Apart from its transcriptional regulation, Grp78/BiP protein levels have also been found as being regulated at the post-transcriptional level. In this context, the discovery of micro-RNA (miRNA) identified this RNA subtype as a crucial player in regulating translation of many genes. miRNAs are a class of small non-coding RNAs that negatively regulate gene expression by binding to target mRNAs. Global alterations in miRNAs can be observed in a number of disease states including cancer 152-154. Several investigations identified specific miRNAs that bind to the 3’-untranslated region in the HSPA5 mRNA, including miR-124 155, miR‑199a‑5p 156, miR-181 157, miR-181a 158, miR-181b 159, miR-376a 160, as well as miR-30a 161 and miR-30a-5p 162. Further post-transcriptional regulatory mechanisms include the activation of the internal ribosome entry sequence (IRES) in the 5′-untranslated region of the HSPA5 mRNA 163, crucially involved in the selective translation of the HSPA5 mRNA 164. Moreover, the activation of the PI3K/AKT pathway in response to ER stress has been found by Dai and colleagues to enhance Grp78/BiP protein stability by a currently unknown mechanism 165.
It has been shown previously that several inflammatory mediators and signaling molecules such as NF-κB and TNF are strictly bound to chaperone / HSP gene expression and protein functions. In this respect, the NF-κB subunit p65/RelA acts as a transcription factor for numerous HSPs 166-168. It should be mentioned here, that combined treatment of leukemic cells with the proteasome inhibitor bortezomib and the casein kinase-2 inhibitor CX-4945 down-regulated Grp78/BiP expression and blocked NF-κB signaling 169, indicative for an NF-κB-dependent regulation of the Grp78/BiP expression.
The activity of Grp78/BiP is regulated by several co- and post-translational modifications. Similar to many other chaperones, Grp78/BiP is a phosphoprotein 41 whose expression and function can be further modulated by methylation 42, 43, AMPylation 44, acetylation 45, O-glycosylation 46, ADP-ribosylation 47 and SUMOylation 48. To what extend phosphorylation especially affects the molecular or biological functions of the chaperone is still unclear. Among the numerous post-translational modifications AMPylation should be highlighted. The group of Seema Mattoo discovered AMPylation of Grp78/BiP as a crucial regulator of signal transduction during UPR which is catalyzed by the ER-resident Fic (filamentation induced by cAMP) protein HYPE (Huntingtin yeast interacting protein E) 44. HYPE has been identified to adenylate Grp78/BiP at Ser365 and Thr366 and consequently to boost the ATPase activity of the chaperone, mandatory for refolding of misfolded polypeptides during ER stress 44.