HSP70: Function

Under physiological conditions, HSP70 family members function as molecular chaperones in an ATP-dependent manner. Depending on their cellular localization HSPs mediate different functions. Intracellular residing HSP70s protect cells against lethal damage induced by stress, and support folding and transport of newly synthesized polypeptides and aberrant proteins as well as the assembly of multi-protein complexes ⁵¹. Extracellular HSPs are considered as molecules with immunomodulatory functions ^{82, 83}, either as cross-presenters of immunogenic peptides via MHC antigens ^{84, 85} or in a peptide-free version as chaperokines ⁸⁶ or stimulators of innate immune responses ⁷⁰. On malignantly transformed cells, HSPs enhance cell growth, suppress senescence, and confer resistance to stress-induced apoptosis including protection against cytostatic drugs and radiation therapy ⁷⁶. With respect to senescence, downregulation of Hsp70-1 has been shown to trigger cell senescence via activation and stabilization of p53 and induction of the cell cycle inhibitor p21 ⁸⁷. Activation of p53 obviously occurs through destabilization and inhibition of the negative p53 regulator Mdm-2 ⁸⁷. Senescence upon Hsp70-1 depletion also involves p53-independent pathways that are described under the Mechanisms and Interactions section. Several data support a cancer-related survival role of HSP70s in stabilizing lysosomal membranes and macro-autophagy ^{79, 88,89,90}. Both, Hsp70-1 and Hsp70-8 (Hsc70) are critical co-chaperones for Hsp90 involved in the delivery of client proteins to Hsp90 ⁹¹. Hsp90 client proteins comprise the human epidermal growth factor receptor 2 (Her-2, also known as Erb-2/Neu), Akt, Cdk-4, Raf-1 and the epidermal growth factor receptor (reviewed by Murphy ⁹²).

Moreover, HSPs serve as tumor-specific target structures for the recognition by activated NK cells. Even in the absence of immunogenic peptides, Hsp70-1 or a peptide derived thereof in combination with proinflammatory cytokines such as IL-2 and IL-15 has the capacity to stimulate the cytolytic activity of NK cells against membrane Hsp70-1 positive tumor cells ^{93, 94}. The mechanism of tumor cell killing has been identified as perforin-independent granzyme B-mediated apoptosis ⁹⁵. Granzyme B derived from activated NK cells specifically binds to membrane Hsp70-1 on tumor cells, and following Hsp70-1 mediated endocytosis apoptosis is induced ^{95, 96}. Hsp70-1 also has been detected on tumor-derived exosomes of membrane Hsp70-1 positive tumors ¹³. These data suggest that NK cells might be attracted to membrane Hsp70-1 positive tumors in vivo via the secretion of Hsp70-1 surface-positive exosomes. Incubation of NK cells with Hsp70-1 protein or a 14mer-peptide derived from the C-terminus of Hsp70-1 (termed TKD) is accompanied by an upregulation of activating receptors on NK cells such as CD94/NKG2C, NKG2D, NKp30, NKp44, and NKp46 ^{95, 97}. Hsp70-1 membrane-positive tumors are thus efficiently eliminated by NK cells that had been pre-stimulated with low dose IL-2 plus Hsp70-1 peptide ⁹⁸. Adoptive transfer of these TKD-stimulated NK cells in tumor-bearing mice revealed identical results in vivo ^99,100,101. It is known that IL-2-activated NK cells are able to induce regression of established lung and liver tumors ^{102,103,104,105}. Recently, a specific migratory capacity of NK cells towards Hsp70 postive tumor cells and supernatants derived thereof could be demonstrated. The same effect could be observed for the Hsp70-1 peptide TKD ¹⁰⁶ implying that killing of Hsp70-1 positive tumors in vivo might be related to an enhanced migratory and cytolytic capacity of preactivated NK cells.