CD44 variant exons induce chemoresistance by modulating cell death pathways

Abstract

Cancer chemoresistance presents a challenge in oncology, often leading to treatment failure and disease progression. CD44, a multifunctional cell surface glycoprotein, has garnered attention for its involvement in various aspects of cancer biology. Through alternative splicing, CD44 can form isoforms with the inclusion of only standard exons, typical for normal tissue, or with the addition of variant exons, frequently expressed in cancer tissue and associated with chemoresistance. The functions of CD44 involved in regulation of cancer signaling pathways are being actively studied, and the significance of specific variant exons in modulating cell death pathways, central to the response of cancer cells to chemotherapy, begins to become apparent. This review provides a comprehensive analysis of the association of CD44 variant exons/total CD44 with clinical outcomes of patients undergoing chemotherapy. The role of CD44 variant exons v6, v9 and others with a significant effect on patient chemotherapy outcomes by means of key cellular death pathways such as apoptosis, ferroptosis and autophagy modulation is further identified, and their impact on drug resistance is highlighted. An overview of clinical trials aimed at targeting variant exon-containing isoforms is provided, and possible directions for further development of CD44-targeted therapeutic strategies are discussed.

Keywords: CD44 variant exon 6; CD44 variant exon 9; apoptosis; autophagy; cancer; chemoresistance; ferroptosis.

FIGURE 1

The genomic organization and protein structure of human CD44. CD44 human gene contains 19 exons. Alternative splicing results in the formation of CD44 standard and variant isoforms. CD44 protein consists of an intracellular and transmembrane domain, consisting of exons 18–19 and 17, and an extracellular domain, consisting of exons 1–16. Inclusion of variant exons (pink) to CD44 isoforms is observed during cancer progression and chemoresistance, with the latter being associated with variant exons v3, v6 and v9 (Created with Biorender.com).

FIGURE 2

Expression profile of CD44 splice variants in patients with several tumor types, including the most common and those associated with the highest mortality. Created by www.gepia2.com.

FIGURE 3

Schematic representation of total CD44 (CD44v and CD44s) involvement in cell death pathways. (A) CD44 is able to form an anti-apoptotic complex with ERBB2. CD44 binds ezrin, Vav2 and GAB1 at the ICD. ERBB2 recruits GRB2, Hsp70/Cdc37 and PI3K. The complex formation results in PI3K/Akt anti-apoptotic signaling promotion upon HA stimulation. (B) Binding of LMW-HA by CD44 leads to inhibitory phosphorylation of MST1/2 by PAR1b and consequent YAP activation, thereby promoting apoptosis inhibition and cell growth and proliferation activation. (C) Lipid raft associated CD44, CD147 and EGFR complex promotes EGFR signaling through the RAS-Erk downstream signaling proteins, conferring anti-apoptotic signaling. (D) Total CD44 is involved in the uptake of HA-bound iron, necessary for the function of iron-dependent proteins including nuclear demethylases. Nuclear demethylase PHF8, activated upon iron binding, promotes the expression of CD44 in a positive-feedback loop. (E) OPN-CD44-ITG-p38MAPK promotes autophagy. OPN-induced autophagy was inhibited by adding an ITG inhibitor RGD and/or anti-CD44 antibody; p38MAPK inhibitor SB203580 significantly attenuated autophagy by decreasing the expression levels of ATG4B, BECLIN1/ATG6, BNIP3 and VPS34 (not shown). (F) OPN-CD44-NF-κB promotes autophagy. NF-κB inhibitor BAY 1170–82 significantly inhibited the OPN-induced LC3-II expression and cell populations with CD44⁺/CD133⁺ expression profile (Created with BioRender.com).

FIGURE 4

Clinical trials aimed at targeting total CD44 (Created with bioRender.com). (A) Antibodies/peptides. (B) Pharmacological inhibitors.

FIGURE 5

Schematic representation of CD44 variant exon v3 interaction with cellular receptors/intracellular proteins involved in cell death signaling. (A) CD44v3 containing isoforms recruit MMP7 and inactive HB-EGF, leading to consequent cleavage, activation and presentation of HB-EGF to ERBB4, thereby promoting anti-apoptotic signaling. (B) CD44v3 containing isoforms form a complex with ERBB2, Vav2 and Grb2 and promote anti-apoptotic Ras-MAPK signaling. (C) The ICD of CD44v3 containing isoforms promotes the formation of Oct4, Sox2 and Nanog complex and its consequent shift to the nucleus. The intranuclear Oct4/Sox2/Nanog complex promotes the expression of miR-302a and miR-302b, promoting anti-apoptotic signaling. (D) CD44v3 promotes antiapoptotic signaling through PI3K/STAT3 pathway. (E) CD44v6 containing isoforms activate c-Met anti-apoptotic signaling: upon binding of HGF, CD44v6 form dimeric complexes (not shown in figure) that are able to laterally diffuse through the plasma membrane for rapid interaction with c-Met with the formation of an active trimeric complex (Created with BioRender.com).

FIGURE 6

Schematic representation of CD44 variant exon v6 interaction with cellular receptors/intracellular proteins involved in cell death signaling. (A) The interference of CD44v6 containing isoforms in apoptosis induction through the extrinsic pathway possibly by blocking the trimerization of FASR, necessary for FASL interaction. (B) CD44v6 containing isoforms activate c-Met anti-apoptotic signaling: upon binding of HGF, CD44v6 form dimeric complexes (not shown in figure) that are able to laterally diffuse through the plasma membrane for rapid interaction with c-Met with the formation of an active trimeric complex. (C) CD44v6 ICD forms a complex with TG2 and ERK1/2 promoting anti-apoptotic signaling. (D) CD44v6 containing isoforms stimulate autophagy induction through increasing the levels of BECN1, one of the key players in autophagic vesicle extension (Created with BioRender.com).

FIGURE 7

Clinical trials aimed at targeting CD44v6 containing isoforms (Created with bioRender.com).

FIGURE 8

Schematic representation of CD44 variant exon v9 involvement in cell death pathways regulation. CD44v9 containing isoforms interact with and stabilize the xCT transporter, a key player in GSH synthesis and ferroptosis regulation. OTUB1 and MUC1-C provide additional stability of CD44-xCT complex (Created with BioRender.com).

FIGURE 9

Clinical trials aimed at targeting CD44v9 containing isoforms (Created with bioRender.com).