HSP70: Disease Relevance

A growing wealth of evidence indicates the pivotal role of Hsp70-1 in tumorigenesis. Constitutively elevated levels of Hsp70-1 can be found in a wide spectrum of cancer cells where Hsp70-1 enhances cell growth, suppresses senescence, and confers resistance to stress-induced apoptosis including protection against cytostatic drugs and radiation therapy ⁷⁶. Hsp70-1 overexpression correlates with elevated tumor cell proliferation, clinical stage or increased grade and poor prognosis ^{180,181,182,183}. Hsp70-1 upregulation is induced through activation of the Her-2/neu pathway during mammary tumorigenesis ^{184, 185}. Upregulation of Hsp70.2 (HspA2), a protein essential for spermatogenesis, has been identified in breast cancer cells as being an important regulatory growth element ¹⁸⁶. Interestingly, concomitant depletion of Hsp70-1 and Hsp70.2 had a synergistic antiproliferative effect on cancer cells thus identifying these HSP70s as essential components for cancer cell growth and survival while the survival of tumorigenic as well as nontumorigenic cells obviously seems to be controlled by Hsp70-8. Nevertheless, the predominant HSP70 overexpressed in cancer is represented by Hsp70-1. Elevated Hsp70-1 expression has been identified as a marker of early-stage prostate and hepatocellular carcinoma ^{187, 188}. Elevated Hsp70-1 levels correlate with poor prognosis not only in patients with breast, endometrial, uterine cervical, and bladder carcinomas ¹³¹ but also in patients with acute myeloid leukemia ¹⁸⁹ and colorectal carcinomas ¹⁹⁰. In colorectal and breast cancer, Hsp70-1 upregulation functions as a marker for advanced disease and lymph node metastasis ^{191, 192}. Elevated Hsp70-1 expression in osteosarcomas correlates with good response to neoadjuvant chemotherapy ¹⁹³. In patients with renal cancer, Hsp70-1 upregulation was associated with better prognosis regardless of stage and histological grade ¹⁹⁴. It is interesting to note that Hsp70-1 expression is frequently reduced in patients with esophageal squamous cell carcinoma and can be considered as an independent negative prognostic factor of this disease ^{195, 196}. The expression of Hsp70-1 has also been found as being upregulated in non-small cell lung carcinoma (NSCLC) specimen where it correlated with the Ki-67 proliferation index ¹⁹⁷. Furthermore, Hsp70-1 expression was statistically significantly associated with malignant rather than benign ovarian tumors ¹⁹⁸. Hsp70-1 levels correlated to the clinical stage of malignant cutaneous melanoma as well as oral cancer ^{181, 182} and grade, stage and survival of patients with bladder cancer ¹⁸⁰.

Phenotypic analyses revealed that Hsp70-1 is found on the plasma membrane in 50 – 70% of colon, lung, pancreas, mammary, head and neck, lung and urogenital carcinomas ^{70, 199, 200}. As demonstrated in a xenograft tumor mouse model, metastases exhibit an elevated Hsp70-1 membrane density compared to primary tumors after orthotopic injection of human tumor cells into immunodeficient animals ^{101, 201}. Interestingly, the corresponding normal tissue of the mice was always found to be membrane Hsp70-1 negative ²⁰¹ suggesting that membrane Hsp70-1 might facilitate metastases, support adherence of tumor cells to endothelial cells and organs, or might confer resistance to an unfavourable milieu during metastasis. In line with these findings overall survival of patients with lower rectal carcinomas and NSCLC exhibiting a membrane Hsp70-1 positive phenotype has been found to be significantly lower than that of their membrane Hsp70-1 negative counterparts ⁷². Apart from solid tumors, also bone marrow samples of patients suffering from acute (AML) and chronic (CML) myeloid leukemia are frequently membrane Hsp70-1 positive ²⁰². Quantitative analysis revealed that approximately 15 – 20% of the total Hsp70-1 is present in tumor cell membranes ¹⁶. Tumor cells expressing Hsp70-1 on the plasma membrane are in general highly resistant to chemoradiotherapy most likely due to blockage of the NF-κB, JNK and ERK signaling pathways ^{203, 204}. They can be eliminated by preactivated NK cells via granzyme B release in a perforin-independent, but Hsp70-1-dependent manner ⁹⁵.

Despite its impact in carcinogenesis, HSP70s have also been shown to play critical roles in other pathological conditions. Overexpression of Hsp70-1 in patients with myelodysplastic syndromes (MDS) was found as being associated with poor prognosis ²⁰⁵. MDS comprise a group of clonal hematopoietic stem cell disorders characterized by varying degrees of pancytopenia and often a progression to acute myeloid leukemia ²⁰⁶. Enhanced plasma levels of Hsp70-1 were recently reported in patients with idiopathic inflammatory myopathy although the plasma levels did not correlate with clinicopathological parameters ²⁰⁷. Hsp70-1 upregulation was also reported for the peripheral lung tissues of chronic obstructive pulmonary disease (COPD) patients where it was closely related to COPD disease severity and smoking status ²⁰⁸. Of note, Hsp70-2 protein content from peripheral blood mononuclear cells (PBMCs) was significantly lower in multiple sclerosis (MS) patients with GG genotype compared to AA genotype, indicating an implication of the G allele of HSP70-2 gene (HSPA1B) polymorphism in the development of MS ²⁰⁹. A single nucleotide polymorphism in HSP70-2 has been shown to be associated with a severe clinical course in Crohn’s disease (CD). A recent study analyzed associations between the Hsp70-2 polymorphism and the clinical courses of CD in the Chinese population. Herein, the allele A of the PstI polymorphism was identified as the association between CD and HSP70-2 gene in this population ²¹⁰. In patients with heart failure (HF), plasma concentrations of Hsp70-1 increased gradually with the progression of HF stages rendering Hsp70-1 a potential screening biomarker for early diagnosis of HF ²¹¹. This finding was extended by a further study identifying elevated extracellular Hsp70-1 levels as an independent prognostic marker of mortality in HF patients ²¹².

A growing body of evidence now indicates that the level and localization of Hsp70-5 (Grp78) is altered in different models of neurodegenerative disorders (ND) including Parkinson’s disease, Alzheimer’s disease and progressive retinal degeneration ^213,214,215. These disorders are characterized by activation of the unfolded protein response (UPR) as a result of ER stress and modified expression of Hsp70-5 (Grp78), the ER homolog of Hsp70 and a key UPR mediator ²¹⁶. Consequently, cell death pathways are activated involving a subsequent crosstalk between the ER and mitochondria leading to apoptosis ²¹⁷. ER stress is therefore considered to be a common mediator of apoptosis in neurodegenerative disorders. The existing literature clearly indicates an age-dependent reduction of the Hsp70-5 expression and activation together with a modification of the ER structure in ND models ^{218, 219}. This raises the question of whether the loss of Hsp70-5 function could be a predisposing factor for many age-associated ND including Parkinson’s disease, Alzheimer’s disease and age-related macular degeneration.

Hsp70-1 is present in the peripheral circulation of healthy nonpregnant and pregnant individuals. In normal pregnancy, circulating Hsp70-1 levels are decreased, and show a positive correlation with gestational age and an inverse correlation with maternal age ²²⁰. In contrast, elevated circulating Hsp70-1 concentrations are associated with an increased risk of several pregnancy complications. In preeclampsia, serum Hsp70-1 levels are increased reflecting systemic inflammation, oxidative stress and hepatocellular injury ^{221, 222}. Moreover, serum Hsp70-1 levels are significantly higher in patients with the syndrome of hemolysis, elevated liver enzymes, and low platelet count (HELLP syndrome) than in severely preeclamptic patients without HELLP syndrome ²²³. In HELLP syndrome, elevated serum Hsp70-1 levels are indicative for tissue damage (hemolysis and hepatocellular injury) and disease severity ²²⁴.

In prokaryotes, DnaK has been shown as being required for survival of bacteria under stressful conditions ²²⁵. Consistent with this notion, mutants of dnaK from S. aureus show reduced viability and susceptibility to stress accompanied by an increased sensitivity towards oxacillin and methicillin in normally resistant S. aureus strains ²²⁶. Moreover, dnaK or dnaJ mutations in E. coli have been reported to sensitize the cells to fluoroquinolones ²²⁷. Since these chaperones are also required for the invasion of epithelial cells by Salmonella enerica and the survival of Listeria monocytogenes in macrophages ^{228, 229}, Hsp70-1/DnaK and its co-chaperones can be considered as potential drug targets that would sensitize prokaryotes to stress. It is interesting to note that in mammalian cells infected with a virulent form of S. choleraesuis, the levels of Hsp70-1 correlated with an increase in TNF-induced cell death ²³⁰ highlighting its role in the host’s immune responses. More recently, Hsp70-peptide complexes (Hsp70.PC-F) extracted from fusions of dendritic cells (DCs) and radiation-enriched tumor cells were used to treat mice with pre-existing lung metastases ²³¹. Immunization of mice with the Hsp70.PC-F vaccine resulted in a T cell-mediated immune response, including the induction of effector T cells capable of targeting radioresistant tumor cells. The combination of chaperone vaccine with radiotherapy inhibited the growth of primary tumors and the number of tumor cells metastasizing to lung suggesting that Hsp70.PC-F vaccine enable the induction of specific immunity to radioresistant populations of mammary tumor cells by complementing radiotherapy, leading to synergistic killing.²³¹