Upregulation of prosurvival cellular pathways
Docetaxel, in addition to stabilizing microtubules, also induces apoptosis by downregulating antiapoptotic proteins.69 BCL2 expression was noted to be an independent predictor of survival in patients treated with taxanes.70,71 BCL2 inhibits mitochondrial release of cytochrome c and subsequently blocks the caspase cascade, thereby inhibiting apoptosis. During treatment with taxanes, BCL2 is phosphorylated, which prevents heterodimerization with other BCL family genes, thereby promoting apoptosis. Inherited resistance is noted in prostate cancer cells that do not express BCL2, indicating that taxanes’ mechanism of action relies at least partly on BCL2 inhibition.70 Mcl1 (myeloid cell leukemia differentiation protein 1) and other members of the BCL family, such as BCL-xl (B-cell lymphoma-extra-large), are also involved in resistance to Interleukin (IL)-6, stromal cell derived factor-1, and cytokine-induced apoptosis.71 Clusterin is a small heat shock glycoprotein overexpressed in most of the solid tumors, which promotes apoptosis by binding to various molecules such as BAX (BCL2-associated X protein)72 and signal transducer and activator of transcription (STAT)−1. BAX and STAT are overexpressed and regulate clusterin expression in docetaxel-treated patients, indicating their role in cytoprotection from chemotherapy.73
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Inhibitors of apoptosis proteins (IAPs), mainly survivin and X-linked inhibitor of apoptosis (XIAP) prevent the processing of procaspase 3 to caspase 3, thereby inhibiting apoptosis.74 Nuclear factor (NF)-κB plays a pivotal role in mounting an inflammatory response. Translocation of NF-κB leads to activation of the genes for IL-6, stress response elements, and many antiapoptotic elements such as IAPs.75,76 Tumor necrosis factor (TNF)–α inhibits apoptosis by activating NF-κB and its downstream pathway, including IL6 and IL8, in androgen-independent prostate cancer cells, whereas it promotes apoptosis in androgen-dependent cancer cells.77 IL-8 acts through chemokine receptors 1 and 2 (CXCR1 and 2) and is involved in promoting angiogenesis through overexpression of vascular endothelial growth factor (VEGF) and fibroblast growth factor (FGF).78,79
Aberrations of AR, erythroblast transformation-specific (ETS) genes, Tumor protein 53 (TP53), and Phosphatase and tensin homolog (PTEN) occurred in 40%–60% of 150 mCRPC cases in a recent study. Additionally, alterations of phosphoinositide-3-kinase, catalytic, alpha polypeptide A/B (PI3KCA A/B), R-spondin, RAF/Rapidly Accelerated Fibrosarcoma1 (BRAF/RAF1), Adenomatous polyposis coli (APC), β-catenin, Zinc finger and BTB domain-containing protein 16/Promyelocytic leukemia zinc finger protein (ZBTB16/PLZF), Breast cancer 2 (BRCA2), Breast cancer 1 (BRCA1), Speckle Type POZ Protein (SPOP), and Ataxia telangiectasia mutated (ATM) were seen. Interestingly, 23% harbored DNA repair pathway aberrations, which appear to correlate with responses to the polyadenosine diphosphate-ribose polymerase (PARP) inhibitor, olaparib, in a recent trial.80,81
Activation of PI3K (phosphoinositide 3 kinase), protein kinase B (PKB), and mammalian target of rapamycin (mTOR) pathways may lead to chemotherapy resistance by either upregulating multidrug resistance protein or by overexpression of various protooncogenes and growth factors such as cyclin D1, VEGF, and c-myc.82,83 Drugs inhibiting pathways such as the Hedgehog, β-catenin, epidermal growth factor receptor (EGFR), endothelin, mitogen-activated protein kinases (MAPK) pathways are also implicated in reviving sensitivity to chemotherapy in chemoresistant cell lines.84–86 The development of neuroendocrine prostate cancer (including small cell cancer) also appears to confer resistance, and a genomic signature correlating with neuroendocrine transformation has been identified.87
Role of tumor microenvironment: angiogenesis and immune mechanisms
The tumor microenvironment plays a substantial role in cancer cell survival and development of resistance to chemotherapy. The majority of solid tumors are composed of tumor cells mixed with noncancerous cells supported by a disorganized vascular network.88 The blood vessels are farther than in normal tissue, which lead to regions of hypoxia and impaired delivery of nutrients as well as improper clearance of metabolic breakdown products. The chaotic blood supply also limits the delivery of cytotoxic drugs including taxanes to the cancer cells.88 The accumulation of metabolic by-products, including lactic acid, increases the acidity of the tumor environment, which also influences the drug uptake by tumor cells.89 The tumor hypoxia may retard tumor proliferation, rendering them more resistant to cell cycle-active chemotherapeutics as well as promoting a more malignant phenotype.90 The hypoxic state also leads to upregulation of genes that promote cell survival including hypoxia–inducible factor (HIF)-1. This inturn leads to suppression of apoptosis, increased receptor tyrosine kinase signaling and increased angiogenesis, thereby promoting cell survival and metastases.91 The intratumoral drug uptake is impeded by high interstitial fluid pressure and absence of lymphatic flow, causing stasis of cytotoxic drugs with in the blood vessels.92 The integrin receptors present on the cancer cells promote adhesion of cancer cells to extracellular matrix to form multidimensional spheres, thereby worsening drug delivery to tumor cells.89
As previously discussed, chemotherapy resistance can occur via overexpression of growth factors and cytokines such as IL-6 and NF-κB produced in the tumor stroma. The TGF-β, FGF, β-catenin, and mTOR pathways, in conjunction with the hypoxic state, are involved in the development of epithelial–mesenchymal transition (EMT).93,94 Indeed, markers of EMT have been strongly associated with docetaxel resistance in preclinical studies.95 This process not only is involved in promoting invasiveness and chemotherapy resistance but also has been linked to development of metastases.96
Additionally, prostate cancer cells secrete various cytokines, such as TGF, VEGF, endothelin-1, FGF, and bone morphogenetic protein, which can influence bone homeostasis either by modifying growth factors present in the osseous microenvironment or exerting direct effect on the osteoblast.97 This leads to osteoblast as well as tumor cell proliferation. Chemokine ligand-2 stimulates tumor cells and osteoclasts, thus promoting bone metastases.98 This paracrine secretion of cytokines may be induced by treatment with chemotherapy, indicating the role of chemokine ligand-2 in chemoresistance.
Drug efflux pump
The multidrug-resistant phenotype, mediated by the ATP-dependent drug efflux pump p-glycoprotein, appears central in the mechanism of chemotherapy resistance.99 Multidrug resistance proteins (MDRPs) belong to ATP-binding cassette transporters and include p-glycoprotein and ABCB4 (encoded by MDR2 gene), and ABCC1 (encoded by MRP1 gene); they act as drug efflux pumps for a variety of chemotherapy agents, including taxanes.100 P-glycoprotein is encoded by the multidrug resistance −1 (MDR1) gene, which is upregulated in cancer cells treated with docetaxel.101
Microtubule alterations
Structural or functional alterations in the microtubules targeted by taxanes provide an additional mechanism of resistance. Upregulation of β-tubulin isotypes III and IV, or β-tubulin mutations that affect docetaxel binding, or posttranslational modifications in β-tubulin that confer the structural changes in microtubules may lead to taxane resistance.102–104 Functional changes such as alterations in the binding site (promoting β-tubulin detyrosination), microtentacles (that enhance endothelial engagement), alterations in γ-actin, and TXTR1-mediated thrombospondin repression may also contribute to taxane resistance.105,106
