Abstract P2083: Erk-mediated Phosphorylation Of Grk2 Leads To Reduced Pyruvate Dehydrogenase Activity And Alters Pyroptotic Signaling

G protein-coupled receptors (GPCRs) are important regulators of cellular functions where agonist binding leads to receptor conformational changes and downstream activation of g-protein-mediated signaling cascades. Signal transduction is terminated by receptor phosphorylation mediated by G-protein coupled receptor kinases (GRKs). GPCR kinase 2 (GRK2) is a main GRK in the heart and is upregulated in heart failure (HF) patients. Recently, we and others have shown that GRK2 can translocate to cardiac mitochondria where it regulates metabolism. Notably, we found that phosphorylated GRK2 at S670 post- ischemia/reperfusion (IR) decreases glucose mitochondrial utilization and pyruvate dehydrogenase (PDH) activity. Furthermore, 2D-SDS PAGE followed by LC/MS/MS revealed a potential link between mitochondrial GRK2 and PDH phosphorylation. Using a novel PDHα KI CRISPR Cas HEK cell line, we tested the hypothesize that PDH phosphorylation of this novel site was important for PDH activity. Utilizing cytosolic and mitochondrially-targeted pyruvate indicators we measured pyruvate levels in WT and KI HEK cells. In parallel, using a novel mouse model GRK2-S670A, we investigated the role of ERK in mediating GRK2 mitochondrial translocation and pyroptotic signaling. Further experiments will determine long-term impact of mitochondrial GRK2 in cell survival in cardiac pathological conditions.

SUMO-mediated regulation of NLRP3 modulates inflammasome activity

The NLRP3 inflammasome responds to infection and tissue damage, and rapidly escalates the intensity of inflammation by activating interleukin (IL)-1β, IL-18 and cell death by pyroptosis. How the NLRP3 inflammasome is negatively regulated is poorly understood. Here we show that NLRP3 inflammasome activation is suppressed by sumoylation. NLRP3 is sumoylated by the SUMO E3-ligase MAPL, and stimulation-dependent NLRP3 desumoylation by the SUMO-specific proteases SENP6 and SENP7 promotes NLRP3 activation. Defective NLRP3 sumoylation, either by NLRP3 mutation of SUMO acceptor lysines or depletion of MAPL, results in enhanced caspase-1 activation and IL-1β release. Conversely, depletion of SENP7 suppresses NLRP3-dependent ASC oligomerisation, caspase-1 activation and IL-1β release. These data indicate that sumoylation of NLRP3 restrains inflammasome activation, and identify SUMO proteases as potential drug targets for the treatment of inflammatory diseases.

Triad3a induces the degradation of early necrosome to limit RipK1-dependent cytokine production and necroptosis

Understanding the molecular signaling in programmed cell death is vital to a practical understanding of inflammation and immune cell function. Here we identify a previously unrecognized mechanism that functions to downregulate the necrosome, a central signaling complex involved in inflammation and necroptosis. We show that RipK1 associates with RipK3 in an early necrosome, independent of RipK3 phosphorylation and MLKL-induced necroptotic death. We find that formation of the early necrosome activates K48-ubiquitin-dependent proteasomal degradation of RipK1, Caspase-8, and other necrosomal proteins. Our results reveal that the E3-ubiquitin ligase Triad3a promotes this negative feedback loop independently of typical RipK1 ubiquitin editing enzymes, cIAPs, A20, or CYLD. Finally, we show that Triad3a-dependent necrosomal degradation limits necroptosis and production of inflammatory cytokines. These results reveal a new mechanism of shutting off necrosome signaling and may pave the way to new strategies for therapeutic manipulation of inflammatory responses.

Ketone body beta-hydroxy butyrate deactivates NLRP3 inflammasome in myeloid cells (CAM1P.153)

The NLRP3 inflammasome is a major regulator of sterile inflammation. It is now known that many age-related diseases are NLRP3-dependent, including metabolic disease, cognitive decline, bone loss, and thymic demise. Therefore, decreasing inflammasome activation during aging may prevent many chronic diseases known to limit healthspan. Calorie restriction and prolonged fasting reduce inflammation. During fasting, the body uses fat for energy, leading to an increase in ketone body production as a byproduct of fatty acid oxidation. We tested the hypothesis that switching the substrates from glucose to ketones could inhibit inflammasome activation in myeloid lineage cells, macrophages and neutrophils. Here we present novel data that ketone body beta-hydroxy butyrate (BHB) inhibits NLRP3-dependent IL-1β secretion. Notably, related short chain fatty acid, butyrate does not impact the inflammasome and oxidation of ketones for energetic utilization was not required for inflammasome deactivation in myeloid cells. Instead, ketones directly inhibit inflammasome complex assembly by blocking ASC oligomerization and prionization. BHB also inhibits NLRP3 dependent inflammasome activation in vivo and in neutrophils of aged mice. Therefore, ketones may represent an anti-inflammatory intervention to prevent chronic inflammation during aging that is normally associated with functional decline and death in the elderly.

The ketone metabolite β-hydroxybutyrate blocks NLRP3 inflammasome-mediated inflammatory disease

Ketone bodies , β-hydroxybutyrate (BHB) and acetoacetate support mammalian survival during states of energy deficit by serving as alternative source of ATP¹. BHB levels are elevated during starvation, high-intensity exercise or by the low carbohydrate ketogenic diet². Prolonged caloric restriction or fasting reduces inflammation as immune system adapts to low glucose supply and energy metabolism switches towards mitochondrial fatty acid oxidation, ketogenesis and ketolysis^2-6. However, role of ketones bodies in regulation of innate immune response is unknown. We report that BHB, but neither acetoacetate nor structurally-related short chain fatty acids, butyrate and acetate, suppresses activation of the NLRP3 inflammasome in response to several structurally unrelated NLRP3 activators, without impacting NLRC4, AIM2 or non-canonical caspase-11 inflammasome activation. Mechanistically, BHB inhibits NLRP3 inflammasome by preventing K⁺ efflux and reducing ASC oligomerization and speck formation. The inhibitory effects of BHB on NLRP3 were not dependent on chirality or classical starvation regulated mechanisms like AMPK, reactive oxygen species (ROS), autophagy or glycolytic inhibition. BHB blocked NLRP3 inflammasome without undergoing oxidation in TCA cycle, independently of uncoupling protein-2 (UCP2), Sirt2, receptor Gpr109a and inhibition of NLRP3 did not correlate with magnitude of histone acetylation in macrophages. BHB reduced the NLRP3 inflammasome mediated IL-1β and IL-18 production in human monocytes. In vivo, BHB attenuates caspase-1 activation and IL-1β secretion in mouse models of NLRP3-mediated diseases like Muckle-Wells Syndrome (MWS), Familial Cold Autoinflammatory syndrome (FCAS) and urate crystal induce body cavity inflammation. Taken together, these findings suggest that the anti-inflammatory effects of caloric restriction or ketogenic diets may be mechanistically linked to BHB-mediated inhibition of the NLRP3 inflammasome, and point to the potential use of interventions that elevate circulating BHB against NLRP3-mediated proinflammatory diseases.

FLIP is expressed in mouse testis and protects germ cells from apoptosis

Apoptosis control in adult testis is crucial to achieve normal spermatogenesis. In this study c-FLIP, an apoptosis-modulating protein, was investigated. In Western blot and immunohistochemical analyses, the 55 KDa c-FLIP long isoform (c-FLIP(L)) was found to be expressed strongly in spermatocytes and spermatids, at low levels in spermatogonia and at almost undetectable levels in Sertoli cells. This expression pattern was confirmed by Northern blot analyses. Further experiments carried out on GC-1spg germ cell line revealed that reducing c-FLIP(L) expression increases Fas-dependent apoptosis. Conversely, restoring c-FLIP(L) expression reduces this response to control levels. Caspase-10 expression was found to match c-FLIP(L) expression pattern; further, caspase-10 activation upon anti-Fas treatment inversely correlated with c-FLIP(L) expression. Finally, TUNEL staining of seminiferous tubules incubated with anti-Fas antibody showed that apoptosis occurs mostly in basally located germ cells, indicating that such cells, expressing low levels of c-FLIP(L), are sensitive to Fas-mediated apoptosis. These data indicate for the first time that c-FLIP(L) might control germ cell apoptosis and caspase activity in the adult testis.

Serum response factor cleavage by caspases 3 and 7 linked to apoptosis in human BJAB cells

Apoptosis involves the cessation of cellular processes, the breakdown of intracellular organelles, and, finally, the nonphlogistic clearance of apoptotic cells from the body. Important for these events is a family of proteases, caspases, which are activated by a proteolytic cleavage cascade and drive apoptosis by targeting key proteins within the cell. Here, we demonstrate that serum response factor (SRF), a transcription factor essential for proliferative gene expression, is cleaved by caspases and that this cleavage occurs in proliferating murine fibroblasts and can be induced in the human B-cell line BJAB. We identify the two major sites at which SRF cleavage occurs as Asp(245) and Asp(254), the caspases responsible for the cleavage and generate a mutant of SRF resistant to cleavage in BJAB cells. Investigation of the physiological and functional significance of SRF cleavage reveals that it correlates with the loss of c-fos expression, whereby neither SRF cleavage fragment retains transcriptional activity. Moreover, the expression of a noncleavable SRF in BJAB cells suppresses apoptosis induced by Fas cross-linking. These results suggest that for apoptosis to proceed, the transcriptional events promoting cell survival and proliferation, in which SRF is involved, must first be inactivated.

Tumor necrosis factor alpha induces BID cleavage and bypasses antiapoptotic signals in prostate cancer LNCaP cells

Survival of cancer cells in response to therapy, immune response, or metastasis depends on interactions between pro- and antiapoptotic signals. Two major proapoptotic pathways have been described: (a) a death receptor pathway; and (b) a mitochondrial pathway. We reported previously that Akt and the epidermal growth factor (EGF) receptor send separate, redundant survival signals that act to inhibit the mitochondrial proapoptotic pathway in prostate cancer LNCaP cells. However, it was unclear at what level the pro- and antiapoptotic signals interact in these cells, and it was also unclear whether these signals would inhibit the death receptor pathway. We found that EGF can protect LNCaP cells from apoptosis induced by LY294002 but not from tumor necrosis factor a (TNF-alpha)-induced apoptosis. Furthermore, TNF-alpha induced apoptosis under conditions in which Akt was active. Treatment with TNF-alpha resulted in activation of caspase 8 and cleavage of BID, which in turn induced cytochrome c release and caspase 9-dependent activation of effector caspases. Thus, proapoptotic signals induced by both TNF-alpha and LY294002 converge on mitochondria and trigger cytochrome c release. Because EGF can inhibit cytochrome c release induced by LY294002 but not cytochrome c release induced by TNF-alpha, we suggest that the EGF survival mechanism operates on the mitochondrial pathway at a site upstream of cytochrome c release. The ability of TNF-alpha to bypass survival signals from activated EGF receptor and Akt in prostate cancer cells makes death receptor signaling a promising avenue for therapeutic intervention.

Synthetic peptide sequence from the C-terminus of the insulin-like growth factor-I receptor that induces apoptosis and inhibition of tumor growth

Although the type 1 insulin-like growth factor receptor (IGF-IR) is a potent inhibitor of apoptosis, its C-terminus sequence sends contradictory signals, including a clearly proapoptotic signal. We have synthesized a peptide, peptide 2, having the sequence of the IGF-IR from residue 1282 to residue 1298 (C-terminus of the beta subunit). To favor its uptake into cells, we linked it to a stearic acid moiety at its NH-terminus. Peptide 2 is taken up by the cells, where it inhibits DNA synthesis and causes apoptosis, while a scrambled peptide (with stearic acid) and peptide 2 without stearic acid are completely ineffective. Peptide 2 is more effective when cells are in anchorage-independent conditions than when they grow in monolayer cultures. Accordingly, we find that peptide 2 can inhibit the growth of a human prostatic cell line in nude mice. The proapoptotic effect of peptide 2 is inhibited by the expression of Bcl-2 or by a dominant negative mutant of caspase 9. These and other data indicate that peptide 2 does not seem to be competing directly with the IGF-IR for common substrates, but that its proapoptotic effect is related to its ability to activate the caspase cascade.

Resistance of actin to cleavage during apoptosis

A small number of cellular proteins present in the nucleus, cytosol, and membrane fraction are specifically cleaved by the interleukin-1beta-converting enzyme (ICE)-like family of proteases during apoptosis. Previous results have demonstrated that one of these, the cytoskeletal protein actin, is degraded in rat PC12 pheochromocytoma cells upon serum withdrawal. Extracts from etoposide-treated U937 cells are also capable of cleaving actin. It was assumed that cleavage of actin represented a general phenomenon, and a mechanism coordinating proteolytic, endonucleolytic, and morphological aspects of apoptosis was proposed. We demonstrate here that actin is resistant to degradation in several different human cells induced to undergo apoptosis in response to a variety of stimuli, including Fas ligation, serum withdrawal, cytotoxic T-cell killing, and DNA damage. On the other hand, cell-free extracts from these cells and the ICE-like protease CPP32 were capable of cleaving actin in vitro. We conclude that while actin contains cleavage sites for ICE-like proteases, it is not degraded in vivo in human cells either because of lack of access of these proteases to actin or due to the presence of other factors that prevent degradation.

Cooperative action of Hsp70, Hsp90, and DnaJ proteins in protein renaturation

The proteins required for the repair of damaged proteins in the eukaryotic cytoplasm remain largely uncharacterized. The renaturation of thermally denatured firefly luciferase readily occurs in rabbit reticulocyte lysate by an ATP-dependent process. Earlier studies had shown that this chaperoning activity could be reconstituted, in part, using purified preparations of hsp70 and hsp90. We have extended the description of this system by clarifying the importance of hsp70 and hsp90 and have tested for additional factors that enhance renaturation. Using mutant hsp70 proteins, we have shown that hsp70 is required for luciferase renaturation. We have also found that hsp70 and hsp90 preparations purified by common procedures were contaminated with low levels of DnaJ proteins that are essential for the renaturing activity. When hsp70 and hsp90 preparations free of DnaJ proteins are used, the system must be supplemented with a DnaJ protein to obtain renaturation activity. The yeast DnaJ protein, YDJ-1, was found to be very effective for this purpose. Although significant renaturation can occur with only hsp70 and DnaJ proteins, hsp90 also contributes to the renaturation process, both in the complex environment of reticulocyte lysate and in a purified system. However, using highly purified hsp90 and geldanamycin, a specific inhibitor of hsp90 function, we have determined that hsp90 is not an essential component of the renaturation system. The contribution of hsp90 to renaturation is only partially blocked by geldanamycin, suggesting that this protein may influence activity in more than one way. This study indicates that hsp70, hsp90, and DnaJ proteins function cooperatively to renature damaged proteins in the eukaryotic cytoplasm and provides a framework by which additional components can be identified and individual chaperone contributions can be investigated.

Interleukin-1 beta-converting enzyme-like protease cleaves DNA-dependent protein kinase in cytotoxic T cell killing

Cytotoxic T cells (CTL) represent the major defense mechanism against the spread of virus infection. It is believed that the pore-forming protein, perforin, facilitates the entry of a series of serine proteases (particularly granzyme B) into the target cell which ultimately leads to DNA fragmentation and apoptosis. We demonstrate here that during CTL-mediated cytolysis the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs), an enzyme implicated in the repair of double strand breaks in DNA, is specifically cleaved by an interleukin (IL)-1 beta-converting enzyme (ICE)-like protease. A serine protease inhibitor, 3,4-dichloroisocoumarin (DCl), which is known to block granzyme B activity, inhibited CTL-induced apoptosis and prevented the degradation of DNA-PKcs in cells but failed to prevent the degradation of purified DNA-PKcs by CTL extracts. However, Tyr-Val-Ala-Asp-CH2Cl (YVAD-CMK) and other cysteine protease inhibitors prevented the degradation of purified DNA-PKcs by CTL extracts. Furthermore, incubation of DNA-PKcs with granzyme B did not produce the same cleavage pattern observed in cells undergoing apoptosis and when this substrate was incubated with either CTL extracts or the ICE-like protease, CPP32. Sequence analysis revealed that the cleavage site in DNA-PKcs during CTL killing was the same as that when this substrate was exposed to CPP32. This study demonstrates for the first time that the cleavage of DNA-PKcs in this intact cell system is exclusively due to an ICE-like protease.

Demonstration of nuclear translocation of the mineralocorticoid receptor (MR) using an anti-MR antibody and confocal laser scanning microscopy

Using a synthetic peptide that corresponds to a unique region of the N-terminal domain of the human mineralocorticoid receptor (MR), amino acids 87-96, we have generated a polyclonal antibody, human (h) MRsN. This sequence shares no homology with the corresponding sequences of the glucocorticoid receptor or other steroid/thyroid hormone receptor superfamily members. Antibody hMRsN cross-reacts with MR from human, rat, and mouse cells and recognizes denatured MR from either crude preparations or partially purified rat kidney cytosol, rat colon, or recombinant hMR overexpressed in baculovirus-infected Sf9 cells. Immunoprecipitation of the native MR from either partially purified or crude preparations of rat kidney cytosol with hMRsN, followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and silver stain, demonstrated a major protein band with a mol wt of 116 kilodaltons. In addition, using confocal laser scanning microscopy and digital image analysis, the immunocytochemical localization of the recombinant hMR over-expressed in Sf9 cells 24 h post-transfection was determined. In the absence of ligand, the MR was detected solely in the cytoplasm, after a 30-min exposure to 100 nM aldosterone the MR was perinuclear, and after 60 min, the MR was predominantly nuclear. To ascertain that this phenomenon was not unique to insect cells, aldosterone induced MR nuclear translocation in mouse macrophage cells was also demonstrated immunocytochemically, clearly indicating a role for nuclear translocation in MR function.