Why Be One Protein When You Can Affect Many? The Multiple Roles of YB-1 in Lung Cancer and Mesothelioma

Abstract

Lung cancers and malignant pleural mesothelioma (MPM) have some of the worst 5-year survival rates of all cancer types, primarily due to a lack of effective treatment options for most patients. Targeted therapies have shown some promise in thoracic cancers, although efficacy is limited only to patients harboring specific mutations or target expression. Although a number of actionable mutations have now been identified, a large population of thoracic cancer patients have no therapeutic options outside of first-line chemotherapy. It is therefore crucial to identify alternative targets that might lead to the development of new ways of treating patients diagnosed with these diseases. The multifunctional oncoprotein Y-box binding protein-1 (YB-1) could serve as one such target. Recent studies also link this protein to many inherent behaviors of thoracic cancer cells such as proliferation, invasion, metastasis and involvement in cancer stem-like cells. Here, we review the regulation of YB-1 at the transcriptional, translational, post-translational and sub-cellular levels in thoracic cancer and discuss its potential use as a biomarker and therapeutic target.

Keywords: Y-box binding protein-1; biomarker; lung cancer; mesothelioma; targeted therapy.

FIGURE 1

YB-1 is altered in NSCLC (ADC and SCC) and MPM patients and high YBX1 mRNA expression correlates with poor prognosis in both diseases. Reported alteration frequencies of YBX1 and commonly altered genes in current TCGA Provisional datasets for all complete tumors with RNASeq V2 RSEM mRNA and RPPA protein Expression for (A) Lung Adenocarcinoma (ADC; n = 584), (B) Lung Squamous Cell Carcinoma (SCC; n = 511) and (C) Mesothelioma (MPM; n = 87). Panels (A–C) were adapted from the open-source platform cBioPortal for Cancer Genomics (cBioPortal.org). (D) High YBX1 expression correlates with poor prognosis in NSCLC patients (p = 1.5 × 10^–10). Kaplan-Meir plot of 1,926 NSCLC patients generated using Lung Cancer KM plotter. Univariate analysis with probe set 20862_s_at (YBX1) using auto-selected cutoff and excluded biased arrays. (E) High YBX1 expression correlates with poor prognosis in MPM patients (p = 8.6 × 10^–3). Kaplan-Meir plot was generated using PROGgene V2 with the TCGA mesothelioma dataset (n = 83) using “DEATH” as the survival measure and median as the cutoff.

FIGURE 2

Control of YB-1 expression. A network of factors controls YBX1 expression at the transcriptional and translational levels. The E-box binding proteins Twist1, Myc and p73 interact with the promoter of YBX1 and initiate transcription of YBX1 mRNA. YBX1 mRNA expression is downregulated by targeting miRNA, including miR-137 and miR-216a. YBX1 translation is stimulated by mTOR, which itself is influenced by proliferation rate. YB-1 protein function and expression are modulated by lncRNA, including MIR22HG and LINC00312. YB-1 is involved in an autoregulatory feedback loop and binds to YBX1 mRNA at two sites (nucleotides 1133–1145 and 1165–1172), inhibiting its own translation. PABP stimulates YBX1 translation by binding to a site located at 1149–1196, overlapping the second YB-1 binding site. Poly(A)-binding protein (PABP) and YB-1 compete for this site and hence regulate the level of YB-1 protein expression. Created with Biorender.com.

FIGURE 3

Post-translational modification of YB-1. The presence of various sites before or after proteasomal cleavage of YB-1 modulates its function and localization, which has implications on antibody use. YB-1 is comprised of a CSD shown in dark gray, an N-terminal domain in white and a CTD, also in white. Within the CTD there are three nuclear localization signals (NLS-1 from amino acid (aa) 149–156, NLS-2 from aa 185 to 194 and NLS-3 from aa 276 to 292), shown in light gray, and one cytoplasmic retention signal (CRS from aa 247 to 267), shown in black. YB-1 is proteolytically cleaved at Glu216 and Glu219 (shown in red and highlighted with a scissors icon), which is thought to stimulate YB-1 translocation. Three commonly used antibodies targeting YB-1 are also shown, two of which have been validated using mass spectrometry (in yellow) and one which is known to cross react with hnRNP1A (in red). If the proteolytic theory of YB-1 translocation is correct, this would also have implications on the use of antibodies. Various post-translational modifications also effect the downstream function and nuclear localization of YB-1. Green dots indicate acetylation, yellow glycosylation, blue phosphorylation, red sumoylation and orange ubiquitination. Solid black arrows indicate a post-translational modification that is produced by a known upstream regulator, or a known function of YB-1. Dotted black arrows indicate a post-translational modification or function that is yet to be fully proven. Blue and dotted blue arrows indicate the movement or supposed movement of YB-1 throughout cellular compartments, respectively. Created with BioRender.com.

FIGURE 4

Subcellular localization of YB-1 – the proteolytic theory of nuclear localization. YB-1 can be found in the nucleus, cytoplasm and extracellular space and its localization is mediated by various factors. Secretion can be preceded by Ubiquitination (orange dot) by HACE1 and acetylation (green dot) by a currently unknown protein. Oxidative stress stimulates stress-granule localization and eventual section of YB-1, where it can bind to the transmembrane protein Notch3 on other cells. YB-1 is cleaved by the proteasome prior to nuclear translocation. Ubiquitination by RBBP6 initiates YB-1 proteolytic cleavage. ΔNp63α prevents full length proteolysis by partially inhibiting YB-1 degradation, resulting in the removal of the CRS. Transportin-1 or WAVE3 bind to NLS of YB-1 and translocate it to the nucleus. RSK can cross into the nucleus, phosphorylating nuclear YB-1 fragments. Solid black arrows indicate a post-translational modification that is produced by a described or known mechanism. Dotted black arrows indicate a post-translational modification whose significance is yet to be realized. Blue and dotted blue arrows indicate the movement or supposed movement of YB-1 throughout cellular compartments, respectively. Created with BioRender.com.

FIGURE 5

Subcellular localization of YB-1 – the phosphorylation theory of nuclear localization. YB-1 can be found in the nucleus, cytoplasm and extracellular space and its localization is mediated by various factors. Secretion can be preceded by Ubiquitination (orange dot) by HACE1 and acetylation (green dot) by a currently unknown protein. Oxidative stress stimulates stress-granule localization and eventual section of YB-1, where it can bind to the transmembrane protein Notch3 on other cells. Phosphorylation is required before nuclear shuttling can take place. Ser102 is phosphorylated by upstream kinases, changing the configuration of YB-1 to block the CRS and allow nuclear shuttling via Transportin-1 or WAVE3. Phosphorylation of Tyr281 by a currently unknown upstream regulator may play a role here too. Solid black arrows indicate a post-translational modification that is produced by a described or known mechanism. Dotted black arrows indicate a post-translational modification whose significance is yet to be realized. Blue and dotted blue arrows indicate the movement or supposed movement of YB-1 throughout cellular compartments, respectively. Created with BioRender.com.

FIGURE 6

Further study required to understand the role of YB-1 and use it in the treatment and management of thoracic cancer patients. Various upstream and downstream regulatory loops and the role of YB-1 in platinum drug resistance, exosomal sorting and proliferation need further study to fully understand the biology of YB-1 in lung cancer and MPM. The mechanism of YB-1 nuclear localization is also under contention and the occurrence and significance of secreted YB-1 is yet to be determined. Standardization of a suitable YB-1 antibody for prognostic application would also be a step forward. Finally, evaluating the current strategies of YB-1 inhibition in vivo further would build a stronger case for translation into humans. Created with BioRender.com.