Nanocarrier based approaches for targeting breast cancer stem cells

Figures & data

Figure 1. The role of BCSCs in tumour relapse and metastasis. (A) Conventional chemotherapeutic agents for treatment of breast cancer eliminate bulk tumour cells (non-BCSCs) while sparing tumour initiating, BCSCs and results in tumour relapse; (B) The transition from an epithelial state to a mesenchymal state (EMT) in BCSCs (or metastatic cells) occurs by activation of transcription factors such as Snail, Slug and Zeb and promotes dissemination. This EMT phenotype of cells enters blood circulation and survives by evading immune surveillance, then enter target organ. These cells can remain in a dormant state, lasting from months to decades or re-acquire epithelial properties by undergoing mesenchymal to epithelial transition (MET) in target organ to initiate secondary tumour growth; (C) Key signalling pathways implicated in the survival of BCSCs.

Figure 2. Possible therapeutic approaches for eradication of BCSCs using nanocarriers. (1) Targeting BCSC surface markers; (2) Silencing self-renewal pathways; (3) inhibiting drug efflux transporters; (4) inhibiting anti-apoptotic proteins; (5) altering metabolism; (6) targeting autophagy process; (7) disruption of vascular niche of the BCSCs to impair the specialized microenvironment housing.

Table 1. Molecular targets of BCSCs.

Molecular targets	Experimental results	References
Surface biomarkers
CD44	Required for cell–cell interaction, cell adhesion, and migration.	[114,115]
CD133	Inhibition of CD133 resulted in elimination of BCSCs and prevented tumour relapse.	[34,116]
EpCAM	EpCAM inhibition using SiRNA resulted in elimination of BCSCs.	[117,118]
Self-renewal pathways
Wnt/β-catenin pathway
Fz receptor	Monoclonal antibody against frizzled receptor inhibited Wnt pathway in BCSCs	[119]
β-catenin	β-catenin (CWP232228) resulted in elimination of BCSCs and non-BCSCs by inhibiting Wnt pathway.	[120]
Axin 2	Axin2 inhibition using small molecule inhibitor results in loss of β-catenin function and down regulates Wnt/β-catenin pathway.	[121,122]
Notch signalling pathway
γ- secretase	γ-secretase enzyme inhibition using MRK-003 (a γ-secretase inhibitor) silenced notch signalling in BCSCs and prevented mammosphere and colony formation.	[123]
MAML	ANTP/DN MAML, a fusion protein targets Notch nuclear co-activator MAML1 to inhibit notch signalling.	[124]
Delta-like 4 ligand (DLL4)	DLL4 inhibition by monoclonal antibodies or small molecule inhibitor resulted in anticancer activity by inhibiting notch signalling pathway.	[125–128]
Hedgehog (Hh) pathway
Smo	Smo antagonist (cyclopamine) prevented drug resistance in MCF-7/ADR cells and eliminates BCSCs.	[129]
Glioma-associated oncogene (Gli)	Gli inhibition using small molecule inhibitor effectively silenced Hh signalling pathway in breast cancer.	[130]
TGF-β	Inhibition of TGF-β signalling resulted in elimination of BCSCs.	[13]
Bmi-1	Bmi-1 silencing using SiRNA inhibited tumour growth and metastasis in breast cancer.	[110]
IL-6/JAK/STAT3 pathway
STAT3	STAT3 inhibition eliminated BCSCs and prevented tumour metastasis.	[131]
Apoptotic/Antiapoptotic proteins
PTEN/PI3/Akt axis	Inhibition of AKT inhibited mammosphere formation and resulted in complete eradication of tumour.	[132]
m-TOR	Inhibition of m-TOR with rapamycin sensitized BCSCs to radiation.	[133]
Bcl2	Bcl2 SiRNA enhanced chemosensitivity of BCSCs.	[134]
Fatty acid synthase (FAS)	Suppression of lipogenesis by modulating FAS expression resulted in apoptosis in BCSCs.	[135]
Death receptor (DR-5)	anti-DR5 antibody resulted in cytotoxicity of BCSCs.	[136]
Autophagy proteins
Beclin1	Depletion of Beclin1 resulted in inhibition of autophagy in BCSCs.	[137]
Metabolic enzymes and transporters
Hexose kinases (HK)	HK inhibition by metformin impairs glucose metabolism in BCSCs and resulted in tumour growth inhibition in breast cancer	[138,139]
Glucose transporters (GLUT)	Inhibition of GLUT1 transporter sensitized breast cancer cells to radiation.	[140]
Microenvironment
Hypoxia inducible factor1α (HIF1α)	HIF1α promotes growth of BCSCs. Inhibitors of HIF1α resulted in chemosensitization of BCSCs.	[141,142]
Carbonic anhydrase-IX (CAIX)	Inhibition of CAIX with novel small-molecule inhibitor eliminated BCSCs.	[143]
CXCR1	CXCR1 inhibition effectively eliminated BCSCs in vitro and in vivo.	[144]

Table 2. Various anti-BCSCs agents and their mechanism of action for elimination of BCSCs.

Anti-BCSC agent	Mechanism of action	References
Disulfiram	Modulator of MAPK signalling pathway	[145]
Flubendazole	Cell cycle arrest at G2/M phase and induced monopolar spindle formation through inhibition of tubulin polymerization	[146]
Metformin	Inhibition of Akt and hexose kinase	[147]
Niclosamide	Inhibition of Wnt, STAT-3, Notch and NF-κB pathways of self-renewal	[148]
Salinomycin	Inhibition of self-renewal	[149]
Simavastatin	Inhibition of mevalonate metabolism	[85]
Thioridazine	Antagonism of dopamine receptor on CSCs	[150]
Tranilast	Activation of aryl hydrocarbon receptor	[151]
Epigallocatechin gallate analogs	Activation of AMPK	[152]
Everolimus	Not established	[153]
Sulforaphine	Down regulation of Wnt-β catenin signalling	[154]
Cyclopamine	Smo Inhibitor	[53]

Figure 3. The preparation procedure of HA modified mesoporous silica nanoparticles (MSN). MSN modified with an amine-containing silane were first loaded with docetaxel or 8-hydroxy quinoline. Liposomes were prepared with different components consisting of DOPC, DOPE, cholesterol and 18:0 PEG-2000 PE (60:5:30:5 mass ratio). HA was conjugated to the liposomes by an EDC-based method. Finally, hyaluronan modified mesoporous silica nanoparticle-supported lipid bilayers (HA-MSS) was constructed by fusing HA-modified liposomes to drug loaded MSN [31] (Reproduced with permission. Copy Right, Elsevier 2013).

Table 3. Targeting BCSC surface markers using nanocarriers.

Surface marker	Therapeutic agent	Nanocarrier	References
CD44	CD44 antibody Paclitaxel	Poly[(d,l-lactide-co-glycolide)-co-PEG](PLGA-co-PEG) micelles	[155]
CD133	CD133 antibody Paclitaxel	CD133 antibody conjugated polymeric nanoparticles	[34]
EpCAM	EpCAM aptamer Doxorubicin	Carbon nanotubes functionalized with EpCAM aptamer and piperazine–polyethylenimine derivative	[36]
	EpCAM SiRNA	Polymeric nanocomplex fabricated with EpCAM aptamer and EpCAM SiRNA	[37]

Table 4. Targeting self-renewal pathways of BCSCs using nanocarriers.

Signalling pathway	Therapeutic agent	Nanocarrier	References
Wnt/β-catenin	Sulforaphane	Self-assembled poly(d, l-lactide-co-glycolide)/hyaluronic acid block copolymer-based nanoparticles	[41]
	Salinomycin	PLGA nanoparticles	[19]
Notch	DAPT	Glucose functionalized mesoporous silica nanoparticles	[20]
	Notch SiRNA	Multifunctional Core/Shell nanoparticles Cross-linked Polyetherimide-folic acid	[47]
TGF-β	LY364947 PLK1 SiRNA	PEG-PLGA nanoparticles	[49]
Hedgehog (Hh)	Cyclopamine Doxorubicin	PLGA nanoparticles	[52]
BMI-1	BMI-1 SiRNA	Polyethylenimine conjugated BMI 1 SiRNA loaded in nanoparticles	[156]
STAT-3	Niclosamide	Polymeric micelles	[61]

Figure 4. Potential nanocarrier systems for efficient targeting of BCSCs. (A) Nanocarriers serve as vehicles for therapeutic agents and increase drug accumulation in tumour tissues via EPR effect. (B) Surface modification of nanocarriers using suitable ligand can reduce off-target effects. (C) Stimuli-responsive nanoparticle systems, which are activated upon exposure to tumour microenvironment may further enhance the accumulation of drugs and also increase cellular internalisation. (D) Nanocarrier systems for co-delivery of dual drugs can improve therapeutic efficacy by simultaneous targeting of both BCSCs and non-BCSCs.

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