Our laboratory focuses on understanding the emerging roles of the heat-shock or stress response in human cancers and longevity. The stress response is an evolutionarily highly conserved adaptive mechanism that enhances cell survival in the face of a large variety of stressful insults from without and from within. In mammals, the master transcriptional regulator of this systemic cellular response is heat shock factor 1, HSF1, a pleiotropic molecule that coordinates a network of cellular pathways to fight stresses. One of the major functions of this stress response is to maintain proteome homeostasis under stressful conditions.
The stress response and tumor evolution
Researchers have learned a tremendous amount about human cancers over the past few decades. Tumorigenesis is a dynamic and multi-step process that involves initiation, promotion, maintenance and progression stages. From normal cell to premalignant lesion to malignant tumor and finally to lethal metastatic tumor, the whole process can be envisioned as a model of somatic evolution following the same Darwinian principles. Cancer is initiated by activation of oncogenes or inactivation of tumor suppressor genes, which affects multiple cellular pathways such as signal transduction, cell cycle control, apoptosis, DNA repair and protein translation. However, numerous other genetic, epigenetic and environmental factors constantly interact with cancer-causing events to exert marked impacts on the susceptibility to cancer development, the ability of tumor cells to invade and metastasize and the tumor responses to therapies.
The stress response and tumor initiation
The HSF1-mediated stress response has been implicated in antagonizing a broad range of human pathophysiological conditions such as hyperthermia, ischemia/reperfusion and neurodegeneration. Surprisingly, its involvement in human malignancies has remained uncertain until very recently. By taking advantage of the Hsf1 knockout mice, we discovered that the stress response plays an enabling role in skin carcinogenesis initiated by a chemical mutagen and multiple tumorigeneses initiated by a hot-spot Trp53 point mutation. These results demonstrate that the HSF1-mediated stress response acts as a potent modifier of tumor initiation by altering cellular susceptibility to oncogenic transformation. These pioneering studies suggest that, like environmental insults, malignant transformation is a "stressful" process accompanied by drastic alterations in cell physiology. The stress response enables cells to survive initial oncogenic stresses and also successfully adapt to the malignant state. Unlike their normal counterparts, tumor cells are enduring stress even without environmental challenges. As a consequence, the stress response that is dispensable for normal cells is constantly activated in tumor cells and becomes an integral aspect of their malignant lifestyle, a novel concept referred as "stress-response addiction."
Exploitation of the stress response as a general cancer target depends on determining whether the enabling role of the stress response is universal and predominates in tumor initiation regardless of causal defects and histological origins. One ongoing project is to investigate the enabling role of the HSF1-mediated stress response in the malignancies associated with Neurofibromatosis type I (NF1), the most common human cancer-predisposition syndrome that afflicts 1 in 3,500 people worldwide. The tumor suppressor NF1 gene encodes a ~ 280KD protein called neurofibromin, a negative regulator of RAS oncoprotein. NF1 patients frequently develop multiple types of malignancy including neurofibromas, brain tumors and myeloid leukemia. Our results clearly indicate that ablation of HSF1 markedly suppresses the tumorigenic process associated with Nf1 deficiency in mice and impairs the growth and survival of human NF1 tumor cells in culture. These studies provide strong evidence that targeting the stress response is an innovative and promising strategy to prevent malignancy in NF1 patients. Our current efforts focus on understanding why the NF1-associated tumor initiation depends on the stress response. We are employing both genetic and biochemical approaches to interrogate aberrant signal transduction pathways and oxidative stress resulted from Nf1 deficiency.
Another interesting direction of the laboratory is investigating the importance of proteome homeostasis to tumorigenesis. We hypothesize that the HSF1-mediated stress response enables tumorigenesis, at least in part, by suppressing the toxicity associated with disrupted cellular proteomic integrity caused by carcinogens. We will first test this novel concept using a mouse photo-carcinogenesis model in which skin cancers are initiated by UV radiation. It has been well known that UV radiation causes DNA damage and mutations that ultimately lead to tumorigenesis. Much less appreciated, UV radiation also damages proteins. However, the impact of protein damage induced by UV radiation on skin carcinogenesis remains unexplored. Our preliminary studies indicated that Hsf1-deficient cells are more sensitive to UV radiation and exhibit increased cell death compared to their wild-type counterparts. We are further examining this effect in vivo.
The stress response and tumor maintenance
Although the current evidence points to an enabling role of the stress response in tumor initiation, whether established tumors become addicted to this powerful adaptive mechanism—the "stress-response addiction" of cancer—remains totally unknown. This question is pivotal to targeting the HSF1-mediated stress response as a novel anti-cancer therapeutic strategy. Our initial studies indicated that established human cancer cell lines are addicted to HSF1 for their continuous growth and survival. Next, two discrete, but complementary, model systems will be incorporated to examine the dependence of established tumors on the HSF1-mediated stress response in vivo. A novel transgenic mouse model in which the Hsf1 gene expression is temporally repressible by doxycycline has been developed in our laboratory. After full tumor induction in these mice, Hsf1 expression will be repressed and tumor responses will be monitored. As a complementary approach, a xenograft model will also be employed using human cancer cells engineered to express HSF1-targeted shRNAs in an inducible fashion. These proof-of-principle experiments will ultimately pave the road to the development of therapeutic reagents that specifically target HSF1 and its mediated stress response in human cancers.
The stress response and tumor progression
Our data indicate that HSF1 expression and activation is positively correlated with cellular malignant state. Nonetheless, it remains unknown whether the stress response affects tumor cells' evolvability and tumor progression. Genomic instability is a major cause of tumor progression; it increases the genetic diversity of tumor cells that can be selected by tumor microenvironments for evolution. We hypothesize that by enhancing the expression of heat shock proteins, which chaperone protein folding and/or orchestrate the transcription of an extensive network of genes, the stress response can allow many otherwise deleterious mutations to accumulate in a silent or tolerable state inside tumor cells and further release them for environmental selections, a phenomenon referred as "genetic buffering." By preventing premature exposure of critical mutations, the stress response may enrich genetic diversity and facilitate tumor progression. Elucidation of these questions may provide crucial evidence that modulating the stress response is a new way to impair tumor cells' ability to evolve. To investigate this question, the skin carcinogenesis model, a classic somatic tumor evolution process, will be adopted in the inducible Hsf1 transgenic mice. Tumor progression is indicated by the rate of malignant conversion of skin tumors and the rate of further generating metastatic lesions. Global genomic abnormalities of skin tumors with and without enhanced HSF1 will be profiled using array Comparative Genomic Hybridization technology (aCGH).
Identification of novel transcriptional targets of HSF1
To fully comprehend the pleiotropic effects of the stress response and uncover novel pathways regulated by HSF1, ChIP--seq, which combines chromatin immunoprecipitation (ChIP) with massively parallel DNA sequencing, will be employed to indentify the binding sites of HSF1 at the whole genome level under both non-transformed and transformed conditions. Particular attention will be given to novel non-heat shock protein genes and non-coding RNAs including microRNAs. Potential regulation of microRNAs by HSF1 will be further validated by microRNA microarray technology. The biology of identified novel targets in the stress response and tumorigenesis will be further investigated by genetic approaches.
The stress response and aging, longevity
Although the stress response has a fascinating role in prolonging the lifespan of lower organisms, its influence on mammalian longevity is still elusive. Particularly in humans, aging is associated with two major diseases, cancer and neurodegeneration, and the stress response happens to be involved in both. Ironically, while the stress response protects neurons from death and antagonizes neurodegeneration, the very same mechanism promotes cancer. Our laboratory is keenly interested in understanding how this ancient mechanism's effects on cancer and neurodegeneration are balanced and how they impact longevity in mammals. These studies may ultimately lead to new ways of fighting aging and increasing longevity. Specifically, our laboratory focuses on investigating how HSF1 and the stress response affect cellular senescence, stem cell self-renewal, and ultimately lifespan in mice. At molecular level, our laboratory is interested in how growth hormone, insulin/IGF-1 signaling and calorie restriction affect HSF1 activation and the stress response.
Postdoctoral Associate: Huawen Li, Ph.D.
Research Assistant I: Junyue Cao, B.S., Siyuan Dai, B.S., Xiogtao Ruan, B.S., Xin Wang, B.S., Wei Zhou, B.S.
Graduate Student: Zijian Tang, B.S., M.S.
Dai C, Santagata S, Tang Z, Shi J, Cao J, Kwon H, Bronson RT, Whitesell L, Lindquist S. 2012. Loss of tumor suppressor NF1 activates HSF1 to promote carcinogenesis. J Clin Invest. 122(10):3742-54.
Dai C, Dai S, Cao J. 2011. Proteotoxic stress of cancer: implications of the heat-shock response in oncogenesis. J Cell Physiol 227(8):2982-2987.Review
Gan N, Wu YC, Brunet M, Garrido C, Chung FL, Dai C, Mi L. 2010. Sulforaphane activates heat shock response and enhances proteasome activity through up-regulation of Hsp27. J Biol Chem 285(46):35528-35536. PMCID: PMC20833711
Dai C, Whitesell L, Rogers A, Lindquist S. 2007. Heat shock factor 1 (HSF1) is a powerful multifaceted modifier of carcinogenesis. Cell 130(6):1005-1018.
Selected Perspectives and Press Coverage:
1. Non-Oncogene Addiction and the Stress Phenotype of Cancer Cells. Cell 2007; 130(6):986-988.
2. Cancer Cells Chill Out to Survive. (http://sciencenow.sciencemag.org/cgi.content/full/2007/920/1)
3. Cancer Biology: Shock Strategy. Nature 2007; 449(7161):380-381.
4. A Shocking Enabler of Tumour Growth. Nature Reviews Cancer 2007; 7(11):817.
5. Aiding and Abetting: A longevity gene also promotes cancer. Science News 2007; 172(12):179-180.
Dai C, Whitesell L. 2005. Hsp90: a rising star on the horizon of anticancer targets. Future Oncol 1:529-540, review.
Dai C, Lyustikman Y, Shih A, Hu X, Fuller GN, Rosenblum M, Holland EC. 2005. The characteristics of astrocytomas and oligodendrogliomas are caused by two distinct and interchangeable signaling formats. Neoplasia 7(4):397-406.
Lassman A, Dai C, Fuller GN, Vickers AJ, Holland EC. 2004. Overexpression of c-MYC promotes an undifferentiated phenotype in cultured astrocyts and allows elevated Ras and Akt signaling to induce gliomas from GFAP-expressing cells in mice. Neuron Glia Biology 1(2):157-163.
Shih A, Dai C, Hu X, Rosenblum MK, Koutcher JA, Holland EC. 2004. Dosage-dependent effects of PDGF-B on glial tumorigenesis. Cancer Res 64(14):4783-4789.
Dai C, Holland EC. 2003. Astrocyte differentiation states and glioma formation. Cancer J 9(2):72-81, review.
Wolf RM, Draghi N, Liang X, Dai C, Uhrbom L, Eklof C, Westermark B, Holland EC, Resh MD. 2003. p190RhoGAP can act to inhibit PDGF-induced gliomas in mice: a putative tumor suppressor encoded on human chromosome 19q13.3. Genes Dev 17(4):476-487.
Uhrbom L, Dai C, Celestino J, Rosenblum MK, Fuller GN, Holland EC. 2002. Ink4a-Arf loss cooperates with Kras activation in astrocytes and neural progenitors to generate glioblastomas of various morphologies depending on amount of Akt activity. Cancer Res 62(19):5551-5558.
Fults D, Pedone C, Dai C, Holland EC. 2002. MYC expression promotes the proliferation of neural progenitor cells in culture and in vivo. Neoplasia 4(1):32-39.
Dai C, Holland EC. 2001. Glioma Models. Biochim Biophys Acta 1551:M19-M2, review.
Dai C, Celestino JC, Okada Y, Louis DN, Fuller GN, Holland EC. 2001. PDGF autocrine stimulation dedifferentiation cultured astroctyes and induces oligodendrogliomas and oligoastrocytomas from neural progenitors and astrocytes in vivo. Genes Dev 15:1913-1925.
Holland EC, Li Y, Celestino JC, Dai C, Schaefer L, Sawaya RE, Fuller GN. 2000. Astrocytes give rise to oligodendrogliomas and astrocytomas after gene transfer of polyoma virus middle T antigen in vivo. Am J Pathol 157(3):1031-1037.
Holland EC, Celestino JC, Dai C, Schaefer L, Sawaya RE, Fuller GN. 2000. Combined activation of Ras and Akt in neural progenitors induces glioblastoma formation in mice. Nat Genet 25(1):55-57.