NGR-TNF, a novel vascular-targeting agent, does not induce cytokine recruitment of proangiogenic bone marrow-derived cells

Background:

Administration of certain chemotherapy drugs at the maximum tolerated dose, vascular-disrupting agents (VDAs) and irradiation can induce mobilisation and tumour homing of proangiogenic bone marrow-derived cells (BMDCs). Increases in cytokines and chemokines contribute to such mobilisation that eventually promotes tumour (re)growth. NGR-TNF is a vascular-targeting agent in advanced clinical development, coupling the CNGRCG angiogenic vessel-homing peptide with tumour necrosis factor-alpha (TNF). We investigated whether NGR-TNF mobilises host BMDCs and growth factors.

Methods:

Blood was obtained from Lewis lung carcinoma and 4T1 tumour-bearing mice at different time points following NGR-TNF, VDA or anti-VEGFR2/flk-1 antibody treatment. Levels of circulating growth factors were assessed by ELISAs. BMDCs were characterised by FACS. Circulating endothelial progenitor cells were defined as CD45⁻/CD13⁺/flk-1⁺/CD117⁺/7AAD⁻, Tie2-expressing monocytes as CD45⁺/CD11b⁺/Tie2⁺ and myeloid-derived suppressor cells as CD45⁺/CD11b⁺/Gr1⁺ cells.

Results:

NGR-TNF decreases tumour blood vessel density-inducing apoptosis of tumour and tumour endothelial cells. Unlike VDAs, or high-dose NGR-TNF, lower doses of NGR-TNF, comparable to those used in clinical trials, neither mobilise nor recruit to the tumour site proangiogenic BMDCs or induce host growth factors.

Conclusion:

Low-dose NGR-TNF exerts antitumour activity without inducing proangiogenic host responses, conceivably important for preventing/overcoming resistance and designing combination therapeutic strategies.

Phase II study of maintenance therapy with temsirolimus (TEM) after response to first-line docetaxel (TAX) chemotherapy in castration-resistant prostate cancer (CRPC)

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Background: TAX is the standard first-line chemotherapy for CRPC. However, a number of questions remain regarding the optimal use of TAX following maximal response. Aside from intermittent TAX, maintenance therapy with well-tolerated agents such as the mTOR inhibitor TEM could be one strategy to prolong treatment response and the chemotherapy-free interval, without significantly compromising quality of life. In fact, the mTOR pathway is involved in many aspects of CRPC, and mTOR inhibitors have demonstrated significant anti-CRPC activity in preclinical testing.

Methods: CRPC pts eligible for this single-arm, multicenter phase II trial must have received between 6 to 8 cycles of first-line TAX (75 mg/m2 q3wks) with documented treatment response by PSA (>50% decline from baseline) or RECIST criteria. 30 pts will be enrolled and administered weekly TEM (25 mg iv × 4/cycle). The primary endpoint is time to treatment failure (TTF, by RECIST or symptomatic progression). Secondary endpoints include safety (NCI-CTCAE v3.0), quality of life (FACT-P, PPI), changes in PSA doubling time and time to PSA progression, objective tumor response rate (RECIST), and overall survival. We will also study correlative endpoints using plasma and peripheral blood mononuclear cells (i.e., markers of mTOR inhibition, of antiangiogenic TEM effects and of intratumoral hypoxia/acidosis, the latter as potential predictive markers).

Results: 10 pts have been enrolled to date: mean age 68 (range 52-80), prior definitive local therapy (7)—radiation (6), prostatectomy (1), mean PSA at entry 99.28 (2.3- 380.7), ECOG 0 (4) or 1 (6), prior cycles of TAX 6 (6) or 8 (4), sites of metastasis—bone (8), lymph nodes (4), visceral (3). 6 pts have been discontinued due to treatment failure after a mean of 5.3±1.9 cycles (range 3-8) - RECIST (1), symptomatic (2), combined (3). 4 pts are on cycle 1-2-8-9, respectively. TEM has been generally well tolerated without unexpected side-effects, but may have contributed to worsening lymphedema in 1 pt.

Conclusions: TEM maintenance therapy in CRPC pts that have responded to first-line TAX is well tolerated and appears to result in meaningful TTF.

No significant financial relationships to disclose.

Clonal changes in tumours during growth and progression evaluated by southern gel analysis of random integrations of foreign DNA

We have exploited random integrations of foreign DNA as a means of genetically tagging tumour cell populations with which to analyse the clonal evolution of tumour growth in vivo. Transfection of a non-metastatic mouse mammary carcinoma called SP1 (or a metastatic variant, SP1HU9L) with the pSV2neo plasmid or retrovirus vector infection with a "clipped-wing' vector (delta p delta eMoTN) was used to generate large numbers of uniquely marked tumour cell clones in single-step selections. The basic approach was to pool large numbers of independently marked transfectants or infectants, inject these cells into mice and analyse the resulting primary tumours and/or metastases later. Overgrowth or derivation of tumour masses by a limited number of clones could be detected by Southern gel analysis. The main findings were: (i) injection of pooled populations containing large numbers of uniquely marked cell clones (up to several thousand) invariably resulted in advanced primary tumours that contained a very limited number of clones, and in some cases only one easily detectable clone; (ii) primary tumours could be overgrown within six weeks by the progeny of the same single metastatic clone when the inoculum contained 1-10% metastatic cells, which suggests that metastatic SP1 cells have a selective growth advantage in primary tumours as well as for metastatic spread; and (iii) spontaneous lung metastases were clonal or biclonal at the time of analysis. The results show that spontaneous metastases can develop from a genetically distinct subpopulation of cells in a non-random (i.e. selective) manner. Because primary tumours can become overgrown by the progeny of a metastatic clone, results of any comparison of the properties of a primary tumour with a distant metastasis could be affected by the stage at which the primary tumour is removed and analysed.

Mechanisms of resistance to low-dose metronomic cyclophosphamide

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Background: It has been postulated that antiangiogenic therapies are less susceptible to acquired resistance compared to conventional treatment strategies, which target genetically unstable tumor cells. However, resistance is commonly seen (pre)clinically. A better understanding of the underlying mechanisms is clearly needed to improve such therapies. Methods: We studied mechanisms of acquired resistance to low-dose metronomic (LDM) cyclophosphamide (CTX), the continuous or frequent administration of comparatively low doses of CTX with no extended breaks resulting primarily in antiangiogenic effects, by using LDM CTX relapsed/resistant variants obtained during prolonged in vivo therapy from the human prostate cancer cell line PC-3, which initially is highly responsive. The variants were analyzed for their growth characteristics, angiogenic “phenotype” and impact on circulating endothelial cells (CEC)/endothelial cell progenitors (CEP). Furthermore, we obtained microvessel density (MVD) counts of corresponding xenografts and analyzed in vitro resistance to 4-hydroxycyclophosphamide (4-OH-CTX), hypoxia, acidosis, and glucose as well as serum deprivation. Results: The resistant variants did not show consistent changes in their angiogenic “phenotype”. The decrease in MVD upon LDM CTX treatment was comparable to that obtained in xenografts from parental PC-3, which was reflected by similar changes in the number of CEC and CEP. Despite in vivo resistance to LDM CTX, there was no evidence of in vitro resistance of the variants to 4-OH-CTX. However, they exhibited increased tolerance to microenvironmental stress resulting from sustained antiangiogenic activity. Conclusions: In this model, resistance to LDM CTX develops despite sustained antiangiogenic activity. This provides further evidence that reduced vascular dependence of the tumor cell population may be a basis for the observed resistance. The administration of CTX in a LDM manner does not result in tumor cell CTX resistance, supporting the notion that LDM protocols target primarily the tumor vasculature. If confirmed in ongoing clinical metronomic chemotherapy trials, our results will have major implications for the management of patients that progress under such treatment.

No significant financial relationships to disclose.

Cyclophosphamide-methotrexate 'metronomic' chemotherapy for the palliative treatment of metastatic breast cancer. A comparative pharmacoeconomic evaluation

BACKGROUND

Metronomic chemotherapy-the chronic administration of chemotherapy at relatively low, minimally toxic doses on a frequent schedule of administration at close regular intervals, with no prolonged drug-free breaks-is a potentially novel approach to the control of advanced cancer disease. It is thought to work primarily through antiangiogenic mechanisms and has, as an advantage, the property of significantly reducing undesirable toxic side-effects. The aim of the present study was to evaluate the cost effectiveness of cyclophosphamide-methotrexate 'metronomic' chemotherapy in the palliative treatment of pretreated metastatic breast cancer.

METHODS

Low-dose cyclophosphamide-methotrexate 'metronomic' chemotherapy was compared with outcome and resource utilisation data of published phase II trials regarding metastatic breast cancer, performed in western countries, mostly in Europe. All direct costs associated with metastatic breast cancer treatment were included and adjusted to year 2003 values. Sensitivity analyses were performed and variations to the values of key parameters were assessed.

RESULTS

Low-dose cyclophosphamide-methotrexate 'metronomic' therapy was assessed to be a cost-effective/cost-saving therapy for palliative treatment for metastatic breast cancer when compared with novel chemotherapy strategies (phase II trials). Compared with the 11 phase II mono- and combination chemotherapies, metronomic treatment showed marked cost savings in each case and improved cost effectiveness. Sensitivity analyses showed the results were robust to variations to the values of key parameters with very few exceptions.

CONCLUSIONS

Metronomic cyclophosphamide-methotrexate is significantly cost effective. If validated by prospective randomized trials, the treatment concept could reduce healthcare costs, especially those associated with the combined use of new, highly expensive, molecularly targeted therapies.

Basic fibroblast growth factor and the complexity of tumour angiogenesis

Intrinsic biochemical properties of basic fibroblast growth factor (bFGF) do not adequately explain its potent pro-angiogenic activity. In vivo, bFGF can act in concert with other modulators of angiogenesis, such as vascular endothelial growth factor (VEGF). It is suggested that due to the complexity of angiogenic growth factor networks the process may be regulated in a non-linear fashion.

A paradigm for therapy-induced microenvironmental changes in solid tumors leading to drug resistance

Intrinsic alterations in the tumor microenvironment are known to contribute to various forms of drug resistance. For example, tumor hypoxia, due to abnormal or sluggish blood flow within areas of solid tumors, can result in both microenvironment-mediated radiation and chemotherapeutic drug resistance. In contrast, acquired resistance to chemotherapy is generally considered to be the result of the gradual selection of mutant subpopulations having genetic mutations and biochemical alterations responsible for the resistant phenotype. Here we present a paradigm for therapyinduced microenvironment-mediated acquired drug resistance. It is based on the results showing that tumor cells appear to be heterogeneous in their relative dependence on adjacent tumor-associated vasculature for survival. Some tumor cells are highly vessel dependent, whereas some are significantly less so, and thus can survive in more hypoxic regions of tumors, distal from such tumor vessels. Hence, it is possible that variant tumor cells that are less vessel dependent may therefore be selected for over time by successful antiangiogenic drug therapies. This results in loss of response or attenuated responses to the therapy. Preliminary evidence is summarized in support of this hypothesis, using paired human colon cancer (HCT116) cell lines that contain two copies of either the wild-type or the disrupted p53 tumor suppressor gene. The mutant cells were found to be less responsive to antiangiogenic therapy, compared to the wild-type cells, and could be progressively selected for in mixed cell populations. Because p53 inactivation can lead to resistance to hypoxia-mediated apoptosis, the results suggest that a protracted and successful antiangiogenic therapy may create more hypoxic tumor microenvironments, thereby creating the necessary conditions to accelerate the selection of mutant tumor cells that are more adept in surviving and growing in such environments. As such, consideration might be given to the combined use of bioreductive hypoxic cell cytotoxic drugs and angiogenesis inhibitors to prolong the efficacy of antiangiogenic therapeutics.

Lineage-specific mechanism of drug and radiation resistance in melanoma mediated by tyrosinase-related protein 2

A major obstacle in the clinical management of malignant melanoma is its intrinsic resistance to chemotherapy and radiation therapy. Consequently, most patients with melanoma often do not respond to conventional anticancer therapy in a clinically significant manner. Recent advances in cancer research have provided new insights into the mechanisms of intrinsic resistance in melanomas. We have recently reported that the over-expression of tyrosinase-related protein 2 (TYRP2), an enzyme that is well characterized for its function in melanin synthesis, is associated specifically with resistance to DNA damaging drugs and radiation treatment. This review will summarize our findings as well as discuss the possible mechanisms by which TYRP2 over-expression contributes to intrinsic resistance in human malignant melanoma.

Possible mechanisms of acquired resistance to anti-angiogenic drugs: implications for the use of combination therapy approaches

The ultimate target of anti-angiogenic drugs is the genetically stable, activated endothelial cell of a newly forming tumor blood vessel, rather than the genetically unstable tumor cell population per se. This led to the notion that acquired resistance to such drugs may not develop as readily, if at all. While there is some evidence that this lack of resistance development may be the case for some direct-acting angiogenesis inhibitors, it is becoming apparent that resistance can develop over time to many types of angiogenesis inhibitors including, possibly, some direct inhibitors, especially when used as monotherapies. Possible mechanisms for such acquired or induced resistance include: (i) redundancy of pro-angiogenic growth factors when the drug used targets a single such growth factor or its cognate endothelial cell-associated receptor tyrosine kinase; (ii) the anti-apoptotic/pro-survival function of growth factors such as VEGF, which, in high local concentrations, can antagonize the pro-apoptotic effects of various angiogenesis inhibitors; (iii) epigenetic, transient upregulation, or induction, of various anti-apoptotic effector molecules in host-endothelial cells; and (iv) heterogeneous vascular dependence of tumor cell populations. It is suggested that long-term disease control with anti-angiogenic drugs can be best achieved by judicious combination therapy. In this regard, the great molecular diversity of anti-angiogenic drug targets, in contrast to chemotherapy, makes this a particularly attractive therapeutic option, especially when approved, commercially available drugs considered to have anti-angiogenic effects are used in such combination treatment strategies.

Dok-R plays a pivotal role in angiopoietin-1-dependent cell migration through recruitment and activation of Pak

Tek/Tie-2 is an endothelial cell (EC)-specific receptor tyrosine kinase that plays a critical role in angiogenesis via its regulation by the angiopoietin family of growth factor ligands. Angiopoietin-1 (Ang1) can promote EC migration; however, the signaling mechanisms underlying this process remain elusive. Here we demonstrate that Dok-R/Dok-2 can associate with Tek in ECs following Ang1 stimulation, resulting in tyrosine phosphorylation of Dok-R and the subsequent recruitment of Nck and the p21-activating kinase (Pak/Pak1) to the activated receptor. Ang1-mediated migration is increased upon Dok-R overexpression and this requires a functional Nck binding site on Dok-R. Localization of this Dok-R-Nck-Pak complex to the activated Tek receptor at the cellular membrane is coincident with activation of Pak kinase. The ability of Dok-R to bind Nck is required for maximal activation of Pak and overexpression of Pak results in increased Ang1-mediated cell motility. Our study outlines a novel signaling pathway underlying Ang1-driven cell migration that involves Dok-R and its recruitment of Nck and the subsequent activation of Pak.

Multicellular gastric cancer spheroids recapitulate growth pattern and differentiation phenotype of human gastric carcinomas

BACKGROUND & AIMS

Advanced gastric cancer has a poor prognosis and is largely unresponsive to currently available chemotherapeutic drugs. The development of more effective therapies would be aided by better preclinical models.

METHODS

An in vitro multicellular gastric cancer spheroid model was established using the liquid overlay technique and compared with the corresponding xenografts in immunodeficient mice.

RESULTS

Twelve of 17 (71%) gastric cancer cell lines reflected growth characteristics of their parental gastric carcinomas in three-dimensional culture. Thus, cell lines derived from peritoneal and pleural carcinomatosis grew as single cells (HSC-39, KATO-II, KATO-III) and cell aggregates (SNU-5, SNU-16). Cell lines representing adenosquamous (MKN-1) and tubular differentiation (MKN-28, MKN-74, N87) formed partly compact multicellular spheroids recapitulating the tumor architecture of the respective original tumor. The differentiated phenotype was lost after subcutaneous implantation of the in vitro spheroids in mice. The degree of morphologic differentiation was reflected by the levels of mucin and constitutive E-cadherin expression. Heterogeneous changes of other adhesion molecules (EpCAM, alpha2beta1, CD44s, Le(x), sLe(x)) were observed. In contrast, cell lines derived from poorly differentiated gastric carcinomas (Hs-746T, RF-1, RF-48) formed fully compact spheroids mimicking the poorly differentiated phenotype, were E-cadherin negative, and showed only CD44s up-regulation.

CONCLUSIONS

Recapitulating some complexity of their in vivo counterparts, multicellular gastric cancer spheroids may represent a physiologically valid model for studying the biology of this cancer, and testing new therapeutic strategies.

Ras regulation of vascular endothelial growth factor and angiogenesis

Given the multifaceted role of Ras in tumor angiogenesis, pharmacologic targeting of such proteins may bring about at least three important consequences: (1) partial obliteration of the angiogenic competence of tumor cells, (2) an increase in vascular dependence and sensitization to apoptosis, and (3) a direct inhibition of endothelial cell responses to proangiogenic stimuli. Exploration of some of these possibilities, using various pharmacological compounds and antibodies, has already begun. An intriguing possibility is that Ras antagonists and signal transduction inhibitors may synergize with a number of other antiangiogenic modalities such as direct acting antiangiogenic agents (e.g., endostatin) or antivascular regimens involving low-dose continuous chemotherapy as a vasculature-targeting strategy.

Acquired resistance to the antitumor effect of epidermal growth factor receptor-blocking antibodies in vivo: a role for altered tumor angiogenesis

Inhibitors of epidermal growth factor receptor (EGFR) signaling are among the novel drugs showing great promise for cancer treatment in the clinic. However, the possibility of acquired resistance to such drugs because of tumor cell genetic instabilities has not yet been explored. Here we report the experimental derivation and properties of such cell variants obtained from recurrent tumor xenografts of the human A431 squamous cell carcinoma, after two consecutive cycles of therapy with one of three different anti-EGFR monoclonal antibodies: mR3, hR3, or C225. Initial response to a 2-week period of treatment was generally total tumor regression and was not significantly different among the three antibody groups. However, tumors often reappeared at the site of inoculation, generally after prolonged latency periods, and most of the tumors became refractory to a second round of therapy. Cell lines established from such resistant tumors retained high EGFR expression, normal sensitivity to anti-EGFR antibody or ligand, and unaltered growth rate when compared with the parental line in vitro. In contrast, the A431 cell variants exhibited an accelerated growth rate and a significantly attenuated response to anti-EGFR antibodies in vivo relative to the parental line. Because of the reported suppressive effect of EGFR inhibitors on vascular endothelial growth factor (VEGF) expression, and the demonstrated role of VEGF in the angiogenesis and growth of A431 tumor xenografts, relative VEGF expression was examined. Five of six resistant variants expressed increased levels of VEGF, which paralleled an increase in both angiogenic potential in vitro and tumor angiogenesis in vivo. In addition, elevated expression of VEGF in variants of A431 cells obtained by gene transfection rendered the cells significantly resistant to anti-EGFR antibodies in vivo. Taken together, the results suggest that, at least in the A431 system, variants displaying acquired resistance to anti-EGFR antibodies can emerge in vivo and can do so, at least in part, by mechanisms involving the selection of tumor cell subpopulations with increased angiogenic potential.

Induction of anoikis and suppression of human ovarian tumor growth in vivo by down-regulation of Bcl-X(L)

Normal or immortal epithelial cells are sensitive to a form of apoptosis, commonly referred to as anoikis, which is induced by detachment from the extracellular matrix (ECM). In contrast, development of carcinomas is associated with acquisition of cellular resistance to anoikis. However, whether human cancer cells deprived of anoikis resistance necessarily display reduced tumorigenic properties in vivo is unknown. We decided to address this question using human ovarian carcinoma cells as a model. Bcl-X(L), an apoptotic factor considered to play an important role in (resistance to) anoikis, is overexpressed in ovarian cancer, and represents an unfavorable prognostic indicator for this type of human malignancy. We therefore evaluated whether Bcl-X(L) can be used as a tool to manipulate anoikis resistance and tumorigenicity of ovarian cancer cells. We show here that when nonmalignant ovarian epithelial cells are detached from the ECM, down-regulation of Bcl-X(L) and apoptotic cell death are observed, although these events do not occur in ovarian carcinoma cells. Moreover, enforced down-regulation of Bcl-X(L) by transfection with antisense cDNA in the anoikis-resistant and highly tumorigenic HEY ovarian carcinoma cell line had no impact on the viability of these cells under adherent conditions but caused significant apoptosis in response to detachment from the ECM. This change was associated with a strong inhibition of tumorigenicity of the Bcl-X(L)-deficient HEY cells in nude mice, both s.c. and in the peritoneal cavity. These results suggest a critical role for Bcl-X(L) in the maintenance of anoikis resistance in ovarian cancer cells. They also serve to establish a functional linkage between this property and the ability of human cancer cells to grow aggressively in vivo. Consequently, targeting molecular mechanisms responsible for anoikis resistance may serve as a potentially effective therapeutic strategy for the treatment of such human malignancies as ovarian cancer.

Oncogenes and angiogenesis: signaling three-dimensional tumor growth

Three-dimensional tumor growth is dependent on the perpetual recruitment of host blood vessels to the tumor site. This recruitment process (mainly via angiogenesis) is thought to be triggered, at least in part, by the very same set of genetic alterations (activated oncogenes, inactivated/lost tumor suppressor genes) as those responsible for other aspects of malignant transformation (e.g., aberrant mitogenesis, resistance to apoptosis). Potent oncogenes are able to deregulate expression of both angiogenesis stimulators and inhibitors in cancer cells. For example, mutant ras expression is associated with increased production of vascular endothelial growth factor (VEGF) and downregulation of thrombospondin-1 (TSP-1). Upregulation of VEGF and angiogenesis can also be induced by constitutive activation of other oncogenic proteins (e.g., EGFR, Raf, MEK, PI3K) acting at various levels on the Ras signaling pathway. The mode and the magnitude of such proangiogenic influences can be significantly modified by cell type (fibroblastic or epithelial origin), epigenetic factors (hypoxia, changes in cell density), and/or presence of additional genetic lesions (e.g., preceding loss of p16 or p53 tumor suppressor genes). Activated oncogenes (e.g., ras, src, HER-2) induce co-expression of angiogenic properties concomitantly with several highly selectable traits (increased mitogenesis, resistance to apoptosis), a circumstance that may accelerate selection of the angiogenic phenotype at the cell population level. On the other hand oncogene-induced reduction in growth requirements may also endow tumor cells with a diminished (albeit not abrogated) dependence on (close) proximity to blood vessels, i.e., with reduced vascular dependence. Thus, oncogenes can impact several interconnected aspects of cellular growth, survival, and angiogenesis. Experimental evidence suggests that, in principle, many of these properties (including angiogenesis) can be simultaneously suppressed (and tumor stasis or regression induced) by effective use of the specific oncogene antagonists and signal transduction inhibitors.

TYRP2-mediated resistance to cis-diamminedichloroplatinum (II) in human melanoma cells is independent of tyrosinase and TYRP1 expression and melanin content

Tyrosinase-related protein-2 (TYRP2) is a melanocyte-specific enzyme that catalyses the non-decarboxylative tautomerization of L-dopachrome to 5,6-dihydroxyindole-2-carboxylic acid (DHICA) in the melanin biosynthetic pathway. We have recently demonstrated that the constitutive expression of TYRP2 in human melanoma cells positively correlates with cis-diamminedichloroplatinum (II) (CDDP) resistance, and that the ectopic expression of TYRP2 in CDDP-sensitive cells rendered them more resistant to CDDP treatment. Here, we demonstrate that this correlation between constitutive TYRP2 expression and CDDP resistance applies to a panel of distinct human melanoma cell lines obtained from patients with melanoma at various stages of disease progression. We further show that CDDP resistance correlates only with TYRP2 expression and is associated neither with tyrosinase and TYRP1 expression, nor with cellular melanin content. Together, these results further support the notion that TYRP2 is a novel mediator of CDDP resistance in melanoma cells and suggest that this function of TYRP2 is independent of cellular melanin content and of the other regulatory enzymes of the melanogenic pathway.

Oncogenes and tumor angiogenesis: the HPV-16 E6 oncoprotein activates the vascular endothelial growth factor (VEGF) gene promoter in a p53 independent manner

Like other types of pre-malignant lesions and carcinoma, angiogenesis is associated with high-grade cervical dysplasia and with invasive squamous carcinoma of the cervix. Vascular endothelial cell growth factor (VEGF) is known to be one of the most important inducers of angiogenesis and is upregulated in carcinoma of the cervix. Human Papilloma Virus 16 (HPV-16) has been etiologically linked to human cervical cancer, and the major oncogenic proteins encoded by the viral genome, E6 and E7, are involved in the immortalization of target cells. Because several oncogenes including mutant ras, EGF receptor, ErbB2/Her2, c-myc and v-src upregulate VEGF expression, we asked whether HVP-16 E6 oncoprotein could act in a similar fashion. We found that HPV-16 E6-positive cells generally express high levels of VEGF message. Furthermore, co-expression of the VEGF promoter-Luc (luciferase) reporter gene with E6 in both human keratinocytes and mouse fibroblast showed that E6 oncoprotein upregulates VEGF promoter activity, and does so in a p53 independent manner. An E6 responsive region which comprises four Sp-1 sites, between -194 and -50 bp of the VEGF promoter, is also necessary for constitutive VEGF transcription. Taken together, our results suggest the possibility that the HPV oncoprotein E6 may contribute to tumor angiogenesis by direct stimulation of the VEGF gene.

Lack of multicellular drug resistance observed in human ovarian and prostate carcinoma treated with the proteasome inhibitor PS-341

Almost all known conventional cytotoxic anticancer drugs are less effective in killing tumor cells grown as multicellular spheroids than in killing tumor cells grown as monolayer cell cultures. This "multicellular resistance" reflects the relative intrinsic drug-resistant phenotype of most solid tumors growing in vivo and is due to factors such as limited drug penetration or reduced fractions of proliferating cells. Proteasome inhibitors such as PS-341, a dipeptide boronic acid analogue, represent an interesting new class of potential anticancer drugs, which are entering early-phase clinical trials. PS-341 has been found to have good broad-spectrum cytotoxic activity in the 60-monolayer cell line National Cancer Institute screen. However, because its relative potency has not been tested in spheroid systems, we analyzed the activity of PS-341 in a spheroid/solid tumor context using four different human ovarian carcinoma cell lines and three prostate carcinoma cell lines, respectively. We found, with one exception, that PS-341 showed equal or greater activity in spheroids than in the respective monolayer cell cultures, even in a prostate cancer spheroid model with a very low growth fraction. PS-341 induced apoptotic cell death in carcinoma cells in both culture systems. We also noted a decrease in XIAP protein, a member of the inhibitor of apoptosis (IAP) family of apoptosis inhibitors, and phosphorylation of Bcl-XL in PS-341-treated ovarian carcinoma cells. Furthermore, DNA fragmentation, a hallmark of apoptosis (in this case, induced by PS-341), was completely inhibited by the caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (Z-VAD). Taken together, the results indicate that unlike most other known anticancer cytotoxic drugs, PS-341 appears to be as effective in killing tumor cells grown in the form of multicell spheroids as in killing tumor cells grown in monolayer cell culture. Hence, this compound has the potential to circumvent multicellular drug resistance and, as such, may show promising activity against solid tumors with low growth fractions in vivo, which are frequently intrinsically resistant to conventional cytotoxic anticancer drugs.

'Accidental' anti-angiogenic drugs. anti-oncogene directed signal transduction inhibitors and conventional chemotherapeutic agents as examples

A number of drugs currently being tested in clinical trials as possible angiogenesis inhibitors were not originally developed with the intention of suppressing tumour angiogenesis. Thalidomide and interferon alpha are obvious examples of such drugs. This list of 'accidental' angiogenesis inhibitors may include established agents such as conventional cytotoxic chemotherapeutic drugs as well as the new generation of anticancer drugs known as anti-oncoprotein signal transduction inhibitors. With respect to the former, the potential of such drugs to inhibit angiogenesis could be the result of their ability to cause collateral damaging effects on cycling endothelial cells found in newly formed blood vessels, or inhibiting other vital endothelial cell functions necessary for angiogenesis. The antitumour vascular side-effects of chemotherapy may be optimised by administering such drugs continuously on a more frequent (e.g. weekly or even daily) basis at levels well below the maximum tolerated dose (MTD), especially when this is done in combination with newly developed anti-angiogenic drugs such as vascular endothelial cell growth factor (VEGF) receptor blocking antibodies. This strategy may minimise or delay the problems of host toxicity and acquired drug resistance. The possibility of anti-angiogenic effects mediated by signal transduction inhibitors such as ras farnesyltransferase inhibitors (ras FTI's), or drugs which block receptor tyrosine kinases (e.g. ErbB2/neu) such as Herceptin, may be the consequence of such oncogenes inducing or upregulating various pro-angiogenic molecules such as VEGF (vascular endothelial cell growth factor) in tumour cells. Hence, treatment of tumour cells with such drugs can lead to downregulation of tumour cell-associated VEGF expression and this can contribute to an anti-angiogenic effect of the drug in vivo. In addition, some of these drugs may also affect certain 'activated' endothelial cell functions directly so as to block angiogenesis. An awareness of the potential of such conventional or experimental anticancer drugs to affect tumour growth through blockade or suppression of angiogenesis has implications for how anticancer drugs may be used clinically, either alone, or in combination with other drugs to optimally treat cancer.

Activated Ras prevents downregulation of Bcl-X(L) triggered by detachment from the extracellular matrix. A mechanism of Ras-induced resistance to anoikis in intestinal epithelial cells

Detachment of epithelial cells from the extracellular matrix (ECM) results in a form of apoptosis often referred to as anoikis. Transformation of intestinal epithelial cells by oncogenic ras leads to resistance to anoikis, and this resistance is required for the full manifestation of the malignant phenotype. Previously, we demonstrated that ras-induced inhibition of anoikis in intestinal epithelial cells results, in part, from the ras-induced constitutive downregulation of Bak, a pro-apoptotic member of the Bcl-2 family. Since exogenous Bak could only partially restore susceptibility to anoikis in the ras-transformed cells, the existence of at least another component of the apoptotic machinery mediating the effect of activated ras on anoikis was suggested. Indeed, here we show that, in nonmalignant rat and human intestinal epithelial cells, detachment from the ECM or disruption of the cytoskeleton results in a significant downregulation of the antiapoptotic effector Bcl-X(L), and that activated H- or K-ras oncogenes completely abrogate this downregulation. In addition, we found that enforced downregulation of Bcl-X(L) in the ras-transformed cells promotes anoikis and significantly inhibits tumorigenicity, indicating that disruption of the adhesion-dependent regulation of Bcl-X(L) is an essential part of the molecular changes associated with transformation by ras. While the ras-induced downregulation of Bak could be reversed by pharmacological inhibition of phosphatidylinositol 3 kinase (PI 3-kinase), the effect of ras on Bcl-X(L) was PI 3-kinase- and mitogen-activated protein kinase (MAP kinase)-independent. We conclude that ras-induced resistance to anoikis in intestinal epithelial cells is mediated by at least two distinct mechanisms: one that triggers downregulation of Bak and another that stabilizes Bcl-X(L) expression in the absence of the ECM.

Continuous low-dose therapy with vinblastine and VEGF receptor-2 antibody induces sustained tumor regression without overt toxicity

Various conventional chemotherapeutic drugs can block angiogenesis or even kill activated, dividing endothelial cells. Such effects may contribute to the antitumor efficacy of chemotherapy in vivo and may delay or prevent the acquisition of drug-resistance by cancer cells. We have implemented a treatment regimen that augments the potential antivascular effects of chemotherapy, that is devoid of obvious toxic side effects, and that obstructs the development of drug resistance by tumor cells. Xenografts of 2 independent neuroblastoma cell lines were subjected to either continuous treatment with low doses of vinblastine, a monoclonal neutralizing antibody (DC101) targeting the flk-1/KDR (type 2) receptor for VEGF, or both agents together. The rationale for this combination was that any antivascular effects of the low-dose chemotherapy would be selectively enhanced in cells of newly formed vessels when survival signals mediated by VEGF are blocked. Both DC101 and low-dose vinblastine treatment individually resulted in significant but transient xenograft regression, diminished tumor vascularity, and direct inhibition of angiogenesis. Remarkably, the combination therapy resulted in full and sustained regressions of large established tumors, without an ensuing increase in host toxicity or any signs of acquired drug resistance during the course of treatment, which lasted for >6 months. This article may have been published online in advance of the print edition. The date of publication is available from the JCI website, http://www.jci.org.

Tumor angiogenesis: past, present and the near future

The concept of treating solid tumors by inhibiting tumor angiogenesis was first articulated almost 30 years ago. For the next 10 years it attracted little scientific interest. This situation changed, relatively slowly, over the succeeding decade with the discovery of the first pro-angiogenic molecules such as basic fibroblast growth factor and vascular endothelial growth factor (VEGF), and the development of methods of successfully growing vascular endothelial cells in culture as well as in vivo assays of angiogenesis. However, the 1990s have witnessed a striking change in both attitude and interest in tumor angiogenesis and anti-angiogenic drug development, to the point where a remarkably diverse group of over 24 such drugs is currently undergoing evaluation in phase I, II or III clinical trials. In this review I will discuss the many reasons for this. These features, together with other recent discoveries have created intense interest in initiating and expanding anti-angiogenic drug discovery programs in both academia and industry, and the testing of such newly developed drugs, either alone, or in various combinations with conventional cytotoxic therapeutics. However, significant problems remain in the clinical application of angiogenesis inhibitors such as the need for surrogate markers to monitor the effects of such drugs when they do not cause tumor regressions, and the design of clinical trials. Also of concern is that the expected need to use anti-angiogenic drugs chronically will lead to delayed toxic side effects in humans, which do not appear in rodents, especially in short-term studies.

Tyrosinase-related protein 2 as a mediator of melanoma specific resistance to cis-diamminedichloroplatinum(II): therapeutic implications

A major obstacle in the systemic treatment of advanced malignant melanoma is its intrinsic resistance to conventionally used chemotherapeutic agents. In order to investigate the mechanisms of this intrinsic resistance, we have previously utilized retroviral insertional mutagenesis on an early-stage, drug sensitive human melanoma cell line (WM35) to establish mutated cell lines that exhibited increased resistance to cis-diammi-nedichloroplatinum(II) (CDDP). Here, we demonstrate that this increased resistance to CDDP is mediated by the over-expression of tyrosinase-related protein-2 (TYRP2), an enzyme that normally functions in the biosynthesis of the pigment, melanin. Northern and Western blot analyses revealed that the expression of TYRP2 in the virally-derived cell lines as well as in a panel of human melanoma cell lines positively correlated with their levels of resistance to CDDP. Furthermore, enforced expression of TYRP2 in WM35 cells by transfection elevated their resistance to CDDP. The increased CDDP resistance in the virally-derived clones and TYRP2 transfectants was accompanied by a reduction in CDDP-induced apoptosis. Interestingly, the virally-derived CDDP-resistant clones also showed cross resistance to carboplatin and methotrexate, but not taxol, suggesting that TYRP2 over-expression may confer resistance specifically to DNA damaging agents. Overall, these results demonstrate a novel mechanism of drug resistance in human melanoma cells that is mediated by the over-expression of TYRP2. Since TYRP2 is expressed only in cells of melanocytic lineage, this may represent the first report of a lineage-specific mechanism of drug resistance. In summary, these findings suggest a significant role for TYRP2 in the intrinsic drug resistance phenotype of human melanoma cells and may have important implications in the development of chemosensitization strategies for the clinical management of this disease.

Oncogenes and tumor angiogenesis: differential modes of vascular endothelial growth factor up-regulation in ras-transformed epithelial cells and fibroblasts

A possible link between oncogenes and tumor angiogenesis has been implicated by the finding that expression of various oncogenes, particularly mutant ras, can lead to a marked induction of a potent paracrine stimulator of angiogenesis, vascular endothelial growth factor (VEGF). We sought to determine how oncogenic ras induction of VEGF is mediated at the molecular level and whether the mechanisms involved differ fundamentally between transformed epithelial cells and fibroblasts. Our results suggest that in a subline (called RAS-3) of immortalized nontumorigenic rat intestinal epithelial cells (IEC-18) that acquired a tumorigenic phenotype upon transfection of mutant ras, up-regulation of VEGF occurs in the absence of an autocrine growth factor circuit. The expression of VEGF mRNA and protein by RAS-3 cells was strongly suppressed in the presence of LY294002, an inhibitor of phosphatidylinositol 3'-kinase, but remained largely unaffected in the same cells treated with an inhibitor (PD98059) of mitogen-activated protein/extracellular signal-regulated kinase kinase 1 (MKK/MEK-1). This is consistent with the observation that overexpression of a constitutively activated mutant of MEK-1 (AN3/ S222D) in the parental IEC-18 cells did not result in up-regulation of VEGF production. The impact of mutant ras on VEGF expression was also significantly amplified at high cell density, conditions under which RAS-3 cells became less sensitive to LY294002-induced VEGF down-regulation. In marked contrast to cells of epithelial origin, ras-transformed murine fibroblasts (3T3RAS) up-regulated VEGF in a manner that was strongly inhibitable by MEK-1 blockade (ie. treatment with PD98059), whereas these cells were relatively unaffected by treatment with the phosphatidylinositol 3'-kinase inhibitor LY294002. In addition, VEGF was up-regulated by 2-3-fold in NIH3T3 cells overexpressing mutant MEK-1. Collectively, the data suggest that the stimulatory effect of mutant ras on VEGF expression is executed in a nonautocrine and cell type-dependent manner and that it can be significantly exacerbated by physiological/ environmental influences such as high cell density.

High-frequency color flow imaging of the microcirculation

The extension of ultrasound (US) color flow imaging (CFI) techniques to high frequencies (> 20 MHz) has the potential to provide valuable noninvasive tools for scientific and clinical investigations of blood flow in the microcirculation. We describe the development of a slow-scan CFI system operating in the 20-100-MHz range that has been optimized to image the microcirculation. The apparatus has incorporated elements of a previously reported pulsed-wave Doppler system and is capable of operating in either CFI or pulsed-wave mode. The performance of the CFI system was evaluated at a center frequency of 50 MHz using two PVDF transducers with -6-dB beam widths of 43 and 60 microm. The -6 dB-axial resolutions were estimated to be 66 and 72 microm, respectively. In vivo validation experiments conducted using the murine ear model demonstrated the detection of flow in vessels down to 15-20 microm in diameter with flow velocities on the order of mm per s. Further experiments examining experimental murine tumors confirmed the successful detection of flow in the tumor microcirculation.

Marked induction of the IAP family antiapoptotic proteins survivin and XIAP by VEGF in vascular endothelial cells

Vascular endothelial growth factor (VEGF) is a potent angiogenic factor that has been shown to act as an endothelial cell mitogen as well as a vascular permeability factor. Several recent reports have also implicated VEGF as a major survival factor for endothelial cells during angiogenesis and vasculogenesis along with other growth factors such as bFGF and angiopoietin-1. VEGF has been shown to mediate this additional function, at least in part through the induction of bcl-2 and the activation of the PI3 kinase-Akt/PKB signaling pathway. We report here that VEGF can also mediate the induction/upregulation of members of a newly discovered family of antiapoptotic proteins, namely the Inhibitors of Apoptosis (IAP), in vascular endothelial cells. We show that VEGF(165) leads to the induction of XIAP (2.9-fold) and survivin (19.1-fold) protein in human umbilical vein endothelial cells (HUVECs). In contrast, bFGF had little effect on XIAP expression, but produced approximately a 10-fold induction on survivin. VEGF-dependent upregulation of survivin could be prevented by cell cycle arrest in the G1 and S phases. These findings implicate that the survival and mitotic functions of VEGF in an angiogenic context may be more intrinsically related than previously anticipated. Moreover, they also raise the possibility of therapeutically targeting XIAP or survivin in antiangiogenic therapy as a means of suppressing tumor growth, in addition to directly targeting tumor cells which express these survival proteins.

Functional dissociation of anoikis-like cell death and activity of stress activated protein kinase

Adhesion to the extracellular matrix is a crucial survival signal for epithelial and endothelial cells. Both cell types activate an endogenous death program termed "anoikis" when detached from the solid substratum. The signaling events culminating in anoikis are still unclear; recent studies have implicated Stress Activated Protein Kinase (SAPK), also known as Jun-N-Terminal kinase, as a potentially crucial signal transducer and mediator of anoikis. However, the generality and the causal role of SAPK in anoikis remain unclear and controversial. For these reasons we decided to examine the relationship between induction of anoikis and SAPK activation in three independent cell systems. We report here that in immortalized rat intestinal epithelial cells (IEC-18) and human umbilical vein endothelial cells (HUVEC), SAPK is activated weakly and transiently upon cell detachment while in canine kidney epithelial cells (MDCK) such induction is strong and protracted. However, cell types fail to commit to anoikis after remaining in three-dimensional culture for the time required for complete activation of SAPK. This suggests that there is no temporal correlation between SAPK activation and the onset of anoikis in any of the cell lines studied. We further examined the potential involvement of SAPK in the IEC-18 system by investigating a ras oncogene-transformed variant of IEC-18 cells (IEC-18 Ras 3) which are highly resistant to anoikis. Ras expression did not abrogate activation of SAPK, although these cells do exhibit altered kinetics of SAPK induction upon cell detachment. These results suggest that SAPK is not involved in anoikis regulation and that SAPK activation is likely a cell-type-specific epi-phenomenon.

What is the optimal rodent model for anti-tumor drug testing?

One of the most serious obstacles facing investigators involved in the development and assessment of new anti-cancer drugs is the failure of preclinical rodent tumor models to predict in a reliable way whether a given drug will have anti-tumor activity and acceptable toxicity in humans. Most previous investigations for assessing drug activity in vivo have utilized rapidly growing non-metastatic transplantable mouse or human tumors injected ectopically in syngeneic or nude mice, respectively. Some of the reasons for the inadequacy of such models are well known and, as a result, there has been a gradual movement toward the use of transgenic oncomouse models for anti-cancer drug testing. It is too early to conclude, one way or the other, whether these will be superior to transplantable tumor models. Moreover, such transgenic models have a number of limitations which are not widely appreciated. It is argued that transplantable tumor models, with various modifications, might be made significantly more predictive than current models, and would thus constitute a more economic alternative to the use of large numbers of transgenic oncomice. These modifications include the use of slower growing and genetically tagged (e.g. LacZ or GFP) tumors which are transplanted initially into orthotopic organ sites. These methods would facilitate the growth and detection of distant microscopic and macroscopic metastases, the response of which to anti-cancer drugs, using 'clinically equivalent doses,' could be evaluated.

'Proteolytic switching': opposite patterns of regulation of gelatinase B and its inhibitor TIMP-1 during human melanoma progression and consequences of gelatinase B overexpression

Although it is generally accepted that proteolytic degradation is an important mechanism used by malignant cells in the process of metastasis, comparatively little is known about the regulation of molecules responsible for proteolysis and how they become de-regulated during human tumour progression. Using a genetically related pair of human melanoma cell lines, derived from the same patient at different stages of disease, we analysed differences in the cytokine-mediated regulation of gelatinase B (MMP-9), an enzyme thought to play an important role in cellular invasiveness, and TIMP-1, a physiological inhibitor of this enzyme. Whereas the advanced stage (i.e. metastatic) partner of this pair (WM 239) could produce gelatinase B upon induction with interleukin (IL)-1beta or tumour necrosis factor alpha (TNF-alpha), the early stage (i.e. primary) partner (WM 115) could not. In sharp contrast, we found that TIMP-1 displayed an opposite pattern of induction in these cell lines. Specifically, the early stage cell line, WM 115, demonstrated a marked increase in the production of TIMP-1 when treated with IL-1beta or TNF-alpha whereas the advanced cell line, WM 239, showed no such increase. Treatment with the DNA demethylating agent, 2-deoxy-5-azacytidine, resulted in a marked up-regulation of both gelatinase B and TIMP-1 in both cell lines. It was further found that constitutive overexpression of gelatinase B in WM 239 cells and an additional melanoma cell line (MeWo), derived from a metastatic lesion, was able to greatly enhance lung colonization in an experimental metastasis assay while we did not observe differences in tumorigenicity. From these results we conclude that an altered responsiveness of gelatinase B and TIMP-1 to induction by similar agents is associated with disease progression in human melanoma and that this altered responsiveness can have consequences to the aggressive nature of the disease.

Adhesion-dependent multicellular drug resistance

Herein we review studies demonstrating resistance manifested at the multicellular level, a phenomenon referred to as intrinsic or acquired multicellular resistance (MR). In addition, due to the fact that such resistance can be recapitulated in vitro only when cells are adhered to one another in a three-dimensional culture context, we examine the roles of cell adhesion molecules and how they may contribute directly or indirectly to MR. Finally, we suggest an experimental approach to circumvent MR in the treatment of advanced, aggressive ascites tumors.

Collateral expression of proangiogenic and tumorigenic properties in intestinal epithelial cell variants selected for resistance to anoikis

Although in vitro anchorage-independent growth is widely used as a marker of cell transformation, the biological implications of this trait are poorly understood. We previously demonstrated that enforced anchorage-independent growth of a nontumorigenic, immortalized epithelial cell line (IEC-18) in multicellular spheroid culture results in massive apoptotic cell death. This death process, termed anoikis, is prevented by expression of transforming oncogenes, which also confer tumorigenic competence. This study examines whether acquisition of an anoikis-resistant phenotype is causally related to the tumorigenic capacity of transformed epithelial cells. Parental IEC-18 cells were subjected to 10 cycles of selection for survival in speroid culture. Unlike parental cells, the resulting anoikis-resistant variants (AR1.10 and AR2.10) formed relatively large tumors in nude mice. Both anoikis-resistant sublines displayed upregulated expression of vascular endothelial growth factor (VEGF), a potent angiogenesis stimulator. VEGF121 overexpression alone did not induce tumorigenic conversion of parental IEC-18 cells, which remained highly susceptible to anoikis. We postulate that both anoikis-resistance and angiogenic-competence contribute to tumor formation. Development of anoikis-resistance can be then viewed as a precondition for expression of the tumorigenic phenotype. Our results suggest that even when angiogenesis is not a rate limiting factor (e.g. in vitro) the selective pressures of solid tumor-like, 3-dimensional growth conditions favoring anoikis resistance result in collateral induction of a proangiogenic phenotype.

Normoxic and hypoxic regulation of vascular endothelial growth factor (VEGF) by astrocytoma cells is mediated by Ras

Vascular endothelial growth Factor (VEGF) has been identified as a key angiogenic factor involved in the growth and malignant progression of tumours. Glioblastoma multiforme (GBM) are the most common primary human brain tumours, histo-pathologically characterized by intense tumour angiogenesis. GBMs do not harbour oncogenic Ras mutations, but there is a functional up-regulation of Ras signaling through activation of receptor tyrosine kinases overexpressed by these tumours. We demonstrate that Ras pathway activation regulates VEGF secretion in astrocytoma cell lines. Ras pathway inhibition was carried out using genetic and pharmacologic techniques. Astrocytoma cells that were transfected to express the dominant inhibitory mutant H-Ras(N17) demonstrated a reduction in VEGF secretion under both normoxic and hypoxic conditions. Cells treated with the farnesyl transferase inhibitor L-744,832 demonstrated similar reductions in VEGF secretion. Furthermore, astrocytoma cells expressing a constitutively phosphorylated and truncated EGF-R common in GBMs (EGFRvIII or p140(EGF-R)) demonstrate further elevations in Ras activation, resulting in a further increase in VEGF secretion. We have previously demonstrated that activation of Ras plays a vital role in transducing mitogenic signals in human malignant astrocytoma cells. Our present results further extend the role of Ras activation in modulating tumour angiogenesis in these tumours. We propose that Ras may contribute to the angiogenic switch in astrocytomas.

Interleukin-6 dependent induction of the cyclin dependent kinase inhibitor p21WAF1/CIP1 is lost during progression of human malignant melanoma

Human melanoma cell lines derived from early stage primary tumors are particularly sensitive to growth arrest induced by interleukin-6 (IL-6). This response is lost in cell lines derived from advanced lesions, a phenomenon which may contribute to tumor aggressiveness. We sought to determine whether resistance to growth inhibition by IL-6 can be explained by oncogenic alterations in cell cycle regulators or relevant components of intracellular signaling. Our results show that IL-6 treatment of early stage melanoma cell lines caused G1 arrest, which could not be explained by changes in levels of G1 cyclins (D1, E), cdks (cdk4, cdk2) or by loss of cyclin/cdk complex formation. Instead, IL-6 caused a marked induction of the cdk inhibitor p21WAF1/CIP1 in three different IL-6 sensitive cell lines, two of which also showed a marked accumulation of the cdk inhibitor p27Kip1. In contrast, IL-6 failed to induce p21WAF1/CIP1 transcript and did not increase p21WAF1/CIP1 or p27kip1 proteins in any of the resistant lines. In fact, of five IL-6 resistant cell lines, only two expressed detectable levels of p21WAF1/CIP1 mRNA and protein, while in three other lines, p21WAF1/CIP1 was undetectable. IL-6 dependent upregulation of p21WAF1/CIP1 was associated with binding of both STAT3 and STAT1 to the p21WAF1/CIP1 promoter. Surprisingly, however, IL-6 stimulated STAT binding to this promoter in both sensitive and resistant cell lines (with one exception), suggesting that gross deregulation of this event is not the unifying cause of the defect in p21WAF1/CIP1 induction in IL-6 resistant cells. In somatic cell hybrids of IL-6 sensitive and resistant cell lines, the resistant phenotype was dominant and IL-6 failed to induce p21WAF1/CIP1. Thus, our results suggest that in early stage human melanoma cells, IL-6 induced growth inhibition involves induction of p21WAF1/CIP1 which is lost in the course of tumor progression presumably as a result of a dominant oncogenic event.

Downregulation of the pro-apoptotic protein Bak is required for the ras-induced transformation of intestinal epithelial cells

Anoikis is a form of programmed cell death induced in normal epithelial cells by detachment from the extracellular matrix [1] [2] [3]. In epithelial cells of the intestine and other organs, activated rasinduces resistance to anoikis [3] [4], but the actual molecular effectors directly involved in the apoptotic machinery that execute or block anoikis have not yet been identified. Bak, a pro-apoptotic member of the Bcl-2 family, is downregulated in a high proportion of colorectal tumours [5]. In addition, Bak is an important regulator of apoptosis in normal intestinal epithelial cells [6] [7]. Here, we show that activated rasinduces the downregulation of Bak in rat and human intestinal epithelial cells. This ras-induced downregulation of Bak expression could be suppressed by an inhibitor of phosphatidylinositol (PI) 3-kinase, an enzyme already implicated in ras-induced resistance to anoikis [8]. Ectopic expression of Bak in ras-transformed rat intestinal epithelial IEC-18 cells inhibited ras-induced resistance to anoikis and significantly reduced their tumorigenicity. We conclude, therefore, that the ability of rasto downregulate Bak, and the consequent resistance to anoikis, are essential components of the transforming capacity of this oncogene in intestinal epithelial cells.

von Hippel-Lindau gene-mediated growth suppression and induction of differentiation in renal cell carcinoma cells grown as multicellular tumor spheroids

Previous results using gene transfection methods have shown that the wild-type (WT) von Hippel-Lindau (VHL) gene can function as a potent tumor suppressor gene in vivo for renal cell carcinoma (RCC) cells in the absence of any suppressive effect on cell growth in monolayer cell culture under serum-rich conditions. Because we had previously found that the function of some oncogenes, such as mutant ras, can be influenced by three-dimensional growth as multicellular spheroids (J. Rak et aL, J. Cell Biol., 131: 1587-1598, 1995), we reasoned the same might be true for suppressor genes as well. We, therefore, decided to compare and study the effects of the WT VHL gene in monolayer versus three-dimensional culture systems of the RCC cell line 786-0, which contains an inactivated VHL gene. We found that the reintroduction of the WT VHL gene into mutant VHL RCC cells resulted in growth suppression in vitro, but only when the cells were grown as spheroid cultures. This decrease in cell proliferation was associated with several features of cell differentiation/morphogenesis, as shown by light and electron microscopy. Thus, in contrast to cultures of mutant VHL RCC cells, which formed very compact and cohesive spheroids, the WT VHL transfectants were loosely arranged and formed a network of tubular and trabecular structures within the spheroids. The morphological changes of the WT VHL spheroids were associated with the deposition of fibronectin in the extracellular space, a feature that was absent in the mutant and inactivated VHL gene-expressing spheroids. The results suggest the VHL gene may be involved in the maintenance of the epithelial phenotype of renal tubular cells, ie., it may act as a differentiation/morphogenetic factor. Moreover, this effect in tumors cells appears to be highly dependent on multicellular growth conditions that mimic the basic nature of solid tumors, such as RCC.

Reversal of intrinsic and acquired forms of drug resistance by hyaluronidase treatment of solid tumors

There are two broad categories of drug resistance encountered during cancer chemotherapy, i.e. intrinsic and acquired. They are observed in virtually every type of tumor with every known anticancer chemotherapeutic drug. As such there is an urgent need to develop innovative approaches of preventing or reversing these types of resistance. One strategy to do so is to develop completely new drugs which may be resistance free, such as direct acting angiogenesis inhibitors (T. Boehm, J. Folkman, T. Browder, M.S. O'Reilly, Antiangiogenic therapy of experimental cancer does not induce acquired drug resistance, Nature 390 (1997) 404-407; R.S. Kerbel, Inhibition of tumor angiogenesis as a strategy to circumvent acquired resistance to anti-cancer therapeutic agents, BioEssays 13 (1991) 31-36; R.S. Kerbel, A cancer therapy resistant to resistance, Nature 390 (1997) 335-336). Another is to devise methods which will improve significantly the effectiveness of those conventional drugs already in use, such as adriamycin, cyclophosphamide and taxol. We have directed efforts towards the latter. They depend on the discovery of a new class of chemosensitizers which act as antiadhesive agents rendering solid tumors more susceptible to such conventional cytotoxic therapeutic drugs. Examples of this concept are illustrated with bovine testicular hyaluronidase and a mouse mammary tumor called EMT-6. When this enzyme preparation is used to treat intact multicellular spheroids of the EMT-6 tumor, the spheroids are substantially disaggregated. Dispersed spheroids are more susceptible to the cytotoxic effects of cyclophosphamide than intact spheroids. Moreover, this antiadhesive chemosensitizing effect can actually be reproduced in BALB/c mice when EMT-6 cells are grown intraperitoneally as an ascites tumor (consisting mostly of multicellular aggregates) and the mice are given injections of hyaluronidase and cyclophosphamide. In a similar fashion, the indifference of P-glycoprotein-positive multidrug-resistant EMT-6 spheroids to the P-glycoprotein reversal agent PSC-833 (a cyclosporin A analogue) can be reversed by disaggregation of the intact spheroids by hyaluronidase. This renders the treated cells highly sensitive to a combination of adriamycin and PSC-833 in a manner similar to the striking chemosensitization effects commonly observed in monolayer culture systems. Thus, hyaluronidase has the potential to reverse forms of both intrinsic and acquired drug resistance in solid tumors, such as EMT-6, which are sensitive to its antiadhesive effects.

E-Cadherin-dependent growth suppression is mediated by the cyclin-dependent kinase inhibitor p27(KIP1)

Recent studies have demonstrated the importance of E-cadherin, a homophilic cell-cell adhesion molecule, in contact inhibition of growth of normal epithelial cells. Many tumor cells also maintain strong intercellular adhesion, and are growth-inhibited by cell- cell contact, especially when grown in three-dimensional culture. To determine if E-cadherin could mediate contact-dependent growth inhibition of nonadherent EMT/6 mouse mammary carcinoma cells that lack E-cadherin, we transfected these cells with an exogenous E-cadherin expression vector. E-cadherin expression in EMT/6 cells resulted in tighter adhesion of multicellular spheroids and a reduced proliferative fraction in three-dimensional culture. In addition to increased cell-cell adhesion, E-cadherin expression also resulted in dephosphorylation of the retinoblastoma protein, an increase in the level of the cyclin-dependent kinase inhibitor p27(kip1) and a late reduction in cyclin D1 protein. Tightly adherent spheroids also showed increased levels of p27 bound to the cyclin E-cdk2 complex, and a reduction in cyclin E-cdk2 activity. Exposure to E-cadherin-neutralizing antibodies in three-dimensional culture simultaneously prevented adhesion and stimulated proliferation of E-cadherin transfectants as well as a panel of human colon, breast, and lung carcinoma cell lines that express functional E-cadherin. To test the importance of p27 in E-cadherin-dependent growth inhibition, we engineered E-cadherin-positive cells to express inducible p27. By forcing expression of p27 levels similar to those observed in aggregated cells, the stimulatory effect of E-cadherin-neutralizing antibodies on proliferation could be inhibited. This study demonstrates that E-cadherin, classically described as an invasion suppressor, is also a major growth suppressor, and its ability to inhibit proliferation involves upregulation of the cyclin-dependent kinase inhibitor p27.

Protein expression of the cell-cycle inhibitor p27Kip1 in malignant melanoma: inverse correlation with disease-free survival

In the present study we analyzed, by immunohistochemistry, a panel of human melanomas for protein expression of the cyclin-dependent kinase (cdk) inhibitor p27Kip1 and evaluated whether deregulated expression correlates with clinical outcome for this type of cancer. We found that p27Kip1 was strongly expressed by normal melanocytes and benign nevi, whereas in malignant melanoma, a heterogeneous expression pattern was observed. In the case of nodular melanomas, the level of p27Kip1 was found to correlate significantly with the thickness of the tumor, with less protein expressed in thicker lesions. We also found that patients having tumors with fewer than 5% p27Kip1-staining cells had a significantly higher risk of early relapse of their disease compared with those expressing moderate or high levels. In contrast, the level of p27Kip1 did not correlate with tumor thickness or disease-free survival in patients with superficial spreading melanomas, suggesting that p27Kip1 may play different roles in these two major pathological subgroups of malignant melanoma. Furthermore, p27Kip1 did not appear to have an influence on overall survival for either subgroup. When we examined the combined effect of p21WAF1/CIP1 (another cdk inhibitor) and p27Kip1 on clinical outcome, we found that analysis of these two cdk inhibitors together may have greater prognostic potential than either alone. In conclusion, our results suggest that virtually complete loss of p27Kip1 protein expression has potential importance as a prognostic indicator of early relapse in patients with nodular melanoma The results, furthermore, underscore the value of analyzing multiple cell cycle regulatory proteins to obtain the most reliable indication of prognosis.

Establishing a link between oncogenes and tumor angiogenesis

We have tried to stress that mutant oncogenes or overexpressed, nonmutated proto-oncogenes, in addition to their direct affect on promoting aberrant tumor cell proliferation (and survival), may possess a crucial indirect means of stimulating tumor cell growth through regulation of angiogenesis. This effect would never be observed in tissue culture studies of oncogene function using pure cultures of tumor cells, which probably helps explain why the pro-angiogenic function of oncogenes has not been appreciated until only relatively recently. Indeed, the very first indication of a possible contributory role of oncogenes, such as ras and myc, to tumor angiogenesis was first reported by Thompson et al. in 1989, who used reconstituted organ cultures of the mouse prostate gland for their studies (69). This potentially important contribution of oncogenes to tumor growth and development may prove to have an impact on how various signal transduction inhibitors that are now in early phase clinical trials, e.g., monoclonal neutralizing antibodies to the human EGF receptor (70), function in vivo as anti-tumor agents.

Impact of oncogenes in tumor angiogenesis: mutant K-ras up-regulation of vascular endothelial growth factor/vascular permeability factor is necessary, but not sufficient for tumorigenicity of human colorectal carcinoma cells

Targeted disruption of the single mutant K-ras allele in two human colorectal carcinoma cell lines (DLD-1 and HCT-116) leads to loss of tumorigenic competence in nude mice with retention of ability to grow indefinitely in monolayer culture. Because expression of the mutant K-ras oncogene in these cell lines is associated with marked up-regulation of vascular endothelial growth factor/vascular permeability factor (VEGF/VPF), we sought to determine whether this potent angiogenesis inducer plays a role in K-ras-dependent tumorigenic competence. Transfection of a VEGF121 antisense expression vector into DLD-1 and HCT-116 cells resulted in suppression of VEGF/VPF production by a factor of 3- to 4-fold. The VEGF/VPF-deficient sublines, unlike the parental population or vector controls, were profoundly suppressed in their ability to form tumors in nude mice for as long as 6 months after cell injection. In contrast, in vitro growth of these sublines was unaffected, thus demonstrating the critical importance of VEGF/VPF as an angiogenic factor for HCT-116 and DLD-1 cells. Transfection of a full-length VEGF121 cDNA into two nontumorigenic mutant K-ras knockout sublines resulted in a weak but detectable restoration of tumorigenic ability in vivo in a subset of the transfectants, with no consistent change in growth properties in vitro. The findings indicate that mutant ras-oncogene-dependent VEGF/VPF expression is necessary, but not sufficient, for progressive tumor growth in vivo and highlight the relative contribution of oncogenes, such as mutant K-ras, to the process of tumor angiogenesis.

Overexpression of Lerk-5/Eplg5 messenger RNA: a novel marker for increased tumorigenicity and metastatic potential in human malignant melanomas

The Lerks, ligands of eph-related receptor tyrosine kinases, are a rapidly expanding family of genes thought to play an important role in the development and oncogenesis of various tissues. However, very little experimental evidence supports this hypothesis. Using RNA fingerprinting, we detected increased expression of Lerk-5 mRNA in human melanocytes as a response to the tumor-promoting drug 12-O-tetradecanoylphorbol-13-acetate, which suggests a possible role of the Lerks in melanoma tumorigenesis and progression. Therefore, we studied Lerk-5 mRNA expression in various melanoma cell lines and tissues of melanocytic tumors by semiquantitative reverse transcription-PCR. Modest expression of Lerk-5 mRNA was found in two melanoma cell lines derived from early primary tumors (WM35 and WM1645B); two metastatic cell lines tested showed a 3.9-fold increased transcript abundance when compared to the primary cell lines (RPMI-7951 and SK-Mel5). Progeny of a melanoma cell line with very low Lerk-5 mRNA abundance (WM35) showed a 5-fold increase in Lerk-5 mRNA expression when it was selected for higher tumorigenicity and multicytokine resistance by passaging in nude mice or repeated high-dose UVB irradiation. Consistent with these experimental data, we found high levels of Lerk-5 mRNA expression in advanced primary malignant melanomas and metastases (n = 22) but significantly lower or undetectable mRNA expression in benign melanocytic nevi (n = 9; P < 0.001). We conclude that increased Lerk-5 expression possibly reflects or induces an increased potential of growth, tumorigenicity, and metastatic abilities in human melanomas. This makes the yet to be elucidated eph-related receptor tyrosine kinase/Lerk signaling system a potential new source for molecular markers as well as a target for new therapies.

The dormant in vivo phenotype of early stage primary human melanoma: termination by overexpression of vascular endothelial growth factor

Early stage primary human cutaneous melanoma is known to remain relatively avascular and dormant for up to a decade, after which it may give rise to more rapidly growing, vascular and metastatically- competent primary tumor. Clinical dormancy of early stage human melanomas can be recapitulated experimentally by injection of cell lines established from such tumors into nude mice. For example, WM1341B cells, which were isolated from a thin vertical growth phase (VGP) human melanoma, are non-tumorigenic in nude mice even though some of the cells remain viable for at least three weeks at the site of orthotopic injection. These cells produce little or no vascular endothelial growth factor/vascular permeability factor (VEGF/VPF), a potent stimulator of angiogenesis. In order to determine whether their in vivo dormant behaviour may therefore be related to an inability to induce tumor angiogenesis, subpopulations of WM1341B cells were engineered to constitutively overexpress the VEGF/VPF121 isoform. This apparently single modification was sufficient to induce overt and progressively growing tumors by several independent VEGF/VPF121 producing clones, which could be largely blocked by systemic treatment of mice with a monoclonal anti-VEGF neutralizing antibody (A 4.6.1). No evidence for an autocrine mechanism of growth stimulation by VEGF was found. Taken together, these results support the notion that defective angiogenesis may, at least in part, account for dormant phenotype of some early stage primary melanomas. Since the induction of an overt tumorigenic phenotype in several VEGF/VPF transfected WM1341B clones appears to depend exclusively on their expression of VEGF/VPF, such sublines should be useful for screening the activity of known or potential VEGF/VPF ligand or VEGF/VPF receptor antagonists in an in vivo context.