff principles which maximize the thermotolerance effect. It therefore provides an effective predictive tool to design preconditioning and fractionation hyperthermia protocols for therapeutic purpose.

Drivers of dynamic intratumor heterogeneity and phenotypic plasticity

Cancer is a clonal disease, i.e., all tumor cells within a malignant lesion trace their lineage back to a precursor somatic cell that acquired oncogenic mutations during development and aging. And yet, those tumor cells tend to have genetic and nongenetic variations among themselves-which is denoted as intratumor heterogeneity. Although some of these variations are inconsequential, others tend to contribute to cell state transition and phenotypic heterogeneity, providing a substrate for somatic evolution. Tumor cell phenotypes can dynamically change under the influence of genetic mutations, epigenetic modifications, and microenvironmental contexts. Although epigenetic and microenvironmental changes are adaptive, genetic mutations are usually considered permanent. Emerging reports suggest that certain classes of genetic alterations show extensive reversibility in tumors in clinically relevant timescales, contributing as major drivers of dynamic intratumor heterogeneity and phenotypic plasticity. Dynamic heterogeneity and phenotypic plasticity can confer resistance to treatment, promote metastasis, and enhance evolvability in cancer. Here, we first highlight recent efforts to characterize intratumor heterogeneity at genetic, epigenetic, and microenvironmental levels. We then discuss phenotypic plasticity and cell state transition by tumor cells, under the influence of genetic and nongenetic determinants and their clinical significance in classification of tumors and therapeutic decision-making.