Tumor-specific Hsp70 plasma membrane localization is enabled by the glycosphingolipid Gb3

Abstract

Background: Human tumors differ from normal tissues in their capacity to present Hsp70, the major stress-inducible member of the HSP70 family, on their plasma membrane. Membrane Hsp70 has been found to serve as a prognostic indicator of overall patient survival in leukemia, lower rectal and non small cell lung carcinomas. Why tumors, but not normal cells, present Hsp70 on their cell surface and the impact of membrane Hsp70 on cancer progression remains to be elucidated.

Methodology/principal findings: Although Hsp70 has been reported to be associated with cholesterol rich microdomains (CRMs), the partner in the plasma membrane with which Hsp70 interacts has yet to be identified. Herein, global lipid profiling demonstrates that Hsp70 membrane-positive tumors differ from their membrane-negative counterparts by containing significantly higher amounts of globotriaoslyceramide (Gb3), but not of other lipids such as lactosylceramide (LacCer), dodecasaccharideceramide (DoCer), galactosylceramide (GalCer), ceramide (Cer), or the ganglioside GM1. Apart from germinal center B cells, normal tissues are Gb3 membrane-negative. Co-localization of Hsp70 and Gb3 was selectively determined in Gb3 membrane-positive tumor cells, and these cells were also shown to bind soluble Hsp70-FITC protein from outside in a concentration-dependent manner. Given that the latter interaction can be blocked by a Gb3-specific antibody, and that the depletion of globotriaosides from tumors reduces the amount of membrane-bound Hsp70, we propose that Gb3 is a binding partner for Hsp70. The in vitro finding that Hsp70 predominantly binds to artificial liposomes containing Gb3 (PC/SM/Chol/Gb3, 17/45/33/5) confirms that Gb3 is an interaction partner for Hsp70.

Conclusions/significance: These data indicate that the presence of Gb3 enables anchorage of Hsp70 in the plasma membrane of tumors and thus they might explain tumor-specific membrane localization of Hsp70.

Figure 1. Quantification of cytosolic and plasma membrane-bound Hsp70 in CX+/CX− and Colo+/Colo− carcinoma sublines.

Western blot analysis of biotinylated whole cell lysates (cytoplasm, upper graph), and plasma membranes (lower graph) of CX+/CX− and Colo+/Colo− tumor sublines. A corresponding Western blot was stained with the Hsp70 specific mAb cmHsp70.1. The figure shows a representative blot from three independent experiments. The Hsp70 surface phenotypes of the tumor sublines, as determined by flow cytometric analysis, are summarized in Table 1.

Figure 2. Hsp70 is associated with detergent resistant microdomains (DRMs).

(A) Definition of the sublethal concentration of the raft disrupting agent methyl-beta cyclodextrin (MbCD). Following incubation of CX+ and CX− tumor sublines with 1 and 10 mM MbCD for 1 h, cells were washed and viability was determined by trypan blue staining after a 12 h recovery period. (B) The images show filipin III binding to untreated (ctrl, left graphs) and to CX+/CX− tumor sublines treated with the sublethal concentration of 1 mM MbCD (right graphs) for 1 h. (C) Cell surface density of Hsp70 in untreated (ctrl) and MbCD (1 mM, 10 mM) treated CX+ cells. Viability was 97.7±0.4 for control and 95±0.7 for MbCD treated cells. The data represent mean fluorescence intensity (mfi) values of three independent flow cytometric analyses. *Indicates values that are significantly different from control (P = 0.006).

Figure 3. Globotriaosylceramide content is significantly higher in Hsp70 membrane-positive tumor sublines compared to their negative counterparts.

Comparative analysis of the cell surface density of glycosphingolipids such as Gb3 (globotriaosylceramide, CD77 Ab), LacCer (lactosylceramide, CDw17 Ab), DoCer (dodecasaccharideceramide, CD65s Ab, data not shown), GalCer (galactosylceramide, MAB 342 Ab), Cer (ceramide, MID15B4 Ab), and GM1 (Cholera Toxin subunit B) in CX+/CX− and Colo+/Colo− tumor sublines. *Indicates values for Hsp70 membrane-positive (CX+, Colo+) and the corresponding Hsp70 membrane-negative (CX−, Colo−) tumor subline that are significantly different (P<0.03).

Figure 4. Bright field and fluorescence microscopic analysis of CX+ and CX− tumor sublines (A), Fabry fibroblasts (B), and Daudi Burkitt's lymphoma cells (C).

Cells were stained either with a relevant isotype-matched control antibody (isotype) or with an Hsp70 (cmHsp70.1-FITC, green) or Gb3 (CD77 plus Cy3-conjugated secondary antibody, red) specific antibody. The co-localization of Hsp70 and Gb3 is visualized in yellow as a merge of red and green in the lowest panel. Scale bar marks 20 µm. In comparison to CX+ and Fabry cells, the density of Gb3 on the cell membrane of Daudi cells is markedly increased, as indicated by a high mean fluorescence intensity of the CD77 staining. A double stain of Hsp70-FITC and Gb3-Cy3 indicates co-localization of both markers on the cell surface (lower right graph). Similar findings were obtained in experiments using the Colo+/Colo− tumor sublines (data not shown). On the right hand panel of Figure 4C, representative flow cytometric profiles of Daudi cells are shown; the upper graph represents the double staining pattern of the isotype-matched antibodies IgM-PE and IgG1-FITC; the graphs below show single staining of Daudi cells with Hsp70-FITC (second graph) and CD77-PE (third graph). The fourth graph represents the double-staining pattern of Daudi cells using Hsp70-FITC and CD77-PE antibodies.

Figure 5. Binding of Hsp70-FITC, and BSA-FITC to viable Gb3 membrane-negative K562 and Gb3 membrane-positive Daudi cells.

(A) Significant binding of Hsp70-FITC was observed to Daudi cells at a concentration of 12 µg/ml and 24 µg/ml. (B) BSA-FITC did neither bind to K562 nor to Daudi cells, at any of the tested concentrations. (C) Specific blocking of Hsp70-FITC binding by anti-Gb3 monoclonal antibody CD77. Daudi cells were kept either untreated or incubated with anti-Gb3 (CD77), or anti-ceramide (MID15B4) antibodies (5 µg/ml) for 30 min on ice prior to incubation with Hsp70-FITC (60 µg/ml, 30 min on ice) and analysis by flow cytometry.

Figure 6. Hsp70 predominantly interacts with artificial raft liposomes containing Gb3.

Quantification of the amount of recombinant Hsp70 protein associated with unilamellar raft-like liposomal pellet fraction consisting of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (PC)/brain sphingomyelin (SM)/cholesterol (Chol), (PC/SM/Chol, 17/50/33); 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (PC)/brain sphingomyelin (SM)/cholesterol (Chol)/brain cerebroside sulfatide (CerSul) (PC/SM/Chol/CerSul, 17/45/33/5); 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (PC)/brain sphingomyelin (SM)/cholesterol (Chol)/galactocerebroside (GalCer) (PC/SM/Chol/GalCer, 17/45/33/5); and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (PC)/brain sphingomyelin (SM)/cholesterol (Chol)/globotriasoylceramide (Gb3) (PC/SM/Chol/Gb3, 17/45/33/5). Data are the total amount of Hsp70 (ng) in the pellet fraction containing liposomes and are means (±SE) of 3 to 5 independent experiments.