Metabolic stress regulates cytoskeletal dynamics and metastasis of cancer cells

TRAP1 regulates proliferation, mitochondrial function, and has prognostic significance in NSCLC

The TNF receptor-associated protein 1 (TRAP1) is a mitochondrial HSP that has been related to drug resistance and protection from apoptosis in colorectal and prostate cancer. Here, the effect of TRAP1 ablation on cell proliferation, survival, apoptosis, and mitochondrial function was determined in non-small cell lung cancer (NSCLC). In addition, the prognostic value of TRAP1 was evaluated in patients with NSCLC. These results demonstrate that TRAP1 knockdown reduces cell growth and clonogenic cell survival. Moreover, TRAP1 downregulation impairs mitochondrial functions such as ATP production and mitochondrial membrane potential as measured by TMRM (tetramethylrhodamine methylester) uptake, but it does not affect mitochondrial density or mitochondrial morphology. The effect of TRAP1 silencing on apoptosis, analyzed by flow cytometry and immunoblot expression (cleaved PARP, caspase-9, and caspase-3) was cell line and context dependent. Finally, the prognostic potential of TRAP1 expression in NSCLC was ascertained via immunohistochemical analysis which revealed that high TRAP1 expression was associated with increased risk of disease recurrence (univariate analysis, P = 0.008; multivariate analysis, HR: 2.554; 95% confidence interval, 1.085-6.012; P = 0.03). In conclusion, these results demonstrate that TRAP1 impacts the viability of NSCLC cells, and that its expression is prognostic in NSCLC.

IMPLICATIONS

TRAP1 controls NSCLC proliferation, apoptosis, and mitochondrial function, and its status has prognostic potential in NSCLC.

The AMP-activated protein kinase (AMPK) and cancer: many faces of a metabolic regulator

The AMP-activated protein kinase (AMPK) is a central regulator of cellular metabolism and energy homeostasis in mammalian tissues. Pertinent to cancer biology is the fact that AMPK is situated in the center of a signaling network involving established tumor suppressors including LKB1, TSC2 and p53. However, recent research suggests that AMPK can exert pro- or anti-tumorigenic roles in cancer depending on context. Loss of AMPK activity has been observed in several tumor types, and can cooperate with oncogenic drivers to reprogram tumor cell metabolism and enhance cell growth and proliferation. However, AMPK activation can also provide a growth advantage to tumor cells by regulating cellular metabolic plasticity, thus providing tumor cells the flexibility to adapt to metabolic stress. Here we discuss the contextual nature of the "two faces" of AMPK in cancer, and discuss the rationale and context for employing AMPK activators versus inhibitors for cancer therapy.

ER stress protection in cancer cells: the multifaceted role of the heat shock protein TRAP1

TRAP1 is an HSP90 chaperone, upregulated in human cancers and involved in organelles’ homeostasis and tumor cell metabolism. Indeed, TRAP1 is a key regulator of adaptive responses used by highly proliferative tumors to face the metabolic stress induced by increased demand of protein synthesis and hostile environments. Besides well-characterized roles in prevention of mitochondrial permeability transition pore opening and in regulating mitochondrial respiration, TRAP1 is involved in novel regulatory mechanisms: i) the attenuation of global protein synthesis, ii) the co-translational regulation of protein synthesis and ubiquitination of specific client proteins, and iii) the protection from Endoplasmic Reticulum stress. This provides a crucial role to TRAP1 in maintaining cellular homeostasis through protein quality control, by avoiding the accumulation of damaged or misfolded proteins and, likely, facilitating the synthesis of selective cancer-related proteins. Herein, we summarize how these regulatory mechanisms are part of an integrated network, which enables cancer cells to modulate their metabolism and to face, at the same time, oxidative and metabolic stress, oxygen and nutrient deprivation, increased demand of energy production and macromolecule biosynthesis. The possibility to undertake a new strategy to disrupt such networks of integrated control in cancer cells holds great promise for treatment of human malignancies.

Pushing tumor cells towards a malignant phenotype: stimuli from the microenvironment, intercellular communications and alternative roads

The tumor microenvironment produces different types of stimuli capable of endowing tumor cells with an aggressive behavior that is characterized by increased motility, invasiveness and propensity to metastasize, gain of a tumor-initiating phenotype, and drug resistance. The following classes of stimuli have been reported to promote such a malignant phenotype: (i) solid- or fluid-induced stress; (ii) altered composition of the extracellular matrix; (iii) hypoxia and low pH; (iv) innate and adaptive immune responses; (v) antitumor drugs. The simultaneous presence of more than one of these stimuli, as likely occurs in vivo, may lead to synergistic interactions in the induction of malignant traits. In many cases, the gain of a malignant phenotype is not the result of a direct effect of the stimuli on tumor cells but, rather, a stimulus-promoted cross-talk between tumor cells and other cell types within the tumor microenvironment. This cross-talk is mainly mediated by two classes of molecules: paracrine factors and adhesion receptors. Stimuli that promote a malignant phenotype can promote additional outcomes in tumor cells, including autophagy and cell death. We summarize here the available evidence about the variables that induce tumor cells to take one or the other of these roads in response to the same stimuli. At the end of this review, we address some unanswered questions in this domain and indicate future directions of research.