Review Role of the Aggresome Pathway in Cancer: Targeting Histone Deacetylase 6–Dependent Protein Degradation Agustin Rodriguez-Gonzalez,1 Tara Lin,1 Alan K. Ikeda,1 Tiffany Simms-Waldrip,1 Cecilia Fu,1 and Kathleen M. Sakamoto1,2,3 1Division of Hematology-Oncology, Mattel Children’s Hospital and 2Department of Pathology and Laboratory Medicine, David Geffen School of Medicine, University of California at Los Angeles, Los Angeles, California; and 3Division of Biology, California Institute of Technology, Pasadena, California Abstract synthesized in the endoplasmic reticulum (ER) are properly folded Misfolded or aggregated proteins have two fates: they are with the help of ER chaperones. Misfolded proteins are disposed either refolded with the help of chaperones or degraded by the of by ER-associated protein degradation (ERAD). When the level proteasome. Cells also have an alternative pathway that of misfolded proteins exceeds the folding capacity of the ER, cells involves intracellular ‘‘storage bins’’ for misfolded intracellu- activate a feedback mechanism known as the ER stress response lar proteins known as aggresomes. Aggresomes recruit motor (4). Expression of ER chaperones and ERAD-associated proteins is proteins that transport misfolded or aggregated proteins to induced to decrease protein synthesis and, hence, the burden on chaperones and proteasomes for subsequent destruction. the ER. There are four classes of agents that induce ER stress; they There is emerging evidence that inhibiting the aggresome are inhibitors of glycosylation, calcium metabolism, reducing agents, and hypoxia. Finally, the ER stress response can result in pathway leads to accumulation of misfolded proteins and apoptosis in tumor cells through autophagy. We discuss the activation of apoptosis (5). role of aggresomes in cancer and the potential to target Three transmembrane proteins regulate the mammalian ER stress response. PERK is a transmembrane kinase, ATF6 is a this pathway for therapy. [Cancer Res 2008;68(8):2557–60] transmembrane transcription factor, and IRE1 is a transmembrane Protein (Mis)Folding and Aggregation RNase. These three proteins maintain integration of the stress response and are critical for cell survival. ER stress-induced Newly synthesized proteins must overcome several obstacles on apoptotic pathways act through proapoptotic and antiapoptotic their way to becoming functional molecules. Small proteins fold proteins, such as bcl-2, p53, and c-abl. Stress-activated protein through a sequence of folding intermediates. During folding, kinase and c-Jun NH2-terminal kinase (JNK) are also activated (5). partially folded proteins expose hydrophobic domains that lead to Recently, the role of UPR in tumorigenesis has been investigated inappropriate associations and protein aggregation. Aggregation is (6). As tumors increase in size, cells are exposed to several toxic to cells and, due to high concentrations of macromolecules, environmental stressors, including hypoxia, limited nutrients, and causes a significant increase in the association constants of acidosis. Exposure to chemotherapy also activates UPR, resulting in unfolded polypeptides over those in dilute solution. Effects of sensitivity to DNA cross-linking agents (e.g., cisplatin); however, protein aggregation are amplified by the fact that stable folding there is also evidence that activation of the stress response could of a domain cannot occur until the entire protein is synthesized. confer resistance to drugs [e.g., topoisomerase II inhibitors (6)]. This is particularly important during synthesis of identical nascent Clearly, the role of the ERAD and ER stress response is a complex polypeptides on polysomes, where numerous polypeptides expose issue due to the heterogeneity of tumor response. the same aggregation-prone domains leading to increased risk of aggregation (1). Chaperones and Ubiquitin-Proteasome System The ultimate fate of a protein is either correct folding or aggregation (1). Whether a polypeptide chain folds correctly or Molecular chaperones have evolved to assist with folding of whether it aggregates is dependent on particular mutations, newly synthesized proteins and refolding of proteins damaged by modification, mistakes during translation, or unequal synthesis of stress and cellular injury. Chaperones bind to and stabilized subunits (1). Misfolding can also be promoted by pH, temperature, exposed hydrophobic residues through ATP-dependent interac- ionic strength, and redox environment. Because a certain level of tions, allowing the protein to achieve proper folding (7). protein misfolding is inevitable, cells have adapted various quality Chaperones do not seem to catalyze folding, but rather, they control mechanisms to minimize misfolding and to eliminate prevent intermolecular and intramolecular interactions between misfolded proteins before aggregation (2, 3). partially folded or misfolded polypeptides. This is an evolutionarily conserved mechanism from bacteria to eukaryotes to maintain Unfolded Protein Response Pathway proper folding after protein synthesis (1). Second, proteins that are unable to fold properly are targeted for degradation by the Another important pathway in cells to regulate misfolded ubiquitin-proteasome system. The proteasome is a multisubunit proteins is the unfolded protein response (UPR). Proteins complex found in the cytosol and nucleus that degrades cytosolic, nuclear, secretory, and transmembrane proteins into smaller Requests for reprints: Kathleen M. Sakamoto, Division of Hematology-Oncology, peptides (8, 9). Misfolded secretory and transmembrane proteins Mattel Children’s Hospital University of California at Los Angeles, David Geffen School are retained in the lumen or membrane of the ER and then of Medicine, Los Angeles, CA 90095-1752. Phone: 310-794-7007; Fax: 310-206-8089; retrotranslocated back to the cytosol and delivered to the E-mail: [email protected]. I2008 American Association for Cancer Research. proteasome (1). When correct folding is difficult or impossible doi:10.1158/0008-5472.CAN-07-5989 and degradation is not performed rapidly, proteins interact with www.aacrjournals.org 2557 Cancer Res 2008; 68: (8). April 15, 2008 Downloaded from cancerres.aacrjournals.org on September 26, 2021. © 2008 American Association for Cancer Research. Cancer Research other unfolded or partially folded proteins, leading to the aggresomes showed that these particles consist of multiple loosely formation of aggregates (3). Cells then destroy protein aggregates associated particles. through the aggresome pathway. The Aggresome Pathway Recruitment of Aggresomes to the Degradation Recent studies have shown a proteasome-independent pathway Machinery that eliminates misfolded polyubiquitinated proteins, known as Aggresomes are not static compartments for misfolded proteins the aggresome (Fig. 1; ref. 1). The initial aggregation process seems (1). They also recruit cytosolic components, including chaperones, to occur cotranslationally, as nascent chains are coming off the ubiquitination enzymes, and proteasome subunits, to facilitate polysome. If nascent peptides do not fold correctly, they will clearance of aggregated proteins. Several chaperones have been coaggregate to form a single aggresomal particle. In cells, these identified [e.g., HSC70, heat shock protein (HSP) 40, and HSP70], particles are uniform in size, supporting the idea that a fixed and some have been targeted for cancer therapy. Chaperones seem number of proteins aggregate to form a single particle (1). These to reduce aggregation; misfolded proteins associate with ER particles are produced throughout the cytosol. After their chaperones first followed by association with cytosolic chaperones formation, the aggresomal particles are transported toward the (1). Recently, heat shock factor 1 (HSF1), the master regulator microtubule organizing center (MTOC), where they are sequestered of the heat shock response in eukaryotes and highly conserved, into a single large cellular garbage bin-like structure known as the was reported to play a major role in carcinogenesis. Elimination of aggresome (1). Movement of the aggresome particle is an active HSF1 protects mice from tumors induced by mutations of the process and requires intact microtubules and association with RAS oncogene or in the tumor suppressor p53 (10). motor dynein. In cells treated with the microtubule-depolarizing Similar to chaperones, proteasomes interact with aggresomes. agent nocodazole, the aggresome remains distributed throughout Proteasomes associate with aggresomes relatively late after small the cytosol. Furthermore, electron microscopy examination of aggresomal particles are delivered to the MTOC. The pathways Figure 1. The aggresome pathway. Unfolded or misfolded proteins can originate from translating polysomes, from proteins retrotranslocated from the ER to the cytosol, or from proteins damaged by stress. If these unfolded/misfolded proteins fail to fold correctly and are not degraded by the proteasome, they can form aggregates throughout the cells. These aggregates are transported in a microtubule-dependent manner to the MTOCthat requires the dynein/dynactin motor compl ex. HDAC6 acetylates a-tubulin and associates with dynein to facilitate transport of aggregated bodies through the cytosol to lysosomes for degradation. HDAC6 coordinates the cell response to protein aggregate formation. Balance between HDAC6 and its partner p97/VCP determines the fate of polyubiquitinated misfolded proteins. The recruitment