Context-dependent role for chromatin remodeling component PBRM1/BAF180 in clear cell renal cell carcinoma

A subset of clear cell renal cell carcinoma (ccRCC) tumors exhibit a HIF1A gene mutation, yielding two ccRCC tumor types, H1H2 type expressing both HIF1α and HIF2α, and H2 type expressing HIF2α, but not functional HIF1α protein. However, it is unclear how the H1H2 type ccRCC tumors escape HIF1's tumor-suppressive activity. The polybromo-1 (PBRM1) gene coding for the BAF180 protein, a component of the SWItch/Sucrose Non-Fermentable (SWI/SNF) chromatin remodeling complex, is inactivated in 40% ccRCCs, the function and mechanism of BAF180 mutation is unknown. Our previous study indicates that BAF180-containing SWI/SNF chromatin remodeling complex is a co-activator for transcription factor HIF to induce HIF target genes. Thus, our questions are if BAF180 is involved in HIF-mediated hypoxia response and if PBRM1/BAF180 mutation has any association with the HIF1A retention in H1H2 type ccRCC. We report here that BAF180 is mutated in H1H2 ccRCC cell lines and tumors, and BAF180 re-expression in H1H2 ccRCC cell lines reduced cell proliferation/survival, indicating that BAF180 has tumor-suppressive role in these cells. However, BAF180 is expressed in HIF1-deficient H2 ccRCC cell lines and tumors, and BAF180 knockdown in H2 type ccRCC cell lines reduced cell proliferation/survival, indicating that BAF180 has tumor-promoting activity in these cells. In addition, our data show that BAF180 functions as co-activator for HIF1- and HIF2-mediated transcriptional response, and BAF180's tumor-suppressive and -promoting activity in ccRCC cell lines depends on co-expression of HIF1 and HIF2, respectively. Thus, our studies reveal that BAF180 function in ccRCC is context dependent, and that mutation of PBRM1/BAF180 serves as an alternative strategy for ccRCC tumors to reduce HIF1 tumor-suppressive activity in H1H2 ccRCC tumors. Our studies define distinct functional subgroups of ccRCCs based on expression of BAF180, and suggest that BAF180 inhibition may be a novel therapeutic target for patients with H2, but not H1H2, ccRCC tumors.

Hypoxia-induced pulmonary vascular remodeling: contribution of the adventitial fibroblasts

Vascular repair in response to injury or stress (often referred to as remodeling) is a common complication of many cardiovascular abnormalities including pulmonary hypertension, systemic hypertension, atherosclerosis, vein graft remodeling and restenosis following balloon dilatation of the coronary artery. It is not surprising that repair and remodeling occurs frequently in the vasculature in that exposure of blood, vessels to either excessive hemodynamic stress (e.g. hypertension), noxious blood borne agents (e.g. atherogenic lipids), locally released cytokines, or unusual environmental conditions (e.g. hypoxia), requires readily available mechanisms to counteract these adverse stimuli and to preserve structure and function of the vessel wall. The responses, which were presumably evolutionarily developed to repair an injured tissue, often escape self-limiting control and can result, in the case of blood vessels, in lumen narrowing and obstruction to blood flow. Each cell type (i. e. endothelial cells, smooth muscle cells, and fibroblasts) in the vascular wall plays a specific role in the response to injury. However, while the roles of the endothelial cells and smooth muscle cells (SMC) in vascular remodeling have been extensively studied, relatively little attention has been given to the adventitial fibroblasts. Perhaps this is because the fibroblast is a relatively ill-defined cell which, at least compared to the SMC, exhibits few specific cellular markers. Importantly though, it has been well demonstrated that fibroblasts possess the capacity to express several functions such as migration, rapid proliferation, synthesis of connective tissue components, contraction and cytokine production in response to activation or stimulation. The myriad of responses exhibited by the fibroblasts, especially in response to stimulation, suggest that these cells could play a pivotal role in the repair of injury. This fact has been well documented in the setting of wound healing where a hypoxic environment has been demonstrated to be critical in the cellular responses. As such it is not surprising that fibroblasts may play an important role in the vascular response to hypoxia and/or injury. This paper is intended to provide a brief review of the changes that occur in the adventitial fibroblasts in response to vascular stress (especially hypoxia) and the role the activated fibroblasts might play in hypoxia-mediated pulmonary vascular disease.

Hypoxia induces cell-specific changes in gene expression in vascular wall cells: implications for pulmonary hypertension

Mammals respond to reduced oxygen concentrations (hypoxia) in many different ways at the systemic, local, cellular and molecular levels. Within the pulmonary circulation, exposure to chronic hypoxia has been demonstrated to illicit increases in pulmonary artery pressure as well as dramatic structural changes in both large and small vessels. It has become increasingly clear that the response to hypoxia in vivo is differentially regulated at the level of specific cell types within the vessel wall. For instance, in large pulmonary blood vessels there is now convincing evidence to suggest that the medial layer is made up of many different subpopulations of smooth muscle cells. In response to hypoxia there are remarkable differences in the proliferative and matrix producing responses of these cells to the hypoxic environment. Some cell populations proliferate and increase matrix protein synthesis, while in other cell populations no apparent change in the proliferative or differentiation state of the cell takes place. In more peripheral vessels, the predominant proliferative changes in response to hypoxia in the pulmonary circulation occur in the adventitial layer rather than in the medial layer. Here again, specific increases in proliferation and matrix protein synthesis take place. Accumulating evidence suggests that the unique responses exhibited by specific cell types of hypoxia in vivo can be modeled in vitro. We have isolated, in culture, specific medial cell populations which demonstrate significant increases in proliferation in response to hypoxia, and others which exhibit no change or, in fact, a decrease in proliferation under hypoxic conditions. We have also isolated and cloned several unique populations of adventitial fibroblasts. There is good evidence that only certain fibroblast populations are capable of responding to hypoxia with an increase in proliferation. We have begun to elucidate the signaling pathways which are activated in those cell populations that exhibit proliferative responses to hypoxia. We show that hypoxia, in the absence of serum or mitogens, specifically activates select members of the protein kinase C isozyme family, as well as members of the mitogen-activated protein kinase (MAPK) family of proteins. This selective activation appears to take place in response to hypoxia only in those cells exhibiting a proliferative response, and antagonists of this pathway inhibit the response. Thus, there appear to be cells within each organ that demonstrate unique responses to hypoxia. A better understanding of why these cells exist and how they specifically transduce hypoxia-mediated signals will lead to a better understanding of how the changes in the pulmonary circulation take place under conditions of chronic hypoxia.

Effect of hypoxia on proximal tubules isolated from nitric oxide synthase knockout mice

Nitric oxide (NO) has been shown to be a mediator of hypoxic injury in rat renal proximal tubules (PT). However, the role of NO in hypoxic injury to mouse. PT has not been examined. The aim of the present study was to determine the effect of knockout of nitric oxide synthase (NOS) isoforms on hypoxic injury in mouse PT. Mouse PTs were isolated by collagenase digestion and Percoll centrifugation. The nonselective NOS inhibitor, N-nitro-L-arginine methyl ester (L-NAME, 10 mM), but not its inactive stereoisomer D-NAME, protected against hypoxic injury as assessed by LDH release. Carboxy-imidazolineoxyl N-oxide (carboxy-PTIO, 100 microM), a stable NO scavenger, also afforded cytoprotection against hypoxic injury. To determine the role of the different NOS isoforms in the hypoxic injury, we examined the effect of hypoxia on PT isolated from knockout mice in which either the inducible NOS (iNOS) endothelial NOS (eNOS) or neuronal NOS (nNOS) gene was lacking. PT isolated from iNOS knockout mice were resistant to hypoxic injury compared to wild-type controls. In contrast, PT isolated from both nNOS and eNOS knockout mice were not protected against hypoxic injury. In conclusion, the present study demonstrates that NO is a mediator of hypoxic PT injury in the mouse and that knockout of the iNOS gene is cytoprotective against this hypoxic PT injury.