Current Cancer Drug Targets - Volume 14, Issue 1, 2014

Understanding the molecular mechanisms and the signaling pathways that underlie the pathology of cancer progression is crucial for the development of novel diagnostic and therapeutic tools. A major common mechanism used by cells to regulate intracellular signal transduction pathways is reversible protein phosphorylation which results in profound changes in cellular responses. This mechanism relies on the coordinated action of two families of proteins: protein kinases and protein phosphatases. Interestingly, there are 3 to 5 times fewer phosphatases than kinases, suggesting that the specificity of substrates is not only due to the variety of the catalytic subunits but also to the diversity of the regulatory subunits. This is particularly true for PhosphoProtein Phosphatase 1 (PPP1) for which more than 200 PPP1 Interacting Proteins (PIPs) have thus far been identified. PIPs can act as targeting subunits, substrates and activity regulators. Many PPP1/PIPs complexes are involved in signaling pathways that regulate cellular growth, cell cycle and apoptosis; processes known to be deregulated in cancer. This review will describe the cellular pathways, many of which involve PPP1/PIP complexes, that when deregulated lead to cancer. Furthermore, the possibility of PPP1/PIP complexes being considered novel targets to cancer diagnostic and therapy will be addressed.

The tumor microenvironment contributes to every aspect of carcinogenesis and therefore offers promising targets for cancer therapy. Compared to chemotherapy alone, targeting tumor cells as well as key components of the tumor microenvironment significantly improve the clinical outcomes of patients. A better understanding of the interaction between tumor cells and the microenvironment could provide new therapeutic options and accelerate the development of novel anti-cancer drugs. In this review, we first defined the tumor microenvironment and then discussed the role of the tumor microenvironment in the initiation and progression of cancer focusing on three major pathways in a tumor cell life cycle: 1) growth and intravasation; in this section, the epithelial-mesenchymal transition (EMT), tumor cell migration, and tumor angiogenesis are reviewed. 2) dissemination; the activation and aggregation of platelets, as an important feature for the survival of tumor cells in the circulation, are reviewed under this section. 3) arrest, extravasation and growth at the secondary sites; the main contents of this section include tissue tropism in metastasis, the formation of the pre-metastatic niche, tumor cell adhesion and extravasation, the mesenchymal to epithelial transition (MET), and the formation of micrometastases and macrometastases. Finally, we briefly introduce the drug resistance mediated by the tumor microenvironment, and also summarize potential drug targets based on the current knowledge of the tumor microenvironment. Although the tumor microenvironment is equally important in the progression of carcinomas, leukemias, and sarcomas, in this review we focus on the common form of malignancy, carcinomas, which represent the malignancy derived from the epithelia.

Platinum-based chemotherapeutics are the mainstay of treatment of a range of tumors achieving high response rates but limited in the course of disease by appearance of drug resistance. Tumor cells respond with reduced uptake and increased intracellular inactivation of the drugs, as well as increased DNA repair and general resistance to chemotherapyinduced cell death. Cisplatin is known to induce expression of cyclophilins, a group of proteins that have peptidyl-prolyl cis-trans isomerase (PPIase) and molecular chaperone activities, as stress response. Cyclophilin A (CypA) and other members of this family are inhibited by cyclosporin A (CsA) which sensitized diverse drug-resistant tumor cell lines in vitro to cisplatin. This effect of CsA was attributed to metabolic changes, inhibition of DNA repair, enhancement of apoptosis, altered intracellular signal transduction or increased production of reactive oxygen species (ROS), although no definitive explanation was provided so far. Several clinical trials employing cisplatin/carboplatin in combination with CsA yielded unsatisfactory results. Since viral replication was found to be dependent on cyclophilins of the host cells, effective new inhibitors, different from CsA or with low or absent immunosuppressive activity, are in development or clinical trials. Sanglifehrins are more potent than CsA and proved to increase toxicity of cisplatin against hepatocellular cancer cells in vitro. These novel cyclophilin inhibitors may offer new opportunities to achieve reversal of resistance to platinumbased drugs in refractory patients. Responsive cancer patients may be enriched in clinical trials by an identification of the downstream targets of Cyps responsible for chemoresistance.

Lung cancer is the most fatal cancer and development of agents that suppress lung tumorigenesis is a crucial strategy to reduce mortality related to this disease. In the present study, we showed, using an in vitro model of lung tumorigenesis, that dimethylamino-parthenolide (DMAPT), a water soluble parthenolide analog, selectively inhibited the growth and survival of premalignant and malignant cells with minimal effects on parental immortalized cells. These effects were paralleled by suppression of pSTAT3, Mcl-1 and cyclin D1 and PARP cleavage, suggesting that the antiproliferative and apoptotic effects of DMAPT could be mediated, at least in part, via suppression of the STAT3 signaling pathway. Moreover, in tobacco smoke carcinogen-induced lung tumor bioassay in mice, intranasal instillation of low doses of DMAPT significantly reduced the overall lung tumor multiplicity by 39%. Interestingly, the drug was specifically effective (62% reduction) against bigger lung tumors (> 2 mm), which have a higher potential to develop into lung adenocarcinoma. Western immunoblotting analyses of mouse lung tissues indicated significantly lower level of pSTAT3 and Mcl-1 in the carcinogen plus DMAPT group relative to the group treated with the carcinogen only. Given the evidence that STAT3 is activated in more than half of lung cancers and it regulates genes involved in cell proliferation, survival and angiogenesis, DMAPT is a promising agent for lung cancer chemoprevention in subjects who are at high risk of developing this devastating disease.

Intraperitoneal (IP) chemotherapy confers significant survival benefits in cancer patients. However, several problems, including local toxicity and ineffectiveness against bulky tumors, have prohibited it from becoming a standard of care. We have developed drug-loaded, polymeric tumor-penetrating microparticles (TPM) to address these problems. Initial studies showed that TPM provides tumor-selective delivery and is effective against ovarian SKOV3 tumors of relatively small size (<50 mg). The present study evaluated whether the TPM activity extends to other tumor types that are more bulky and have different morphologies and disease presentation. We evaluated TPM in mice bearing two IP human pancreatic tumors with different growth characteristics and morphologies (rapidly growing, large and porous Hs766T vs. slowly growing, smaller and densely packed MiaPaCa2), and at different disease stage (early stage with smaller tumors vs. late stage with larger tumors plus peritoneal carcinomatosis). Comparison of treatments with TPM or paclitaxel in Cremophor micelles, at equi-toxic doses, shows, in all tumor types: (a) higher paclitaxel levels in tumors (up to 55-fold) for TPM, (b) greater efficacy for TPM, including significantly longer survival and higher cure rate, and (c) a single dose of TPM was equally efficacious as multiple doses of paclitaxel/Cremophor. The results indicate tumor targeting property and superior antitumor activity of paclitaxel-loaded TPM are generalizable to small and large peritoneal tumors, with or without accompanying carcinomatosis.

Cancer cell lines selected for resistance to microtubule targeting agents (MTA) often have acquired mutations in the β-tubulin binding sites for these agents. Despite strong correlational evidence, the functional and quantitative significance of such mutations in the resistance to MTA remains unknown. We recently showed that peloruside A (PLA) and laulimalide (LAU)-resistant cancer cell lines, 1A9-R1 (R1) and 1A9-L4 (L4), generated through multi-step selection of human 1A9 ovarian cancer cells with high concentrations of either PLA (for R1) or LAU (for L4) have single distinct mutations in their βI-tubulin gene. The R1 cells have a mutation at amino acid position 296 (A296T), and the L4 cells have a mutation at position 306 (R306H/C), both of which lie at the putative binding sites of PLA and LAU. To gain insights on the functional role of these mutations in the resistance phenotype, R1 and L4 cells were transfected with wild type βI-tubulin. MTT cell proliferation assays revealed that restoration of wild type βI-tubulin expression partially sensitized the R1 and L4 cells to PLA and LAU. Cell cycle analysis and intracellular tubulin polymerization assays demonstrated that the increased sensitivity was correlated with an increased ability of PLA and LAU to induce G2-M arrest and tubulin polymerization in the cells. Unlike paclitaxel-selected clones of 1A9 cells, both R1 and L4 cells exhibited a functional p53 gene, and the abundance of the mismatch repair gene hMSH2 (human mutS homolog 2) was comparable to the parental 1A9 cells. This study provides the first direct evidence that A296 and R306 of βI-tubulin are important determinants of the PLA and LAU response in cancer cells.

Gambogic acid (GA) has been approved by the Chinese Food and Drug Administration for the treatment of lung cancer in clinical trials. However, whether GA has chemosensitizing properties when combined with other chemotherapy agents in the treatment of lung cancer is not known. Here we investigated the effects of GA combined with adriamycin (ADM), a common chemotherapy agent, in regard to their activities and the possible mechanisms against lung cancer in vitro and in vivo. Cell viability results showed that sequential GA-ADM treatment was synergistic, while the reverse sequence and simultaneous treatments were antagonistic or additive, in lung cancer cells and ADM resistant cells, but not in normal cells. The combined use of GA and ADM synergistically displayed apoptosis-inducing activities in lung cancer cells. Moreover, GA in combination with ADM could promote PARP cleavage, enhance caspases activation and decrease the expression of anti-apoptotic proteins in lung cancer cells. The combined use of GA and ADM decreased the expression of P-glycoprotein and increased the accumulation of ADM in lung cancer cells. Furthermore, it was found that, prior to ADM treatment, GA could inhibit NF-κB signaling pathways, which have been validated to confer ADM resistance. The critical role of NF-κB was further confirmed by using PDTC, a NF-κB inhibitor, which significantly increased apoptosis induction by the combination of GA and ADM and inhibited ADM-induced ABCB1 upregulation. Importantly, our results indicated that the combination of GA and ADM exerted enhanced anti-tumor effects on A549 xenograft models through inhibiting NF-κB and P-glycoprotein, and attenuated ADM-induced cardiotoxicity. Collectively, these findings indicate that GA sensitizes lung cancer cells to ADM in vitro and in vivo, providing a rationale for the combined use of GA and ADM in lung cancer chemotherapy.