The Multifunctional Protein BAG3: A Novel Therapeutic Target in Cardiovascular Disease

The B-cell lymphoma 2–associated anthanogene (BAG3) protein is expressed most prominently in the heart, the skeletal muscle, and in many forms of cancer. In the heart, it serves as a co-chaperone with heat shock proteins in facilitating autophagy; binds to B-cell lymphoma 2, resulting in inhibition of apoptosis; attaches actin to the Z disk, providing structural support for the sarcomere; and links the α-adrenergic receptor with the L-type Ca²⁺ channel. When BAG3 is overexpressed in cancer cells, it facilitates prosurvival pathways that lead to insensitivity to chemotherapy, metastasis, cell migration, and invasiveness. In contrast, in the heart, mutations in BAG3 have been associated with a variety of phenotypes, including both hypertrophic/restrictive and dilated cardiomyopathy. In murine skeletal muscle and vasculature, a mutation in BAG3 leads to critical limb ischemia after femoral artery ligation. An understanding of the biology of BAG3 is relevant because it may provide a therapeutic target in patients with both cardiac and skeletal muscle disease.

Role of BAG3 in cancer progression: A therapeutic opportunity

BAG3 is a multifunctional protein that can bind to heat shock proteins (Hsp) 70 through its BAG domain and to other partners through its WW domain, proline-rich (PXXP) repeat and IPV (Ile-Pro-Val) motifs. Its intracellular expression can be induced by stressful stimuli, while is constitutive in skeletal muscle, cardiac myocytes and several tumour types. BAG3 can modulate the levels, localisation or activity of its partner proteins, thereby regulating major cell pathways and functions, including apoptosis, autophagy, mechanotransduction, cytoskeleton organisation, motility. A secreted form of BAG3 has been identified in studies on pancreatic ductal adenocarcinoma (PDAC). Secreted BAG3 can bind to a specific receptor, IFITM2, expressed on macrophages, and induce the release of factors that sustain tumour growth and the metastatic process. BAG3 neutralisation therefore appears to constitute a novel potential strategy in the therapy of PDAC and, possibly, other tumours.

BAG3: a new player in the heart failure paradigm

BAG3 is a cellular protein that is expressed predominantly in skeletal and cardiac muscle but can also be found in the brain and in the peripheral nervous system. BAG3 functions in the cell include: serving as a co-chaperone with members of the heat-shock protein family of proteins to facilitate the removal of misfolded and degraded proteins, inhibiting apoptosis by interacting with Bcl2 and maintaining the structural integrity of the Z-disk in muscle by binding with CapZ. The importance of BAG3 in the homeostasis of myocytes and its role in the development of heart failure was evidenced by the finding that single allelic mutations in BAG3 were associated with familial dilated cardiomyopathy. Furthermore, significant decreases in the level of BAG3 have been found in end-stage failing human heart and in animal models of heart failure including mice with heart failure secondary to trans-aortic banding and in pigs after myocardial infarction. Thus, it becomes relevant to understand the cellular biology and molecular regulation of BAG3 expression in order to design new therapies for the treatment of patients with both hereditary and non-hereditary forms of dilated cardiomyopathy.

Regulatory T-cell therapy in the induction of transplant tolerance: the issue of subpopulations

Clinical tolerance induction to permit minimization or cessation of immunosuppressive drugs is one of the key research goals in solid organ transplantation. The use of ex vivo expanded or manipulated immunologic cells, including CD4CD25FOXP3 regulatory T cells (Tregs), to achieve this aim is already a reality, with several trials currently recruiting patients. Tregs are a highly suppressive, nonredundant, population of regulatory cells that prevent the development of autoimmune diseases in mammals. Data from transplanted humans and animal models support the notion that Tregs can mediate both induction and adoptive transfer of transplantation tolerance. However, human Tregs are highly heterogeneous and include subpopulations with the potential to produce the proinflammatory cytokine interleukin-17, which has been linked to transplant rejection. Tregs are also small in number in the peripheral circulation, thus they require ex vivo expansion before infusion into man. Selection of the most appropriate Treg population for cell therapy is, therefore, a critical step in ensuring successful clinical outcomes. In this review, we discuss Treg subpopulations, their subdivision based on nonmutually exclusive criteria of origin, expression of immunologic markers and function, availability in the peripheral blood of patients awaiting transplantation, and their suitability for programs of cell-based therapy.

Damage-associated molecular patterns derived from mitochondria may contribute to the hemodialysis-associated inflammation

PURPOSE

Inflammation is common in hemodialysis (HD) patients. Mitochondrial damage-associated molecular patterns (DAMPs) are released during cell necrosis or apoptosis and induce inflammation. Cell apoptosis is increased in HD patients. The mitochondrial protein cytochrome c, as a marker of released mitochondrial DAMPs, and interleukin-6 (IL-6), as a marker of inflammation, were evaluated in HD patients.

METHODS

Thirty-four HD patients and 20 controls were enrolled in the study. Serum cytochrome c and IL-6 were measured by means of enzyme-linked immunosorbent assay.

RESULTS

Compared to controls, cytochrome c was markedly increased in HD patients (1392.88 ± 905.24 pg/mL vs. 212.95 ± 91.71 pg/mL). IL-6 was also significantly increased in HD patients (50.32 ± 35.89 pg/mL vs. 14.27 ± 6.83 pg/mL). In HD patients serum IL-6 was positively related to serum cytochrome c (r = 0.458).

CONCLUSION

Both circulating cytochrome c and IL-6 are markedly increased in HD patients. Cytochrome c is positively related to IL-6.

BAG3: a multifaceted protein that regulates major cell pathways

Bcl2-associated athanogene 3 (BAG3) protein is a member of BAG family of co-chaperones that interacts with the ATPase domain of the heat shock protein (Hsp) 70 through BAG domain (110–124 amino acids). BAG3 is the only member of the family to be induced by stressful stimuli, mainly through the activity of heat shock factor 1 on bag3 gene promoter. In addition to the BAG domain, BAG3 contains also a WW domain and a proline-rich (PXXP) repeat, that mediate binding to partners different from Hsp70. These multifaceted interactions underlie BAG3 ability to modulate major biological processes, that is, apoptosis, development, cytoskeleton organization and autophagy, thereby mediating cell adaptive responses to stressful stimuli. In normal cells, BAG3 is constitutively present in a very few cell types, including cardiomyocytes and skeletal muscle cells, in which the protein appears to contribute to cell resistance to mechanical stress. A growing body of evidence indicate that BAG3 is instead expressed in several tumor types. In different tumor contexts, BAG3 protein was reported to sustain cell survival, resistance to therapy, and/or motility and metastatization. In some tumor types, down-modulation of BAG3 levels was shown, as a proof-of-principle, to inhibit neoplastic cell growth in animal models. This review attempts to outline the emerging mechanisms that can underlie some of the biological activities of the protein, focusing on implications in tumor progression.