1. ¹Department of Evolutionary Biology, University of Siena, Siena I-53100, Italy
2. ²Department of Biomedical Sciences and CNR Institute of Neuroscience, University of Padova, Padova I-35121, Italy
3. ³Department of Molecular Oncology, European Institute of Oncology, Milano I-20141, Italy

Correspondence: CT Baldari, Department of Evolutionary Biology, University of Siena, Via Aldo Moro 2, 53100 Siena, Italy. Tel. +39 0577 234400; Fax +39 0577 234476; E-mail: baldari@unisi.it

⁴The first two authors contributed equally to this work.

Received 3 March 2006; Revised 18 May 2006; Accepted 19 May 2006; Published online 23 June 2006.

Abstract

p66Shc, a redox enzyme that enhances reactive oxygen species (ROS) production by mitochondria, promotes T cell apoptosis. We have addressed the mechanisms regulating p66Shc-dependent apoptosis in T cells exposed to supraphysiological increases in [Ca²⁺]_c. p66Shc expression resulted in profound mitochondrial dysfunction in response to the Ca²⁺ ionophore A23187, as revealed by dissipation of mitochondrial transmembrane potential, cytochrome c release and decreased ATP levels. p66Shc expression also caused a dramatic alteration in the cells' Ca²⁺-handling ability, which resulted in Ca²⁺ overload after A23187 treatment. The impairment in Ca²⁺ homeostasis was ROS dependent and caused by defective Ca²⁺ extrusion due at least in part to decreased plasma membrane ATPase (PMCA) expression. Both effects of p66Shc required Ca²⁺-dependent serine-36 phosphorylation. The mitochondrial effects of p66Shc were potentiated by but not strictly dependent on the rise in [Ca²⁺]_c. Thus, Ca²⁺-dependent p66Shc phosphorylation causes both mitochondrial dysfunction and impaired Ca²⁺ homeostasis, which synergize in promoting T cell apoptosis.