Role of β4 integrin in HER-3-negative, K-RAS wild-type metastatic colorectal tumors receiving cetuximab

Role of Integrins in Resistance to Therapies Targeting Growth Factor Receptors in Cancer

Integrins contribute to cancer progression and aggressiveness by activating intracellular signal transduction pathways and transducing mechanical tension forces. Remarkably, these adhesion receptors share common signaling networks with receptor tyrosine kinases (RTKs) and support their oncogenic activity, thereby promoting cancer cell proliferation, survival and invasion. During the last decade, preclinical studies have revealed that integrins play an important role in resistance to therapies targeting RTKs and their downstream pathways. A remarkable feature of integrins is their wide-ranging interconnection with RTKs, which helps cancer cells to adapt and better survive therapeutic treatments. In this context, we should consider not only the integrins expressed in cancer cells but also those expressed in stromal cells, since these can mechanically increase the rigidity of the tumor microenvironment and confer resistance to treatment. This review presents some of these mechanisms and outlines new treatment options for improving the efficacy of therapies targeting RTK signaling.

Epidermal growth factor-mediated Rab25 pathway regulates integrin β1 trafficking in colon cancer

Background

Integrins play a critical role in carcinogenesis. Integrin β1 localization is regulated by the guanosine-5′-triphosphate hydrolase Rab25 and integrin β1 levels are elevated in the serum of colon cancer patients; thus, the present study examined the effects of epidermal growth factor (EGF) and Rab25 on integrin β1 localization in colon cancer cells.

Methods

HCT116 human colon cancer cells were treated with increasing concentrations of EGF, and cell proliferation and protein expression were monitored by MTT and western blot analyses, respectively. Cell fractionation was performed to determine integrin β1 localization in the membrane and cytosol. Integrin β1 extracellular shedding was monitored by enzyme-linked immunosorbent assays (ELISAs) with culture supernatants from stimulated cells. HCT116 cells were transfected with Rab25-specific siRNA to determine the significance of Rab25 in integrin β1 trafficking in the presence of EGF.

Results

Total integrin β1 expression increased in response to EGF and subsequently decreased at 24 h post-stimulation. A similar decrease was observed in purified membrane fractions, whereas no changes were observed in cytosolic levels. ELISAs using media from stimulated cell cultures demonstrated increased integrin β1 levels corresponding to the decrease observed in membrane fractions, suggesting that EGF induces integrin receptor shedding. EGF stimulation in Rab25-knockdown cells resulted in integrin β1 accumulation in the membrane, suggesting that Rab25 promotes integrin endocytosis.

Conclusions

Integrin β1 is shed from colon cancer cells in response to EGF stimulation in a Rab25-dependent manner. These results further the present understanding of the role of integrin β1 in colon cancer progression.

Reduced-gliotoxin induces ROS-mediated anoikis in human colorectal cancer cells

Reduced-gliotoxin is a small molecule derived from the secondary metabolites of marine fungi; compared to other gliotoxin analogues, it exhibits potent anticancer effects. However, the molecular basis of the death of colorectal cancer (CRC) cells induced by reduced-gliotoxin is unclear. Thus, the aim of this study was to investigate the potency of reduced-gliotoxin against CRC cells and to elucidate the underlying mechanisms. Cell morphology, flow cytometric analysis and western bolt analysis were performed to examine the functions and mechanisms of cell death induced by reduced-gliotoxin. Our findings demonstrated that reduced-gliotoxin triggered rapid cell detachment and induced anoikis in CRC cells. Mechanistically, our data indicated that the anoikis induced by reduced-gliotoxin was associated with the disruption of integrin-associated cell detachment and multiple signaling pathways. Furthermore, reduced-gliotoxin induced the excessive production of reactive oxygen species (ROS) and the disruption of mitochondrial membrane potential (MMP), resulting in the activation of both endogenous and exogenous apoptotic pathways and eventually, in the apoptosis of CRC cells. The blockage of ROS generation with N-acetylcysteine (NAC) attenuated the anoikis induced by reduced-gliotoxin. Taken together, these results suggest that reduced-gliotoxin may prove to be a potential candidate in the treatment of CRC.

Biomarkers predicting resistance to epidermal growth factor receptor-targeted therapy in metastatic colorectal cancer with wild-type KRAS

EGFR pathway is an important therapeutic target in human tumors, including metastatic colorectal cancer (mCRC). The advent of EGFR-targeted monoclonal antibodies panitumumab and cetuximab has generated promise for the treatment of mCRC and has largely improved patients' progression-free survival (PFS) and overall survival (OS). However, treatment with anti-EGFR monoclonal antibodies is only effective in a subset of mCRC patients with wild-type KRAS. This indicates that there are other factors affecting the efficacy of anti-EGFR monoclonal antibodies. Existing studies have demonstrated that among colorectal cancer patients with wild-type KRAS, harboring mutations of BRAF, PIK3CA, NRAS, or PTEN-null may demonstrate resistance to anti-EGFR-targeted therapy, and biomarkers detection can provide better-personalized treatment for mCRC patients. How to identify and reverse the secondary resistance to anti-EGFR monoclonal antibody therapy is also another great challenge to improve the anti-EGFR efficacy in wild-type KRAS mCRC patients. Finally, both of the molecular mechanisms of response and acquired resistance would be important for the directions of future research. This review focuses on how to further improve the predictive value of anti-EGFR therapies and how to also try and avoid futile treatment for wild-type KRAS colorectal cancer patients.

Keratins Stabilize Hemidesmosomes through Regulation of β4-Integrin Turnover

Epidermal integrity and wound healing depend on remodeling of cell-matrix contacts including hemidesmosomes. Mutations in β4-integrin and plectin lead to severe epidermolysis bullosa (EB). Whether mutations in keratins K5 or K14, which cause EB simplex, also compromise cell-matrix adhesion through altering hemidesmosomal components is not well investigated. In particular, the dependence of β4-integrin endocytosis and turnover on keratins remains incompletely understood. Here, we show that the absence of keratins causes loss of plectin-β4-integrin interaction and elevated β4-integrin phosphorylation at Ser1354 and Ser1362. This triggered a caveolin-dependent endocytosis of β4-integrin but not of other integrins through Rab5 and Rab11 compartments in keratinocytes. Expressing a phospho-deficient β4-integrin mutant reduces β4-integrin endocytosis and rescues plectin localization in keratin-free cells. β4-integrin phosphorylation in the absence of keratins resulted from elevated Erk1/2 activity downstream of increased EGFR and PKCα signaling. Further, increased Erk1/2 phosphorylation and altered plectin localization occur in keratin-deficient mouse epidermis in vivo. Strikingly, expression of the K14-R125P EBS mutant also resulted in plectin mislocalization and elevated β4-integrin turnover, suggesting disease relevance. Our data underscore a major role of keratins in controlling β4-integrin endocytosis involving a plectin-Erk1/2-dependent mechanism relevant for epidermal differentiation and pathogenesis.