Clients and Oncogenic Roles of Molecular Chaperone gp96/grp94

Abstract

As an endoplasmic reticulum heat shock protein (HSP) 90 paralogue, glycoprotein (gp) 96 possesses immunological properties by chaperoning antigenic peptides for activation of T cells. Genetic studies in the last decade have unveiled that gp96 is also an essential master chaperone for multiple receptors and secreting proteins including Toll-like receptors (TLRs), integrins, the Wnt coreceptor, Low Density Lipoprotein Receptor-Related Protein 6 (LRP6), the latent TGFβ docking receptor, Glycoprotein A Repetitions Predominant (GARP), Glycoprotein (GP) Ib and insulin-like growth factors (IGF). Clinically, elevated expression of gp96 in a variety of cancers correlates with the advanced stage and poor survival of cancer patients. Recent preclinical studies have also uncovered that gp96 expression is closely linked to cancer progression in multiple myeloma, hepatocellular carcinoma, breast cancer and inflammation-associated colon cancer. Thus, gp96 is an attractive therapeutic target for cancer treatment. The chaperone function of gp96 depends on its ATPase domain, which is structurally distinct from other HSP90 members, and thus favors the design of highly selective gp96-targeted inhibitors against cancer. We herein discuss the strategically important oncogenic clients of gp96 and their underlying biology. The roles of cell-intrinsic gp96 in T cell biology are also discussed, in part because it offers another opportunity of cancer therapy by manipulating levels of gp96 in T cells to enhance host immune defense.

Fig. 1. Model of gp96 cancer-associated clientele

Gp96, a resident ER protein chaperones TLR1, TLR2, TLR4, TLR5 and TLR6 through the Golgi apparatus to the cell surface (i) and TLR7, TLR8 and TLR9 to endosomes (ii). Gp96 also chaperones multiple integrins (αβ subunits) (iii) and participates in canonical Wnt signaling by folding the fizzled co-receptor, LRP6 (iv). Recently, gp96 was also shown to be the key molecular chaperone for GARP (v). For clarity only relevant molecules are depicted.

Fig. 2. Summary of the roles of membrane-bound and soluble gp96 in adaptive immunity

(i) Tumor-bearing membrane-bound gp96, or soluble gp96 released by tumor cells (ii), can induce maturation of immature dendritic cells (iDCs) into fully matured DCs (mDCs) via interaction with gp96 receptors on the surface of DCs. Matured DCs can induce activation of both CD4⁺ and CD8⁺ T cells, leading to apoptosis of tumor cells (iii). In some cases, LPS-activated B cells express surface gp96 (iv), which interacts with gp96 receptors such as CD91 on activated CD4⁺ T cells (v). Here, surface gp96 acts as a co-stimulatory molecule for T cell differentiation into the Th2 cell lineage (vi). These cells release Th2-specific cytokines which provide support for CD8 T cells in mediating tumor cell killing (vii).

Fig. 3. Roles of molecular chaperone gp96 in oncogenesis from the vantage point of immunology

Upon TCR engagement in regulatory T cells (Tregs), gp96 mediates the surface expression of the latent TGF-β docking receptor, GARP (i). Expression of surface GARP provides a site for further binding and activation of latent TGF-β via multiple mechanisms including, cleavage by integrins such as α_vβ₆ integrins, which are clients of gp96 (ii). Tregs then mediate suppression of effector CD4⁺ and CD8⁺ T cells via cell-cell contact using membrane-bound active TGF-β (iii), or soluble active TGF-β released upon integrin cleavage of latent TGF-β from GARP (iv). Treg suppression of the adaptive immune response promotes tumor progression and the release of active TGF-β mediates epithelial-mesenchymal transition of normal epithelial cells (v). Furthermore in macrophages, chronic inflammation coupled with carcinogens triggers gp96 upregulation, which in-turn chaperones TLRs and augments the induction of tumor-associated macrophages (TAMs) (vi). TAMs support tumor progression by releasing cytokines including IL-6, IL-17, IL-23 and TNFα (vii). In addition, gp96 plays a major role in the oncogenesis of multiple myeloma (viii). Gp96 chaperones the Wnt signaling co-factor, LRP6, to the cell surface and this augments Wnt signaling to inhibit GSK-3β and mediate nuclear translocation of β-catenin (ix). This mechanism promotes excessive proliferation of multiple myeloma cells (x).