Endoplasmic reticulum chaperone BiP (GRP78) (GRP78)

Endoplasmic reticulum chaperone BiP, also known as GRP78 or HSPA5, is a 78 kDa member of the heat shock protein 70 (HSP70) family that primarily resides in the endoplasmic reticulum (ER). It serves as a master regulator of ER homeostasis, facilitating protein folding and orchestrating the unfolded protein response (UPR) to mitigate cellular stress (NIH, 2025; NIH, 2010). In many cancers, including hepatocellular carcinoma (HCC), GRP78 is overexpressed and translocated to the cell surface, where it functions as a signaling receptor that promotes tumor cell survival, proliferation, and chemoresistance (NIH, 2024; ACS, 2020). The SP94 peptide (SFSIIHTPILPL) was identified through phage display as a high-affinity ligand that specifically recognizes this cell-surface GRP78 on HCC cells (NIH, 2008; AACR, 2008). This specificity allows SP94 to be used as a targeting moiety for the delivery of various therapeutic payloads, such as liposomal doxorubicin or sorafenib-loaded nanoparticles, directly to tumor cells while sparing normal hepatocytes (NIH, 2020; ResearchGate, 2020). Beyond its utility in drug delivery, GRP78 is a direct therapeutic target; its inhibition can trigger apoptosis and reverse the pro-survival signaling pathways that drive HCC progression (NIH, 2024; NIH, 2013).

Other names

78 kDa glucose-regulated proteinHSPA5BiPImmunoglobulin heavy chain-binding proteinHepatocellular carcinoma cell-surface marker recognized by SP94 peptideSP94-binding protein

The SP94 peptide acts as a targeting ligand that binds to cell-surface GRP78, facilitating receptor-mediated endocytosis of conjugated therapeutic agents (e.g., doxorubicin or sorafenib) specifically into hepatocellular carcinoma cells (NIH, 2024; ACS, 2020). Additionally, small molecule inhibitors like HA15 target the ATPase activity of GRP78 to induce ER stress and apoptosis in cancer cells (NIH, 2025).