New paradigms in anticancer therapy: targeting multiple signaling pathways with kinase inhibitors

Signal transduction in cancer cells is a sophisticated process that involves receptor tyrosine kinases (RTKs) that eventually trigger multiple cytoplasmic kinases, which are often serine/threonine kinases. A number of tumor models have identified several key cellular signaling pathways that work independently, in parallel, and/or through interconnections to promote cancer development. Three major signaling pathways that have been identified as playing important roles in cancer include the phosphatidyl inositol-3-kinase (PI3K)/AKT, protein kinase C (PKC) family, and mitogen-activated protein kinase (MAPK)/Ras signaling cascades. In clinical trials, highly selective or specific blocking of only one of the kinases involved in these signaling pathways has been associated with limited or sporadic responses. Improved understanding of the complexity of signal transduction processes and their roles in cancer has suggested that simultaneous inhibition of several key kinases at the level of receptors and/or downstream serine/threonine kinases may help to optimize the overall therapeutic benefit associated with molecularly targeted anticancer agents. Using targeted agents to inhibit multiple signaling pathways has emerged as a new paradigm for anticancer treatment based on preclinical and clinical data showing potent anti-tumor activity of single drugs inhibiting multiple molecular targets or combination therapies involving multiple drugs with selective or narrow target specificity. Preclinical and clinical studies point to molecules on vascular endothelial cells and pericytes as being important targets for anticancer therapies, as well as molecules on or within tumor cells themselves. This suggests that optimal therapeutic approaches to cancer may involve targeting multiple molecules found in both the tumor and supportive tissues. In this review, we will use the most recent preclinical and clinical data to describe this emerging paradigm for anticancer therapy involving targeting multiple signaling pathways with tyrosine or serine/threonine kinase inhibitors.

Differential protein expression, DNA binding and interaction with SV40 large tumour antigen implicate the p63-family of proteins in replicative senescence

P53 activity plays a key role in mammalian cells when they undergo replicative senescence at their Hayflick limit. To determine whether p63 proteins, members of the family of p53-related genes, are also involved in this process, we examined their expression in serially passaged rat embryo fibroblasts. Upon senescence, two truncated DeltaNp63 proteins decreased in abundance whereas two TAp63 isoforms accumulated. 2-D gel analysis showed that the DeltaNp63 proteins underwent post-translational modifications in both proliferating and senescent cells. Direct binding of DeltaNp63 proteins to a p53 consensus motif was greater in proliferating cells than senescent cells. In contrast p63alpha isoforms bound to DNA in a p53 dependent manner and this was higher in senescent cells than proliferating cells. An interaction of p63alpha proteins with SV40 large tumour antigen was also detected and ectopic expression of DeltaNp63alpha can extend the lifespan of rat embryo fibroblasts. Taken together the results indicate that p63 proteins may play a role in replicative senescence either by competition for p53 DNA binding sites or by direct interaction with p53 protein bound to DNA.

Antitumor activity of oxaliplatin in combination with 5-fluorouracil and the thymidylate synthase inhibitor AG337 in human colon, breast and ovarian cancers

Oxaliplatin, classical [5-fluorouracil (5-FU)] and non-classical (AG337) thymidylate synthase inhibitors have shown promising activity in the treatment of cancer. This study investigates the cytotoxic effects of oxaliplatin in combination with 5-FU and AG337 in cultured human colon (HT29, CaCo2), breast (MCF-7, MDA-MB-231) and ovarian (2008) cancer cell lines, and their derived counterparts selected for their resistance to 5-FU (HT29-5-FU), doxorubicin (MCF-7mdr) or cisplatin (2008C13). Therapeutic experiments were conducted in mice bearing colon-HT29 xenografts and in the GR hormone-independent mammary carcinoma model. In vitro, oxaliplatin shows potent cytotoxic activity in colon (IC50 from 2.1 +/- 1.1 to 5.9 +/- 1.7 microM), ovarian (IC50 = 10 +/- 1.6 microM) and breast cancer cells (IC50 from 7.4 +/- 2.7 to 17.9 +/- 7.1 microM). Oxaliplatin was a potent inhibitor of DNA synthesis and bound to cellular DNA. Surprisingly, the overall amount of oxaliplatin DNA binding was significantly inferior to that induced by isocytotoxic concentrations of cisplatin in HT29 (p=0.026). In vitro, synergistic antiproliferative effects were observed when oxaliplatin was added to 5-FU and AG337. Those synergistic effects of combinations were maintained in colon HT29-5-FU cancer cells. In vivo, 5-FU increased significantly the antitumor activity of oxaliplatin in HT29 xenografts (p=0.0036), and similarly 5-FU and AG337 increased the activity of oxaliplatin in the GR tumor model (p=0.0012). These data may encourage further clinical investigation of oxaliplatin in combination with classical and non-classical thymidylate synthase inhibitors in the treatment of human cancers.

Enterocyte differentiation is compatible with SV40 large T expression and loss of p53 function in human colonic Caco-2 cells. Status of the pRb1 and pRb2 tumor suppressor gene products

Transfer of the SV40 large-T (LT) oncogene into isolated human and murine intestinal epithelial cells induced alterations of the ultrastructural organization and polarization of the resulting immortalized cell lines. We now demonstrate that the functional expression of the SV40 LT antigen in Caco-2 cells did not alter phenotypic markers of differentiation, including expression of villin, sucrase-isomaltase, brush border and dome formation. As compared to parental cells, the transfected Caco-2 LT9 cells exhibited similar growth curves and no invasive properties in vitro. The major oncogenic function of the SV40 LT antigen in transfected Caco-2 cells is associated with reduced latency times necessary for the manifestation of tumors in athymic nude mice. The Caco-2 cell line contained deleted and mutant p53 alleles (stop codon in position 204) and has no detectable truncated p53 protein by Western blot. Molecular complexes between the SV40 LT antigen and the retinoblastoma-related proteins pRb1 and Rb2 were clearly identified at the different phases of the growth curve. When compared to normal human colonic crypts, Caco-2 cell differentiation is related to partial redistribution of pRb1 into hypophosphorylated, antiproliferative forms. The pRb2 protein is found elevated in a subset of human colorectal tumors and their corresponding liver metastases. We conclude that: (1) Caco-2 cells exert a dominant control against the oncogenic functions of the LT antigen; (2) loss of p53 function is not restrictive for the establishment of polarity and differentiation of the enterocyte lineage; (3) the levels and phosphorylation status of the Rb1 and Rb2 proteins may play important roles in the proliferation and differentiation of normal and neoplastic human colonic mucosa.

Evidence for a role of Rho-like GTPases and stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) in transforming growth factor beta-mediated signaling

Transforming growth factor beta (TGF-beta) is a multifunctional factor that induces a wide variety of cellular processes which affect growth and differentiation. TGF-beta exerts its effects through a heteromeric complex between two transmembrane serine/threonine kinase receptors, the type I and type II receptors. However, the intracellular signaling pathways through which TGF-beta receptors act to generate cellular responses remain largely undefined. Here, we report that TGF-beta initiates a signaling cascade leading to stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) activation. Expression of dominant-interfering forms of various components of the SAPK/JNK signaling pathways including Rho-like GTPases, mitogen-activated protein kinase (MAPK) kinase kinase 1 (MEKK1), MAPK kinase 4 (MKK4), SAPK/JNK, and c-Jun abolishes TGF-beta-mediated signaling. Therefore, the SAPK/JNK activation contributes to TGF-beta signaling.

Progression of familial adenomatous polyposis (FAP) colonic cells after transfer of the src or polyoma middle T oncogenes: cooperation between src and HGF/Met in invasion

Little is known about the the signalling pathways driving the adenoma-to-carcinoma sequence in human colonic epithelial cells. Accumulation and activation of the src tyrosine kinase in colon cancer suggest a potential role of this oncogene in this early progression. Therefore, we introduced either activated src (m-src), polyoma-MT alone or combined with normal c-src in the adenoma PC/AA/C1 cell line (PC) to define the function and phenotypic transformations induced by these oncogenes in familial adenomatous polyposis (FAP) colonic epithelial cells. Functional expression of these oncoproteins induced the adenoma-to-carcinoma conversion, overexpression of the hepatocyte growth factor (HGF) receptor Met, but failed to confer invasiveness in vivo and in vitro, or to produce alterations in cell proliferation and differentiation. In contrast, PC-msrc cells became susceptible to the HGF-induced invasion of collagen gels and exhibited sustained activation of the pp60src tyrosine kinase and Tyr phosphorylation of the 120-kDa E-cadherin, which was further increased by HGF Transcripts of HGF were clearly identified by reverse transcription-polymerase chain reaction (RT-PCR) and Southern blot in the parental and transformed PC cells, suggesting an autocrine mechanism. Taken together, the data indicate that: (1) experimental activation of src and PyMT pathways directly induces tumorigenicity and Met upregulation in a colon adenoma cell line; (2) HGF-activated Met and src cooperate in inducing invasion; (3) in view of the molecular associations between catenins and cadherin or the tumour-suppressor gene product APC, the cell adhesion molecule E-cadherin may constitute a downstream effector of src and Met.

Synergy between the non-classical thymidylate synthase inhibitor AG337 (Thymitaq) and cisplatin in human colon and ovarian cancer cells

AG337 is the recent non-classical thymidylate synthase inhibitor with promising activity and manageable toxicity in phase I clinical trials. In this study, we investigated the cytotoxic activity of AG337 alone and in combination with cisplatin in cultured human colon (HT29) and ovarian (2008) cancer cell lines and their derived counterparts selected for their resistance to 5-fluorouracil (5-FU) (HT29-5-FU) and cisplatin (2008C13). We observed that AG337 had potent cytotoxic effects in colon (IC50 = 0.17 MicroM) and ovarian cancer cells (IC50 = 0.65 microM). The cytotoxic activity of AG337 was higher than that of 5-FU in the two models. The Activity of AG337 was not significantly affected in 5-FU-resistant HT29-5-FU colon cancer cells characterized by an amplification of the thymidylate synthase gene (IC50 = 0.27 microM, p = 0.15). Combinations of cisplatin and AG337 exert synergistic activity in both ovarian and colon cancer cells. Interestingly, this synergism was maintained in 5-FU- and cisplatin-resistant cells. Therefore, our data encourage further examination of combinations of AG337 with cisplatin in cancer chemotherapy.

Resistance to TGF beta in SV40 large T-immortalized rat intestinal epithelial cells is associated with down-regulation of TGF beta type I receptor

A new continuous cell line designated ESKI-1 was established by transfection of rat fetal intestinal epithelial cells with ecotropic retroviruses containing SV40 large T oncogene. The ESKI-1 cell line exhibits morphologic features of an epithelial cell line and expresses the OCI-5 and cytokeratin 8 transcripts associated with epithelial cells in the small intestine. Signal transduction and proliferation responses to TGF beta has been characterized in ESKI-1 cells, in comparison with the spontaneously-immortalized IEC cell lines originating from neonatal rat duodenum and ileum. ESKI-1 express both TGF alpha and TGF beta. However, despite a marked increase in TGF beta-stimulated p78 kinase activity observed in ESKI-1 and IEC cells, TGF beta did not modulate growth, or extracellular matrix expression in ESKI-1 cells. Resistance to growth modulation was associated with downregulation of TGF beta. Type I receptor expression in the SV40 large T-immortalized cells. Thus, proliferative resistance to TGF beta inhibition can result from depletion of the TGF beta type I receptor and disruption of the TGF beta signaling pathway downstream the p78 serine/threonine kinase. These molecular defects constitute two early events during the SV40LT-mediated immortalization and neoplastic progression of the intestinal epithelia.

Antiproliferative effects of the arotinoid Ro 40-8757 in human gastrointestinal and pancreatic cancer cell lines: combinations with 5-fluorouracil and interferon-alpha

The arotinoid Ro 40-8757 was previously shown to inhibit the growth of a variety of human cancer cell lines derived from breast, lung and uterus. In view of the high incidence of human digestive cancers, and the slow progress in the development of new therapy, we examined in this paper several combinations between the new arotinoid Ro 40-8757, 5-fluorouracil (5FU) and interferon alpha-2a on the growth of nine human cancer cell lines derived from the gastrointestinal and pancreatic system. Half-maximal inhibition of cell proliferation by Ro 40-8757 was observed at concentrations ranging between 0.18 and 0.57 microM, and increased up to 4.7 microM in retinoid-resistant CAPAN 620 pancreatic cells. All-trans-retinoic acid was 70 times less potent. The sensitivity of HT29-5FU-resistant colonic cells was similar to that observed in the parental cells, suggesting an action independent of pyrimidine metabolism. Ro 40-8757 did not induce any differentiation on HT29 cells, as suggested by ultrastructural analysis. The arotinoid did not interact with receptor signal transduction pathways under the control of serum components, such as growth factors as half-maximal inhibiton of growth was similar in HT29-S-B6 cells cultured in the absence or presence of serum. Cell cycle analysis showed that Ro 40-8757 was not acting at a phase-specific transition in HT29 cells and, accordingly, did not induce overexpression of the protein kinase C (PKC)alpha isoform, or conversion of hyperphosphorylated p105 Rb into hypophosphorylated forms. However, the arotinoid induced significant accumulation of the dephosphorylated, active form of the tumour-suppressor protein. Combinations of Ro 40-8757 with 5FU and interferon alpha 2a resulted in an additive but not synergistic antiproliferative action in HT29 cells. Our data support the interest in Ro 40-8757 as a potent anti-cancer drug, especially in combination therapy with 5FU and interferon, in gastrointestinal and pancreatic cancers, where new active therapeutic modalities are urgently needed.