Heat shock proteins (HSPs) are highly conserved molecules, which support folding of proteins under physiological conditions and mediate protection against lethal damage after various stress stimuli. Five HSP families exist defined by their molecular size (i.e. HSP100, HSP90, HSP70, HSP60, and the small HSPs). Hsp70 belongs to the HSP70 family and is expressed at low levels in normal non-stressed cells. Its expression is however induced by different cellular stresses, such as heat shock and oxidative stress.

The function of Hsp70 depends on its cellular location: Intracellular it has cytoprotective and anti-apoptotic functions, whereas it exerts immunostimulatory functions extracellularly. Secreted Hsp70 is for example involved in cross-presentation of cancer-derived antigenic peptides, a function which is currently explored in immunotherapeutic approaches against cancer. Additionally, membrane-bound Hsp70 can stimulate antigen presenting cells (APCs) to release proinflammatory cytokines and can provide a target structure for NK cell-mediated lysis.

Human cancer cells frequently express Hsp70 on their cell surface, whereas the corresponding normal tissues do not. In addition, several clinically applied reagents, such as alkyl-lysophospholipides, chemotherapeutic agents, and anti-inflammatory reagents, have been found to enhance Hsp70 surface expression on cancer cells.

We have found that inhibition of histone deacetylase (HDAC) activity leads to surface expression of Hsp70 on various hematopoietic cancer cells, an occurance that was not observed on naïve or activated peripheral blood cells. HDAC-inhibitor mediated Hsp70 surface expression was confined to the apoptotic Annexin V positive cells and blocked by inhibition of apoptosis. Other chemotherapeutic inducers of apoptosis such as Etoposide and Camptothecin also led to a robust induction of Hsp70 surface expression. Hsp70 expression was however not caused by induction of apoptosis per se, since activated CD4 T cells remained Hsp70 surface negative despite effective induction of apoptosis.

Interestingly, inhibition of endolysosomes or normal ER/Golgi transport did not affect Hsp70 surface expression. Intracellular Calcium and the transcription factor Sp1, that has previously been shown to be important for the intracellular stress mediated by HDAC-inhibitors, were not involved in Hsp70 surface expression. We also found that HDAC-inhibitors decreased cellular Plasma Membrane Electron Transport (PMET) activity and that a selective inhibition of PMET activity with extracellular NADH induced a robust Hsp70 surface expression.

Our data suggest that inhibition of HDAC activity selectively induces surface expression of Hsp70 on hematopoietic cancer cells and that this may increase immunorecognition of these cells.