Adam H. Kramer

The balance between cellular lineages can be controlled by reactive oxygen species (ROS). Cellular differentiation into adipocytes is highly dependent on the production of ROS to initiate the process through activation of multiple interlinked factors that stimulate mitotic clonal expansion and cellular maturation. The signal transducer and activator of transcription family of signaling proteins have accepted roles in adipogenesis and associated lipogenesis. Non-canonical mitochondrial localization of STAT3 and other members of the STAT family however opens up new avenues for investigation of its role in the aforementioned processes. Following recent observations of differences in mitochondrially localized serine 727 phosphorylated STAT3 (mtSTAT3-pS727) in preadipocytes and adipocytes, here, we hypothesize and speculate further on the role of mitochondrial STAT3 in adipogenesis.

A mechanistic relationship exists between protein localisation, activity and cellular differentiation. Understanding the contribution of these molecular mechanisms is required for elucidation of conditions that drive development. Literature suggests non‐canonical translocation of the Signal Transducer and Activator of Transcription 3 (STAT3) to the mitochondria contributes to the regulation of the electron transport chain, cellular respiration and reactive oxygen species production. Based on this we investigated the role of mitochondrial STAT3, specifically the serine 727 phosphorylated form, in cellular differentiation using the well‐defined mouse adipogenic model 3T3‐L1. Relative levels of reactive oxygen species (ROS) and the levels and dynamic localization of pSTAT3S727 were investigated during the initiation of adipogenesis. As a signalling entity, ROS is known to regulate the activation of C/EBPβ to stimulate a critical cascade of events prior to differentiation of 3T3‐L1. Results indicate that upon induction of the differentiation programme, relative levels of mitochondrial pSTAT3S727 dramatically decrease in the mitochondria; in contrast the total cellular pSTAT3S727 levels increase. A positive correlation between increasing levels of ROS and dynamic changes in C/EBPβ indicate that mitochondrial STAT3 plays a potential critical role as an initiator of the process. Based on these findings we propose a model for mitochondrial STAT3 as a regulator of ROS in adipogenesis. J. Cell. Biochem. 116: 1232–1240, 2015.

Mitochondria are key to eukaryotic cell survival and their activity is linked to generation of reactive oxygen species (ROS) which in turn acts as both an intracellular signal and an effective executioner of cells with regards to cellular senescence. The mitochondrial molecular chaperone tumor necrosis factor receptor associated protein 1 (TRAP1) is often termed the cytoprotective chaperone for its role in cancer cell survival and protection from apoptosis. Here, we hypothesize that TRAP1 serves to modulate mitochondrial activity in stem cell maintenance, survival and differentiation. © 2013 IUBMB Life, 66(1):42–45, 2014

Heat shock protein 90 (Hsp90) functionally modulates signal transduction. The signal transducer and activator of transcription 3 (STAT3) mediates interleukin‐6 family cytokine signaling. Aberrant activation and mutation of STAT3 is associated with oncogenesis and immune disorders, respectively. Hsp90 and STAT3 have previously been shown to colocalize and coimmunoprecipitate in common complexes. Surface plasmon resonance spectroscopy revealed a direct, high affinity specific interaction between recombinant Hsp90β and STAT3β in the presence and absence of adenosine triphosphate (ATP) in molar excess. Furthermore, comparative analysis using a phosphomimetic mutation at tyrosine 705 showed that the direct interaction appeared to favor neither unactivated nor activated STAT3. Destabilizing mutation of STAT3 at arginine residues 414/417 to alanine in the DNA‐binding domain, previously shown to disrupt nuclear translocation in vivo, reduced interaction with a STAT3 DNA binding site oligonucleotide and Hsp90β in vitro, indicating that STAT3 requires a functional DNA‐binding domain for full direct interaction with Hsp90. Site‐directed mutagenesis of a mammalian STAT3–EGFP‐N1 fusion construct at RR414/417 and subsequent transfection into human MCF7 epithelial breast cancer cells showed no impaired nuclear translocation when observed by confocal laser scanning microscopy. However, costaining for Hsp90α/β isoforms and colocalization analysis revealed a defined decrease in pixel‐on‐pixel colocalization compared with the wild‐type confirming the requirement of the DNA‐binding domain for high‐affinity interaction. © 2012 IUBMB IUBMB Life, 2012