Mechanisms of drug resistance in colon cancer and its therapeutic strategies

Drug resistance develops in nearly all patients with colon cancer, leading to a decrease in the therapeutic efficacies of anticancer agents. This review provides an up-to-date summary on over-expression of ATP-binding cassette (ABC) transporters and evasion of apoptosis, two representatives of transport-based and non-transport-based mechanisms of drug resistance, as well as their therapeutic strategies. Different ABC transporters were found to be up-regulated in colon cancer, which can facilitate the efflux of anticancer drugs out of cancer cells and decrease their therapeutic effects. Inhibition of ABC transporters by suppressing their protein expressions or co-administration of modulators has been proven as an effective approach to sensitize drug-resistant cancer cells to anticancer drugs in vitro. On the other hand, evasion of apoptosis observed in drug-resistant cancers also results in drug resistance to anticancer agents, especially to apoptosis inducers. Restoration of apoptotic signals by BH3 mimetics or epidermal growth factor receptor inhibitors and inhibition of cancer cell growth by alternative cell death pathways, such as autophagy, are effective means to treat such resistant cancer types. Given that the drug resistance mechanisms are different among colon cancer patients and may change even in a single patient at different stages, personalized and specific combination therapy is proposed to be more effective and safer for the reversal of drug resistance in clinics.

Chronic inflammation and colorectal cancer: the role of vascular endothelial growth factor

Colorectal cancer (CRC) is the third most frequently diagnosed cancer in males and the second in females worldwide. Accumulating evidence has demonstrated that patients with chronic inflammation in bowels have an increased risk to develop CRC. Various inflammatory cells and mediators produced during chronic inflammation are orchestrated through different molecular signaling pathways and lead to the formation of a microenvironment in favor of tumorigenesis. Vascular endothelial growth factor (VEGF), which can be induced by chronic inflammation, plays a pivotal role in tumor angiogenesis as well as tumor growth and metastasis. Antiangiogenic therapy targeting VEGF and its signaling pathways represents a promising strategy to inhibit colorectal tumorigenesis. Indeed, anti-angiogenic agents modulating VEGF ligands and their receptors have already exhibited great potential in treating patients with CRC, especially when combined with conventional chemotherapeutic agents. This review discusses the promoting role of chronic inflammation in colorectal tumorigenesis at different stages including tumor initiation, promotion, progression and metastasis, highlighting the contributory role of VEGF in angiogenesis during the development from chronic inflammation to CRC. It also describes the clinical significance of anti- VEGF therapy in the treatment of such disease.

Sensitivity of apoptosis-resistant colon cancer cells to tanshinones is mediated by autophagic cell death and p53-independent cytotoxicity

BACKGROUND

Multidrug resistance (MDR) develops in nearly all patients with colon cancer. The reversal of MDR plays an important role in the success of colon cancer chemotherapy. One of the commonest mechanisms conferring MDR is the suppression of apoptosis in cancer cells.

PURPOSE

This study investigated the sensitivity of cryptotanshinone (CTS) and dihydrotanshinone (DTS), two lipophilic tanshinones from a traditional Chinese medicine Salvia miltiorrhiza, in apoptosis-resistant colon cancer cells.

METHODS

Cell viability was measured by MTT assay. Cell cycle distribution and apoptosis were determined by flow cytometry. Protein levels were analyzed by western blot analysis. The formation of acidic vesicular organelles was visualized by acridine orange staining.

RESULTS

Experimental results showed that multidrug-resistant colon cancer cells SW620 Ad300 were sensitive to both CTS and DTS in terms of cell death, but with less induction of apoptosis when compared with the parental cells SW620, suggesting that other types of cell death such as autophagy could occur. Indeed, the two tanshinones induced more LC3B-II accumulation in SW620 Ad300 cells with increased autophagic flux. More importantly, cell viability was increased after autophagy inhibition, indicating that autophagy induced by the two tanshinones was pro-cell death. Besides, the cytotoxic actions of the two tanshinones were p53-independent, which could be useful in inhibiting the growth of apoptosis-resistant cancer cells with p53 defects.

CONCLUSION

The current findings strongly indicate that both CTS and DTS could inhibit the growth of apoptosis-resistant colon cancer cells through induction of autophagic cell death and p53-independent cytotoxicity. They are promising candidates to be further developed as therapeutic agents in the adjuvant therapy for colon cancer, especially for the apoptosis-resistant cancer types.