- Email: [email protected]
Murine models of malignant melanoma
Disease models
MOLECULAR MEDICINE TODAY, OCTOBER 2000 (VOL. 6)
Murine models of malignant melanoma Maja K. Tietze and Lynda Chin
Little is known about the precise genetic underpinning of malignant melanoma and how the few recognized genetic lesions relate to disease progression. Nevertheless, a survey of putative melanoma genes/loci has revealed several consistent themes: loss or mutation of the 9p21 locus, activation of a receptor tyrosine kinase (RTK) or mutations in their signaling surrogates (PTEN and RAS), and rarity of p53 mutations1. The 9p21 locus in humans and mice contains the closely linked INK4a and INK4b genes, encoding p16INK4a and p15INK4b, respectively. p16 INK4a and p15 INK4b are cyclin-dependent kinase inhibitors that constrain transit through retinoblastoma (RB)-regulated G1/S transition. The INK4a gene encodes a second distinct
product, p19ARF, via an alternative exon and reading frame (ARF). p19ARF acts to suppress tumorigenesis partly by signaling aberrant proliferative signals to p53, which in turn activates p53-dependent cell-cycle arrest or apoptosis. Thus, mutations at the INK4a/ARF locus are particularly threatening because of the strategic linkage of this gene to the two most important tumor suppressor pathways in human cancers: RB and p53. Indeed, human melanomas often sustain homozygous 9p21 deletions that eliminate the INK4a/ARF genes. Melanocytes depend on complex RTK signaling for the appropriate orchestration of basic cellular functions essential for development and homeostasis. This provides a basis for the frequent involvement of deregulated
RTK [for example, epidermal growth factor receptor (EGFR) or MET], or their signaling components (for example, PTEN or RAS) in melanomas1. For instance, overexpression of MET RTK has been described in human melanomas, and mice with inappropriate expression of its ligand (hepatocyte growth factor/scatter factor) developed diverse cancer types, including melanomas, in a variety of tissues2. In the case of RAS, activating mutations have been observed in approximately 30% of the nodular amelanotic melanoma.
Mouse models of malignant melanoma Although spontaneous melanoma occurs rarely in rodents, melanomas can be induced
Table 1. Transgenic mouse models of malignant melanomaa Model
Genetics
Clinical features
Histology
Ref.
Tyr-Tag
Melanocyte-specific expression of viral oncoprotein SV40 early region (small and large T antigens) under control of mouse tyrosinase promoter
Predominantly metastatic ocular melanomas, except for one low-susceptibility transgenic line that developed spontaneous cutaneous melanomas after long latency
Epithelioid and spindle cells; nodular type lesions; hypomelanotic to melanotic
Tpras
Melanocyte-specific expression of activated H-RASV12G under control of the mouse tyrosinase gene promoter
Hyperpigmentation without melanoma formation Melanocytic hyperplasia without malignant 4 in untreated animals; Upon treatment with the transformation in skin; pleiomorphic cells with carcinogen DMBA, metastatic cutaneous hyperchromatic nuclei and pigmented melanomas develop granules in DMBA-treated tumors
MT-HGF/SF
Ubiquitous expression of HGF/SF under control of general promoter (methallothionein)
Development of multiple tumor types, including metastatic melanomas, but with long latency
Amelanotic, epitheloid and spindle cells
2
Tyr-RAS, INK4aD2/3 knockout
Melanocyte-specific H-RASV12G expression via chimeric promoter consists of mouse tyrosinase gene promoter and 59 enhancer element on p16INK4a and p19ARF null background
Develop invasive non-metastatic cutaneous, and rarely ocular, melanomas
Amelanotic dermal melanomas composed of spindle cells with prominent epithelioid features
5
Tyr/Tetop-RAS, INK4aD2/3 knockout
Melanocyte-specific expression of H-RASV12G is under control of tetracycline- or doxycyclineresponsive promoter
Induction of H-RASV12G expression by doxycycline leads to development of melanoma similar to above
As above
6
3
a
Abbreviations: ARF, alternative reading frame; DMBA, dimethylbenz[a]arthracine; HGF, hepatocyte growth factor; SF, scatter factor.
408
1357-4310/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: 51357-4310(00)01781-0
Disease models
MOLECULAR MEDICINE TODAY, OCTOBER 2000 (VOL. 6)
Table 2. Comparison of transgenic mouse models of malignant melanoma with the human diseasea Model
Similarities to human disease
Tyr-Tag
High frequency of metastasis in ocular Developed predominantly ocular melanomas; UVB exposure in one melanomas; requires expression of founder line increases the incidences viral oncoprotein in melanocytes of cutaneous metastatic melanomas
3
Tpras
Develops metastatic lesions; RAS mutations and loss of p16INK4a, 1/2 p15INK4b and p19ARF expression in melanomas have been described in human melanomas
Requires carcinogen treatment to induce development of cutaneous melanomas
4
MT-HGF/SF
Develop metastatic lesions
Inappropriate expression of HGF/SF not 2 described in human melanomas, but over-expression of its receptor, c-MET, has been observed in ocular melanomas
Tyr-RAS, INK4aD2/3 knockout
Activated RAS, loss of INK4a/ARF, and wild-type p53 status are all features of human melanomas; human melanomas with RAS mutation are usually amelanotic
Melanomas arise from dermal melanocytes; resultant melanomas are non-metastatic
5
As above
6
Tyr/Tetop-RAS, Simulates somatic acquisition of INK4aD2/3 dominant acting oncogenic lesion, knockout H-RASV12G, in postnatal life
Differences from human disease
Ref.
a
Abbreviations: ARF, alternative reading frame; DMBA, dimethylbenz[a]arthracine; HGF, hepatocyte growth factor; SF, scatter factor.
in mice by carcinogens with low penetrance, long latencies and limited metastatic potential. In humans, melanomas arise within the epidermal micro-environment as in situ lesions (radial growth phase). Invasion into the dermis (vertical growth phase) is thought to require additional genetic alterations. Since melanocytes of the adult mice reside in the dermis, it is reasonable to assume that multiple pro-survival/ growth-stimulatory signals are required to reach a critical transformation threshold in the mouse, providing a basis for the observed resistance of rodents to melanoma. Mintz and colleagues have produced a transgenic mouse model that develops primarily ocular melanoma by targeting expression of SV40 T antigens to melanocytes with the tyrosinase promoter (Tables 1 and 2). Cutaneous melanomas with metastatic potential have been observed, after long latency or with ultraviolet B (UVB) irradiation, in a founder line that has low susceptibility and does not succumb to ocular melanomas3. This model will find use in the biological analysis of disease progression but may be less applicable as a system for melanoma gene discovery owing to the broad and dominant impact of
T antigens on many cancer control pathways. Thus, several efforts have been directed towards the construction of mouse models harboring genetic lesions that are encountered in human melanomas. To this end, a transgenic mouse strain was constructed to express activated H-RASV12G in melanocytes (Tables 1 and 2). Upon exposure to carcinogen 7,12dimethylbenz[a]anthracene (DMBA), these mice developed cutaneous and metastatic melanomas4. Notably, tumor-derived cell lines sustain deletions in the INK4a locus, resulting in loss or reduced expression of p16INK4a, as well as p19ARF and p15INK4b in some cases4. In a separate series of experiments, a newly isolated tyrosinase promoter/enhancer element was used to direct melanocyte-specific activated H-RASV12G expression in mice harboring a germline INK4a?2/3 allele, null for both p16INK4a and p19ARF (Tables 1 and 2). These mice are prone to the development of spontaneous nonmetastatic amelanotic cutaneous melanomas, and sustain loss of the remaining wildtype INK4a allele on a heterozygous background5. From a genetic standpoint, this model parallels human melanoma as demonstrated by the lack of p53 mutations and the presence of
Figure 1. Regression of primary cutaneous melanomas upon doxycycline withdrawal. (a) Photograph of cutaneous melanomas in a Tyr/Tetop-RAS INK4a D2/3 mouse that was initiated and maintained on doxycycline-supplemented drinking water during postnatal life. The presence of doxycycline initiates expression of the H-RASV12G oncogene in melanocytes, leading to development of melanomas at a cutaneous site. Note the vascular appearance and amelanotic nature of the tumors. (b) Photograph of the same mouse as depicted in (a), taken 13 days after doxycycline was removed from the drinking water. Removal of doxycycline switches off expression of H-RASV12G in melanoma cells and melanocytes, leading to regression of the tumor, demonstrating that H-RASV12G plays a role not only in tumorigenesis, but also in maintenance of established tumors. Note that complete clinical and histological regression is possible upon long-term withdrawal from doxycycline.
genomic alterations in regions often involved in human melanoma5. These observations imply that this model will find use in the discovery of genes/loci relevant to the genesis and progression of melanoma. More recently, the tet-ON regulatory system has been utilized as an in vivo molecular switch to regulate expression of H-RASV12G in premalignant melanocytes and established melanomas by adding or removing tetracycline or doxycycline to the system6 (Tables 1 and 2). This allows one to simulate somatic acquisition of an oncogenic mutation in adult life or remediation of a genetic lesion in established tumors. Most significant was the observation that loss of activated RAS in established melanomas leads to complete clinical and histological regression, indicating that activated RAS plays a role in both tumor genesis and 409
Disease models
MOLECULAR MEDICINE TODAY, OCTOBER 2000 (VOL. 6)
maintenance (Fig. 1). Notably, regression was accompanied by a robust apoptotic response in host-derived endothelial cells, raising the possibility that H-RASV12G plays a key role in the maintenance of a pro-angiogenic environment. Together, these transgenic studies in the mouse have provided unequivocal proof that both activated RAS and INK4a deficiency play causal roles in the pathogenesis of melanoma.
Future directions In the new era of functional genomics, a mouse model that faithfully reproduces human disease progression on clinico-pathological and genetic levels will serve as a genetic platform for characterization of several aspects of the disease: the impact of cooperating mutations; the decoding of molecular signals between the host and tumor compartments; the identification of novel targets; and, ultimately, the development and preclinical testing of rational therapeutics and chemoprevention strategies.
Acknowledgments. We apologize to colleagues whose work was not discussed or cited owing to space constraints. We thank Norman Sharpless, Steven Artandi and Ron DePinho for critical reading of the manuscript. L.C. is a V foundation scholar and is supported by grants from the NIH and NCI.
References
4 Broome-Powell, M. et al. (1999) Induction of melanoma in Tpras transgenic mice. Carcinogenesis 20, 1747–1753 5 Chin, L. et al. (1997) Cooperative effects of INK4a and ras in melanoma susceptibility in vivo. Genes Dev. 11, 2822–2834 6 Chin, L. et al. (1999) Essential role for oncogenic RAS in tumour maintenance. Nature 400, 468–472
1 Bardeesy, N. et al. Animal models of malignant melanomas: recent advances and future prospects. In Advances in Cancer Research (Vol. 79) (Vande Woude, G. and Klein, G., eds), pp. 123–156 (in press) 2 Takayama, H. et al. (1997) Diverse tumorigenesis associated with aberrant development in mice overexpressing hepatocyte growth factor/ scatter factor. Proc. Natl. Acad. Sci. U. S. A. 94, 701–706 3 Kelsall, S.R. and Mintz, B. (1998) Metastatic Cutaneous Melanoma promoted by ultraviolet radiation in mice with transgene-inititated low melanoma susceptibility. Cancer Res. 58, 4061–4065
Maja K. Tietze MD Research Fellow Lynda Chin* MD Assistant Professor of Dermatology Department of Dermatology, Harvard Medical School, Department of Adult Oncology, Dana-Farber Cancer Institute, 44 Binney Street, Boston, MA 02115, USA. Tel: 1617 632 6091 Fax: 1617 632 6069 *e-mail: [email protected]
Molecular Medicine Today covers a range of cancer-related topics. Here’s a selection of recent articles… PI 3-kinase inhibition: a target for drug development? by Robert C. Stein and Michael D. Waterfield: Mol. Med. Today (2000) 6, 347–357 TP53 gene therapy: a key to modulating resistance to anticancer therapies by Esther H. Chang et al.: Mol. Med. Today (2000) 6, 358–364 The influence of the microenvironment on the malignant phenotype by Catherine C. Park et al.: Mol. Med. Today (2000) 6, 324–329 Neurofibromatosis type II: mouse models reveal broad roles in tumorigenesis and metastasis by Andrea I. McClatchey: Mol. Med. Today (2000) 6, 252–253 Cervical cancer risk: is there a genetic component? by Patrik K.E. Magnusson and Ulf B. Gyllensten: Mol. Med. Today (2000) 6, 145–148 Matrix metalloproteinases: multifunctional contributors to tumor progression by Lisa J. McCawley and Lynn M. Matrisian: Mol. Med. Today (2000) 6, 149–156 Exploiting the hypoxic cancer cell: mechanisms and therapeutic strategies by J. Martin Brown: Mol. Med. Today (2000) 6, 157–162 Dynamics of intercellular communication during melanoma development by Gang Li and Meenhard Herlyn: Mol. Med. Today (2000) 6, 163–169
410
MOLECULAR MEDICINE TODAY, OCTOBER 2000 (VOL. 6)
Murine models of malignant melanoma Maja K. Tietze and Lynda Chin
Little is known about the precise genetic underpinning of malignant melanoma and how the few recognized genetic lesions relate to disease progression. Nevertheless, a survey of putative melanoma genes/loci has revealed several consistent themes: loss or mutation of the 9p21 locus, activation of a receptor tyrosine kinase (RTK) or mutations in their signaling surrogates (PTEN and RAS), and rarity of p53 mutations1. The 9p21 locus in humans and mice contains the closely linked INK4a and INK4b genes, encoding p16INK4a and p15INK4b, respectively. p16 INK4a and p15 INK4b are cyclin-dependent kinase inhibitors that constrain transit through retinoblastoma (RB)-regulated G1/S transition. The INK4a gene encodes a second distinct
product, p19ARF, via an alternative exon and reading frame (ARF). p19ARF acts to suppress tumorigenesis partly by signaling aberrant proliferative signals to p53, which in turn activates p53-dependent cell-cycle arrest or apoptosis. Thus, mutations at the INK4a/ARF locus are particularly threatening because of the strategic linkage of this gene to the two most important tumor suppressor pathways in human cancers: RB and p53. Indeed, human melanomas often sustain homozygous 9p21 deletions that eliminate the INK4a/ARF genes. Melanocytes depend on complex RTK signaling for the appropriate orchestration of basic cellular functions essential for development and homeostasis. This provides a basis for the frequent involvement of deregulated
RTK [for example, epidermal growth factor receptor (EGFR) or MET], or their signaling components (for example, PTEN or RAS) in melanomas1. For instance, overexpression of MET RTK has been described in human melanomas, and mice with inappropriate expression of its ligand (hepatocyte growth factor/scatter factor) developed diverse cancer types, including melanomas, in a variety of tissues2. In the case of RAS, activating mutations have been observed in approximately 30% of the nodular amelanotic melanoma.
Mouse models of malignant melanoma Although spontaneous melanoma occurs rarely in rodents, melanomas can be induced
Table 1. Transgenic mouse models of malignant melanomaa Model
Genetics
Clinical features
Histology
Ref.
Tyr-Tag
Melanocyte-specific expression of viral oncoprotein SV40 early region (small and large T antigens) under control of mouse tyrosinase promoter
Predominantly metastatic ocular melanomas, except for one low-susceptibility transgenic line that developed spontaneous cutaneous melanomas after long latency
Epithelioid and spindle cells; nodular type lesions; hypomelanotic to melanotic
Tpras
Melanocyte-specific expression of activated H-RASV12G under control of the mouse tyrosinase gene promoter
Hyperpigmentation without melanoma formation Melanocytic hyperplasia without malignant 4 in untreated animals; Upon treatment with the transformation in skin; pleiomorphic cells with carcinogen DMBA, metastatic cutaneous hyperchromatic nuclei and pigmented melanomas develop granules in DMBA-treated tumors
MT-HGF/SF
Ubiquitous expression of HGF/SF under control of general promoter (methallothionein)
Development of multiple tumor types, including metastatic melanomas, but with long latency
Amelanotic, epitheloid and spindle cells
2
Tyr-RAS, INK4aD2/3 knockout
Melanocyte-specific H-RASV12G expression via chimeric promoter consists of mouse tyrosinase gene promoter and 59 enhancer element on p16INK4a and p19ARF null background
Develop invasive non-metastatic cutaneous, and rarely ocular, melanomas
Amelanotic dermal melanomas composed of spindle cells with prominent epithelioid features
5
Tyr/Tetop-RAS, INK4aD2/3 knockout
Melanocyte-specific expression of H-RASV12G is under control of tetracycline- or doxycyclineresponsive promoter
Induction of H-RASV12G expression by doxycycline leads to development of melanoma similar to above
As above
6
3
a
Abbreviations: ARF, alternative reading frame; DMBA, dimethylbenz[a]arthracine; HGF, hepatocyte growth factor; SF, scatter factor.
408
1357-4310/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: 51357-4310(00)01781-0
Disease models
MOLECULAR MEDICINE TODAY, OCTOBER 2000 (VOL. 6)
Table 2. Comparison of transgenic mouse models of malignant melanoma with the human diseasea Model
Similarities to human disease
Tyr-Tag
High frequency of metastasis in ocular Developed predominantly ocular melanomas; UVB exposure in one melanomas; requires expression of founder line increases the incidences viral oncoprotein in melanocytes of cutaneous metastatic melanomas
3
Tpras
Develops metastatic lesions; RAS mutations and loss of p16INK4a, 1/2 p15INK4b and p19ARF expression in melanomas have been described in human melanomas
Requires carcinogen treatment to induce development of cutaneous melanomas
4
MT-HGF/SF
Develop metastatic lesions
Inappropriate expression of HGF/SF not 2 described in human melanomas, but over-expression of its receptor, c-MET, has been observed in ocular melanomas
Tyr-RAS, INK4aD2/3 knockout
Activated RAS, loss of INK4a/ARF, and wild-type p53 status are all features of human melanomas; human melanomas with RAS mutation are usually amelanotic
Melanomas arise from dermal melanocytes; resultant melanomas are non-metastatic
5
As above
6
Tyr/Tetop-RAS, Simulates somatic acquisition of INK4aD2/3 dominant acting oncogenic lesion, knockout H-RASV12G, in postnatal life
Differences from human disease
Ref.
a
Abbreviations: ARF, alternative reading frame; DMBA, dimethylbenz[a]arthracine; HGF, hepatocyte growth factor; SF, scatter factor.
in mice by carcinogens with low penetrance, long latencies and limited metastatic potential. In humans, melanomas arise within the epidermal micro-environment as in situ lesions (radial growth phase). Invasion into the dermis (vertical growth phase) is thought to require additional genetic alterations. Since melanocytes of the adult mice reside in the dermis, it is reasonable to assume that multiple pro-survival/ growth-stimulatory signals are required to reach a critical transformation threshold in the mouse, providing a basis for the observed resistance of rodents to melanoma. Mintz and colleagues have produced a transgenic mouse model that develops primarily ocular melanoma by targeting expression of SV40 T antigens to melanocytes with the tyrosinase promoter (Tables 1 and 2). Cutaneous melanomas with metastatic potential have been observed, after long latency or with ultraviolet B (UVB) irradiation, in a founder line that has low susceptibility and does not succumb to ocular melanomas3. This model will find use in the biological analysis of disease progression but may be less applicable as a system for melanoma gene discovery owing to the broad and dominant impact of
T antigens on many cancer control pathways. Thus, several efforts have been directed towards the construction of mouse models harboring genetic lesions that are encountered in human melanomas. To this end, a transgenic mouse strain was constructed to express activated H-RASV12G in melanocytes (Tables 1 and 2). Upon exposure to carcinogen 7,12dimethylbenz[a]anthracene (DMBA), these mice developed cutaneous and metastatic melanomas4. Notably, tumor-derived cell lines sustain deletions in the INK4a locus, resulting in loss or reduced expression of p16INK4a, as well as p19ARF and p15INK4b in some cases4. In a separate series of experiments, a newly isolated tyrosinase promoter/enhancer element was used to direct melanocyte-specific activated H-RASV12G expression in mice harboring a germline INK4a?2/3 allele, null for both p16INK4a and p19ARF (Tables 1 and 2). These mice are prone to the development of spontaneous nonmetastatic amelanotic cutaneous melanomas, and sustain loss of the remaining wildtype INK4a allele on a heterozygous background5. From a genetic standpoint, this model parallels human melanoma as demonstrated by the lack of p53 mutations and the presence of
Figure 1. Regression of primary cutaneous melanomas upon doxycycline withdrawal. (a) Photograph of cutaneous melanomas in a Tyr/Tetop-RAS INK4a D2/3 mouse that was initiated and maintained on doxycycline-supplemented drinking water during postnatal life. The presence of doxycycline initiates expression of the H-RASV12G oncogene in melanocytes, leading to development of melanomas at a cutaneous site. Note the vascular appearance and amelanotic nature of the tumors. (b) Photograph of the same mouse as depicted in (a), taken 13 days after doxycycline was removed from the drinking water. Removal of doxycycline switches off expression of H-RASV12G in melanoma cells and melanocytes, leading to regression of the tumor, demonstrating that H-RASV12G plays a role not only in tumorigenesis, but also in maintenance of established tumors. Note that complete clinical and histological regression is possible upon long-term withdrawal from doxycycline.
genomic alterations in regions often involved in human melanoma5. These observations imply that this model will find use in the discovery of genes/loci relevant to the genesis and progression of melanoma. More recently, the tet-ON regulatory system has been utilized as an in vivo molecular switch to regulate expression of H-RASV12G in premalignant melanocytes and established melanomas by adding or removing tetracycline or doxycycline to the system6 (Tables 1 and 2). This allows one to simulate somatic acquisition of an oncogenic mutation in adult life or remediation of a genetic lesion in established tumors. Most significant was the observation that loss of activated RAS in established melanomas leads to complete clinical and histological regression, indicating that activated RAS plays a role in both tumor genesis and 409
Disease models
MOLECULAR MEDICINE TODAY, OCTOBER 2000 (VOL. 6)
maintenance (Fig. 1). Notably, regression was accompanied by a robust apoptotic response in host-derived endothelial cells, raising the possibility that H-RASV12G plays a key role in the maintenance of a pro-angiogenic environment. Together, these transgenic studies in the mouse have provided unequivocal proof that both activated RAS and INK4a deficiency play causal roles in the pathogenesis of melanoma.
Future directions In the new era of functional genomics, a mouse model that faithfully reproduces human disease progression on clinico-pathological and genetic levels will serve as a genetic platform for characterization of several aspects of the disease: the impact of cooperating mutations; the decoding of molecular signals between the host and tumor compartments; the identification of novel targets; and, ultimately, the development and preclinical testing of rational therapeutics and chemoprevention strategies.
Acknowledgments. We apologize to colleagues whose work was not discussed or cited owing to space constraints. We thank Norman Sharpless, Steven Artandi and Ron DePinho for critical reading of the manuscript. L.C. is a V foundation scholar and is supported by grants from the NIH and NCI.
References
4 Broome-Powell, M. et al. (1999) Induction of melanoma in Tpras transgenic mice. Carcinogenesis 20, 1747–1753 5 Chin, L. et al. (1997) Cooperative effects of INK4a and ras in melanoma susceptibility in vivo. Genes Dev. 11, 2822–2834 6 Chin, L. et al. (1999) Essential role for oncogenic RAS in tumour maintenance. Nature 400, 468–472
1 Bardeesy, N. et al. Animal models of malignant melanomas: recent advances and future prospects. In Advances in Cancer Research (Vol. 79) (Vande Woude, G. and Klein, G., eds), pp. 123–156 (in press) 2 Takayama, H. et al. (1997) Diverse tumorigenesis associated with aberrant development in mice overexpressing hepatocyte growth factor/ scatter factor. Proc. Natl. Acad. Sci. U. S. A. 94, 701–706 3 Kelsall, S.R. and Mintz, B. (1998) Metastatic Cutaneous Melanoma promoted by ultraviolet radiation in mice with transgene-inititated low melanoma susceptibility. Cancer Res. 58, 4061–4065
Maja K. Tietze MD Research Fellow Lynda Chin* MD Assistant Professor of Dermatology Department of Dermatology, Harvard Medical School, Department of Adult Oncology, Dana-Farber Cancer Institute, 44 Binney Street, Boston, MA 02115, USA. Tel: 1617 632 6091 Fax: 1617 632 6069 *e-mail: [email protected]
Molecular Medicine Today covers a range of cancer-related topics. Here’s a selection of recent articles… PI 3-kinase inhibition: a target for drug development? by Robert C. Stein and Michael D. Waterfield: Mol. Med. Today (2000) 6, 347–357 TP53 gene therapy: a key to modulating resistance to anticancer therapies by Esther H. Chang et al.: Mol. Med. Today (2000) 6, 358–364 The influence of the microenvironment on the malignant phenotype by Catherine C. Park et al.: Mol. Med. Today (2000) 6, 324–329 Neurofibromatosis type II: mouse models reveal broad roles in tumorigenesis and metastasis by Andrea I. McClatchey: Mol. Med. Today (2000) 6, 252–253 Cervical cancer risk: is there a genetic component? by Patrik K.E. Magnusson and Ulf B. Gyllensten: Mol. Med. Today (2000) 6, 145–148 Matrix metalloproteinases: multifunctional contributors to tumor progression by Lisa J. McCawley and Lynn M. Matrisian: Mol. Med. Today (2000) 6, 149–156 Exploiting the hypoxic cancer cell: mechanisms and therapeutic strategies by J. Martin Brown: Mol. Med. Today (2000) 6, 157–162 Dynamics of intercellular communication during melanoma development by Gang Li and Meenhard Herlyn: Mol. Med. Today (2000) 6, 163–169
410