Caspase-8 expression and its Src-dependent phosphorylation on Tyr380 promote cancer cell neoplastic transformation and resistance to anoikis.

Caspase-8 expression is lost in a small percentage of tumors suggesting that the retention of its functionality may positively contribute to tumor progression. Consistently, several non-apoptotic functions of Caspase-8 have been identified and Caspase-8 has been shown to modulate cell adhesion, migration and to promote tumor progression. We have previously identified the Src-dependent phosphorylation of Caspase-8 on Tyr380 as a molecular mechanism to downregulate the proapoptotic function of Caspase-8; this phosphorylation occurs in colon cancer and may promote cell migration in neuroblastoma cell lines. However, the occurrence of Caspase-8 phosphorylation on Tyr380 and its significance in different carcinoma cellular models, have not been clarified yet. Here we show that Caspase-8 expression may promote cell transformation in glioblastoma and in hepatocarcinoma cell lines. In these systems Caspase-8 is phosphorylated on Tyr380 in a Src kinase dependent manner and this phosphorylation is required for transformation and it is enhanced by hypoxic conditions. Using a cancer cellular model characterized by Src constitutive activation engineered to express either Caspase-8-wt or Caspase-8-Y380F we could show that Caspase-8 expression and its phosphorylation on Tyr380, but not its enzymatic activity, promote in vitro cell transformation and resistance to anoikis. This work demonstrates a dual role for Caspase-8 in cancer, suggesting that Tyr380 phosphorylation may represent a molecular switch to hijack its activity from tumor suppressor to tumor promoter.

ATM kinase activity modulates ITCH E3-ubiquitin ligase activity.

Ataxia Telangiectasia Mutated (ATM) kinase, a central regulator of the DNA damage response, regulates the activity of several E3-ubiquitin ligases, and the ubiquitination-proteasome system is a consistent target of ATM. ITCH is an E3-ubiquitin ligase that modulates the ubiquitination of several targets, therefore participating to the regulation of several cellular responses, such as the DNA damage response, tumor necrosis factorα (TNFα), Notch and Hedgehog signaling, and the differentiation of 'naive' lymphocytes into T helper type 2 cells. Here we uncover ATM as a novel positive modulator of ITCH E3-ubiquitin ligase activity. A single residue on ITCH protein, S161, which is part of an ATM SQ consensus motif, is required for ATM-dependent activation of ITCH. ATM activity enhances ITCH enzymatic activity, which in turn drives the ubiquitination and degradation of c-FLIP-L and c-Jun, previously identified as ITCH substrates. Importantly, ATM-deficient mice show resistance to hepatocyte cell death, similarly to Itch-deficient animals, providing in vivo genetic evidence for this circuit. Our data identify ITCH as a novel component of the ATM-dependent signaling pathway and suggest that the impairment of the correct functionality of ITCH caused by Atm deficiency may contribute to the complex clinical features linked to Ataxia Telangiectasia.