Thiol redox transitions in cell signaling: a lesson from N-acetylcysteine

The functional status of cells is under the control of external stimuli affecting the function of critical proteins and eventually gene expression. Signal sensing and transduction by messengers to specific effectors operate by post-translational modification of proteins, among which thiol redox switches play a fundamental role which is just beginning to be understood. The maintenance of the redox status is, indeed, crucial for cellular homeostasis, and its dysregulation towards a more oxidized intracellular environment is associated to aberrant proliferation, ultimately related to diseases such as cancer, cardiovascular disease and diabetes. Redox transitions occur in sensitive cysteine residues of regulatory proteins relevant to signaling, their evolution to metastable disulfides accounting for the functional redox switch. N-acetylcysteine (NAC) is a thiol-containing compound which is able to interfere with redox transitions of thiols, and thus, in principle, able to modulate redox signaling. We here review the redox chemistry of NAC, then we screen possible mechanisms to explain the effects we observed in NAC-treated normal and cancer cells; such effects involve a modification of global gene expression, thus of functions and morphology, with a leitmotif of a switch from proliferation to terminal differentiation. The regulation of thiol redox transitions in cell signaling is therefore proposed as a new tool, holding promise not only for a deeper explanation of mechanisms but indeed for innovative pharmacological interventions.