Neuronal NLRP1 inflammasome activation of Caspase-1 coordinately regulates inflammatory interleukin-1-beta production and axonal degeneration-associated Caspase-6 activation

Neuronal active Caspase-6 (Casp6) is associated with Alzheimer disease (AD), cognitive impairment, and axonal degeneration. Caspase-1 (Casp1) can activate Casp6 but the expression and functionality of Casp1-activating inflammasomes has not been well-defined in human neurons. Here, we show that primary cultures of human CNS neurons expressed functional Nod-like receptor protein 1 (NLRP1), absent in melanoma 2, and ICE protease activating factor, but not the NLRP3, inflammasome receptor components. NLRP1 neutralizing antibodies in a cell-free system, and NLRP1 siRNAs in neurons hampered stress-induced Casp1 activation. NLRP1 and Casp1 siRNAs also abolished stress-induced Casp6 activation in neurons. The functionality of the NLRP1 inflammasome in serum-deprived neurons was also demonstrated by NLRP1 siRNA-mediated inhibition of speck formation of the apoptosis-associated speck-like protein containing a caspase recruitment domain conjugated to green fluorescent protein. These results indicated a novel stress-induced intraneuronal NLRP1/Casp1/Casp6 pathway. Lipopolysaccharide induced Casp1 and Casp6 activation in wild-type mice brain cortex, but not in that of Nlrp1(-/-) and Casp1(-/-) mice. NLRP1 immunopositive neurons were increased 25- to 30-fold in AD brains compared with non-AD brains. NLRP1 immunoreactivity in these neurons co-localized with Casp6 activity. Furthermore, the NLRP1/Casp1/Casp6 pathway increased amyloid beta peptide 42 ratio in serum-deprived neurons. Therefore, CNS human neurons express functional NLRP1 inflammasomes, which activate Casp1 and subsequently Casp6, thus revealing a fundamental mechanism linking intraneuronal inflammasome activation to Casp1-generated interleukin-1-β-mediated neuroinflammation and Casp6-mediated axonal degeneration.

TrkA overexpression enhances growth and metastasis of breast cancer cells

The Trk family of neurotrophin tyrosine kinase receptors is emerging as an important player in carcinogenic progression in non-neuronal tissues. Here, we show that breast tumors present high levels of TrkA and phospho-TrkA compared to normal breast tissues. To further evaluate the precise functions of TrkA overexpression in breast cancer development, we have performed a series of biological tests using breast cancer cells that stably overexpress TrkA. We show that (1) TrkA overexpression promoted cell growth, migration and invasion in vitro; (2) overexpression of TrkA per se conferred constitutive activation of its tyrosine kinase activity; (3) signal pathways including PI3K-Akt and ERK/p38 MAP kinases were activated by TrkA overexpression and were required for the maintenance of a more aggressive cellular phenotype; and (4) TrkA overexpression enhanced tumor growth, angiogenesis and metastasis of xenografted breast cancer cells in immunodeficient mice. Moreover, recovered metastatic cells from the lungs exhibited enhanced anoikis resistance that was abolished by the pharmacological inhibitor K252a, suggesting that TrkA-promoted breast tumor metastasis could be mediated at least in part by enhancing anoikis resistance. Together, these results provide the first direct evidence that TrkA overexpression enhances the tumorigenic properties of breast cancer cells and point to TrkA as a potential target in breast cancer therapy.

Amplification of apoptosis through sequential caspase cleavage of the MET tyrosine kinase receptor

Activation of the MET tyrosine kinase receptor by hepatocyte growth factor/scatter factor is classically associated with cell survival. Nonetheless, stress stimuli can lead to a caspase-dependent cleavage of MET within its juxtamembrane region, which generate a proapoptotic 40 kDa fragment (p40 MET). We report here that p40 MET is in fact generated through an additional caspase cleavage of MET within its extreme C-terminal region, which removes only few amino acids. We evidenced a hierarchical organization of these cleavages, with the C-terminal cleavage favoring the juxtamembrane one. As a functional consequence, the removal of the last amino acids of p40 MET increases its apoptotic capacity. Finally, cells expressing a MET receptor mutated at the C-terminal caspase site are unable to generate p40 MET and are resistant to apoptosis, indicating that generation of p40 MET amplifies apoptosis. These results revealed a two-step caspase cleavage of MET resulting in the reshaping of this survival receptor to a proapoptotic factor.

Regulation of the Ets-1 transcription factor by sumoylation and ubiquitinylation

Sumoylation and ubiquitinylation reversibly regulate the activity of transcription factors through covalent attachment to lysine residues of target proteins. We examined whether the Ets-1 transcription factor is modified by sumoylation and/or ubiquitinylation. Among four potential SUMO motifs in Ets-1, we identified lysines 15 and 227 within the LK(15)YE and IK(227)QE motifs, as being the sumoylation acceptor sites. Using transfection of Ets-1 wildtype (WT) or its sumoylation deficient version (Ets-1 K15R/K227R), as well as WT or mutant proteins of the SUMO pathway, we further demonstrated that the E2 SUMO-conjugating enzyme Ubc9 and a E3 SUMO ligase, PIASy, can enhance Ets-1 sumoylation, while a SUMO protease, SENP1, can desumoylate Ets-1. We also found that Ets-1 is modified by K48-linked polyubiquitinylation independently of the sumoylation acceptor sites and is degraded through the 26S proteasome pathway, while sumoylation of Ets-1 does not affect its stability. Finally, sumoylation of Ets-1 leads to reduced transactivation and we demonstrated that previously identified critical lysine residues in Synergistic Control motifs are the sumoylation acceptor sites of Ets-1. These data show that Ets-1 can be modified by sumoylation and/or ubiquitinylation, with sumoylation repressing transcriptional activity of Ets-1 and having no clear antagonistic action on the ubiquitin-proteasome degradation pathway.