Biomedical subjects

Abhijit Guha

Publications and source records attributed to Abhijit Guha.

32 records · Page 2Linked to original sources

PubMed2003

Neoangiogenesis in human astrocytomas: expression and functional role of angiopoietins and their cognate receptors.

Since the introduction of the concept of an "angiogenic switch" driving tumor growth and malignant progression by Judah Folkman in 1971, there have been numerous scientific reports confirming the central concept that tumor growth is angiogenesis dependent. Various angiogenic genes and gene products, from both neoplastic and normal tissues, have been isolated, purified, and cloned that contribute to the 'angiogenic switch'. Of these various molecules, two have been identified that act specifically on endothelial cells. First is Vascular Endothelial Growth Factor (VEGF), the cognate receptors of which are almost specifically expressed on endothelial cells. VEGF plays a crucial role in the development of the embryonic vasculature by providing differentiation and mitogenic signals to endothelial cells and their mesodermal precursors. Second are the Angiopoietins and their cognate receptor, Tie2. Angiopoietins are primarily involved in maturation of both embryonal and adult vasculature, with Angiopoietin 1 2 being naturally occurring agonists and antagonists of Tie2 respectively, indicating a very precise level of regulation in-vivo. In this review we summarize what is known of the biological role of Angiopoietins and Tie2, their interaction with VEGF in normal and tumor related angiogenesis, with emphasis on their functional consequence in the progression and growth of malignant human astrocytomas.

Angiopoietins↗

PubMed2003

Role of the Rap1 GTPase in astrocyte growth regulation.

Tuberous sclerosis complex (TSC) is an autosomal dominant syndrome in which affected individuals develop nervous system abnormalities that might reflect astrocyte dysfunction. The TSC2 gene product, tuberin, encodes a GTPase-activating protein (GAP) domain, which regulates the activity of Rap1 in vitro. To determine whether dysregulated Rap1, resulting from TSC2 inactivation, leads to increased astrocyte proliferation in vivo, we generated transgenic mice expressing activated Rap1(G12V) specifically in astrocytes. We observed no statistically significant difference in the number of astrocytes between wild-type and GFAP-Rap1(G12V) littermates in vivo; however, during log-phase growth, we observed a 25% increase in GFAP-Rap1(G12V) astrocyte doubling times compared to wild-type controls. This decreased proliferation was associated with delayed MAP kinase, but not AKT, activation. Lastly, to determine whether constitutive Rap1 activation could reverse the increased astrocyte proliferation observed in transgenic mice expressing oncogenic Ras(G12V), we generated transgenic mice expressing both Ras(G12V) and Rap1(G12V) in astrocytes. These double transgenic mice showed a striking reversion of the Ras(G12V) astrocyte growth phenotype. Collectively, these results argue that the tumor suppressor properties of tuberin are unlikely to be related to Rap1 inactivation and that Rap1 inhibits mitogenic Ras pathway signaling in astrocytes.

Animals↗

PubMed2003

Metaplastic bone formation in a low grade conus glioma: case report and review of the literature.

Calcifications associated with both benign and neoplastic intra-axial lesions of the central nervous system (CNS) are well recognized. Bony metaplasia in the CNS, where there is formation of mature trabecular bone, is a much more rare entity and has not been reported in the spinal cord. We present a case of bony metaplasia in the conus medullaris associated with a low grade astrocytoma. Radiological, pathological and clinical features of this unique case are discussed.

Astrocytoma↗

PubMed2003

Angiogenesis in nervous system disorders.

Angiogenesis is a crucial requirement for embryonal development and new vessel formation during adult life. Various disease processes such as cancer, ischemia, vascular malformations, and inflammatory processes also depend on pathological angiogenesis. A better understanding of the complex and coordinated interactions among various angiogenic pathways involved in pathological angiogenesis is necessary to improve our therapeutic approaches to the various disease processes observed in the central nervous system. This review summarizes the current understanding of the role of principal angiogenic factors relevant to neurosurgical abnormalities.

Angiogenesis Modulating Agents↗

PubMed2003

Rap1 activity is elevated in malignant astrocytomas independent of tuberous sclerosis complex-2 gene expression.

Increased small GTPase protein mitogenic signaling is common in tumors. We have previously demonstrated that sporadic astrocytomas exhibit high levels of activated Ras important for tumor growth. Individuals with tuberous sclerosis complex (TSC) develop astrocytoma-like tumors resulting from mutations in the TSC2 protein, tuberin, which is hypothesized to function as a Rap1 GTPase activating protein (GAP). Since we have previously reported that high-grade astrocytomas frequently exhibit loss of tuberin expression or increased Rap1 levels, we sought to determine whether there is a correlation between decreased tuberin Rap1-GAP function or Rap1 overexpression and tumor Rap1 activity. In this study, we compared levels of Rap1-GTP, Rap1 and tuberin levels in normal brain tissue and 24 grade II-IV astrocytoma specimens. Whereas Rap1 overexpression was observed in astrocytomas of all malignancy grades, tuberin loss was seen most frequently in the higher-grade astrocytomas. In the grade IV glioblastoma multiforme tumors, Rap1 activity was 2-3-fold higher than in lower grade or non-neoplastic brain. However, there was no correlation between tuberin expression or Rap1 overexpression and the levels of Rap1 activity in the tumors studied, suggesting that the increased Rap1 activation is not the direct result of reduced tuberin Rap1-GAP function or elevated Rap1 protein expression.

Astrocytoma↗

PubMed2003

Molecular regulators of angiogenesis in the developing nervous system and adult brain tumors (review).

Angiogenesis involves a highly regulated and coordinated interaction of multiple angiogenic factors and is critical for both embryonal development and physiological vessel formation in adults. Angiogenesis is also proven to significantly contribute to the progression of various disease processes. Pathological angiogenesis is thought to recapitulate physiological angiogenesis, however the precise molecular mechanisms are not understood. Various angiogenic genes and gene products have been isolated, purified, and cloned from both neoplastic and normal tissues that contribute to the angiogenic process. In this review we summarize the role of the main angiogenic factors in both normal embryonal vascularization of the embryonal nervous system together with the main brain tumors seen in adults.

Angiogenesis Inducing Agents↗

PubMed2002

Neuropathology of genetically engineered mice: consensus report and recommendations from an international forum.

The Mouse Models of Cancer Consortium of the NCI sponsored a meeting of neuropathologists and veterinary pathologists in New York City in November of 2000. A rapidly growing number of genetically engineered mice (GEM) predisposed to tumors of the nervous system have led to a concomitant need for neuropathological evaluation and validation of these models. A panel of 13 pathologists reviewed material representing most of the available published and unpublished GEM models of medulloblastoma, primitive neuroectodermal tumor, astrocytoma, oligodendroglioma, mixed glioma, and tumors of the peripheral nerve. The GEM tumors were found to have many similarities and some distinct differences with respect to human disease. After review of the biology and pathology for all models presented, participants were split into groups reflective of clinical expertise in human pathology, tumor biology, neuroimaging, or treatment/intervention. Recommendations were made detailing an extensive and complete neuropathological characterization of animals. Importance was placed on including information on strains, tumor clonality, and examination for genetic mutation or altered gene expression characteristics of the corresponding human malignancy. Specific proposals were made to incorporate GEM models in emerging neuroradiological modalities. Recommendations were also made for preclinical validation of these models in cancer therapeutics, and for incorporation of surrogate markers of tumor burden to facilitate preclinical evaluation of new therapies.

Animals↗

PubMed2002

Mouse glioma gene expression profiling identifies novel human glioma-associated genes.

Based on previous studies demonstrating increased RAS activity in human astrocytomas, we developed a transgenic mouse model (B8) that targets an activated RAS molecule to astrocytes. Within 3 to 4 months after birth, these mice develop high-grade astrocytomas that are histologically identical to human astrocytomas. To characterize genetic events associated with B8 mouse astrocytoma formation, we employed comparative gene expression profiling of wild-type cultured mouse astrocytes, non-neoplastic B8 astrocytes, B8 astrocytoma cultures, and two other astrocytoma cultures from independently derived RAS transgenic mouse lines. We identified several classes of gene expression changes, including those associated with the non-neoplastic state in the B8 transgenic mouse, those associated with astrocytoma formation, and those specifically associated with only one of the three independently derived transgenic mouse astrocytomas. Differential expression of several unique genes was confirmed at the protein level in both the RAS transgenic mouse astrocytomas and two human glioblastoma multiforme cell lines. Furthermore, reexpression of one of these downregulated astrocytoma-associated proteins, GAP43, resulted in C6 glioma cell growth suppression. The use of this transgenic mouse model to identify novel genetic changes that might underlie the pathogenesis of human high-grade astrocytomas provides a unique opportunity to discover future targets for brain tumor therapy.

Animals↗

PubMed2002

Proteolytic cleavage of the CD44 adhesion molecule in multiple human tumors.

Cell surface adhesion molecules are crucial for the development and/or pathogenesis of various diseases including cancer. CD44 has received much interest as a major adhesion molecule that is involved in tumor progression. We have previously demonstrated that the ectodomain of CD44 undergoes proteolytic cleavage by membrane-associated metalloproteases in various tumor cell lines. The remaining membrane-bound CD44 cleavage product can be detected using antibodies against the cytoplasmic domain of CD44 (anti-CD44cyto antibody). However, the cleavage of CD44 in primary human tumors has not been investigated. Using Western blots with anti-CD44cyto antibody to assay human tumor tissues, we show that the CD44 cleavage product can be detected in 58% (42 of 72) of gliomas but not in normal brain. Enhanced CD44 cleavage was also found in 67% (28 of 42) of breast carcinomas, 45% (5 of 11) of non-small cell lung carcinomas, 90% (9 of 10) of colon carcinomas, and 25% (3 of 12) of ovarian carcinomas. Tumors expressing a CD44 splice variant showed a significantly higher incidence of enhanced CD44 cleavage. The wide prevalence of CD44 cleavage suggests that it plays an important role in the pathogenesis of human tumors.

Brain↗

PubMed2002

Phosphatidylinositol 3'-kinase and MAPK/ERK kinase 1/2 differentially regulate expression of vascular endothelial growth factor in human malignant astrocytoma cells.

Malignant astrocytomas are characterized by extensive vascularization attributed to increased expression of the angiogenic cytokine vascular endothelial growth factor (VEGF). VEGF is elevated in astrocytomas under normal oxygen conditions and undergoes induction in hypoxic stress. Prior studies have shown that both the phosphatidylinositol 3'-kinase (PI3-kinase) and MEK1/2 (MAPK/ERK kinase 1/2) pathways promote proliferation of astrocytoma cells and growth of astrocytic tumors. Whether these pathways regulate growth by modulating angiogenesis as well as proliferation is not clear. In this study, pharmacologic inhibitors were used to specifically inhibit PI3-kinase and MEK1/2 activity in human malignant astrocytoma cell lines, and their effects on VEGF expression were determined. Northern blot analysis of VEGF messenger RNA (mRNA) from cells treated with inhibitors demonstrated cell line-specific responses. The PI3-kinase pathway regulated both the normoxic expression and hypoxic induction of VEGF in 2 cell lines, whereas MEK1/2 regulated only the normoxic expression in the same 2 lines. The third cell line showed no change in VEGF mRNA with inhibition of either of these 2 pathways. This study suggests that modulation of signaling pathways implicated in proliferation of astrocytoma cell lines may have varying effects in vivo depending on the role these pathways play in regulating tumor angiogenesis.

Astrocytoma↗

PubMed2002

Orthopaedic manifestations of neurofibromatosis in children: an update.

Neurofibromatosis is one of the most common genetic disorders affecting mankind. Despite extensive basic science research, the diagnosis still is based largely on well-defined clinical criteria, which often present gradually during childhood. Approximately 50% of patients have significant musculoskeletal manifestations, with scoliosis and congenital pseudarthrosis of the tibia most common. Approximately 20% of children with Type I neurofibromatosis present with scoliosis with or without the classic dystrophic features, such as vertebral scalloping and rib penciling. Dystrophic curves portend rapid progression and require early fusion. Surgical treatment often is challenging because of the common presence of neurofibromas adjacent to the spinal cord, significant multiplanar deformity, and poor bone quality. Congenital pseudarthrosis of the tibia also continues to present significant difficulties. The use of a brace is the mainstay of early treatment, whereas intramedullary rodding commonly is used for operative fixation. Grafting of the free fibula and correction using techniques of distraction and compression histiogenesis with Ilizarov fixators have been reported for refractory cases with varying degrees of success. Multiple heroic, operative attempts may have a tremendous toll on the quality of life of affected children through their early childhood. In addition to these and other distinctive musculoskeletal lesions, affected children often suffer from various medical problems.

Bone and Bones↗

PubMed2002

Aberrant G protein signaling in nervous system tumors.

OBJECT: Guanosine triphosphate (GTP)-binding proteins, also known as G proteins, play important roles in the regulation of cell growth and differentiation by transmitting intracellular signals from cell surface receptors. In this paper, the authors review G protein signaling in general and its aberrations in four human nervous system tumors. METHODS: In the nervous system, four tumor types have been associated with aberrant G protein signaling. The first tumor type includes astrocytomas, which have increased levels of the activated form of the small G protein, p21-ras, without primary oncogenic p21-ras mutations. The likely source for increased p21-ras activity in sporadically occurring astrocytomas is overexpressed or constitutively activated growth factor receptors, whereas in neurofibromatosis Type 1 (NF1)-associated astrocytomas, the source is a loss of expression of neurofibromin, a major inactivator of p21-ras (ras-GTPase activating protein [GAP]). The second type of tumor associated with aberrant G protein signaling includes sporadic and NF1-associated neurofibromas and malignant peripheral nerve sheath tumors, which also have increased p21-ras activity due to a loss of neurofibromin expression. The third tumor type includes subependymal giant cell astrocytomas as part of the tuberous sclerosis complex (TSC). These tumors display a loss of tuberin expression due to germline mutations in the TSC2 gene. Tuberin functions as an inactivator of the small G protein rap1B (rap1-GAP) and, hence, loss of its expression could lead to increased rap1B activity. In addition to TSC-associated tumors, the authors demonstrate that the majority of sporadically occurring astrocytomas display either loss of tuberin or overexpression of rap1B. This suggests that increased rap1B activity, which can augment p21-ras-mediated signals, also contributes to G protein-mediated aberrant signaling in sporadically occurring astrocytomas. The fourth tumor type includes a significant subset of pituitary adenomas that show constitutive activation of the G alpha subunit of the large heterotrimeric G s protein, which is involved in hormone receptor signaling. The net result of this aberrant activation is increased cyclic adenosine monophosphate and mitogenic tumor-promoting signals. CONCLUSIONS: The authors' review of G protein signaling and aberrations in this process is made with the long-term view that increased understanding of relevant signaling pathways will eventually lead to novel biological targeted therapies against these tumors.

Adenoma↗

PubMedDate not supplied

High-resolution longitudinal screening with magnetic resonance imaging in a murine brain cancer model.

One of the main limitations of intracranial models of diseases is our present inability to monitor and evaluate the intracranial compartment noninvasively over time. Therefore, there is a growing need for imaging modalities that provide thorough neuropathological evaluations of xenograft and transgenic models of intracranial pathology. In this study, we have established protocols for multiple-mouse magnetic resonance imaging (MRI) to follow the growth and behavior of intracranial xenografts of gliomas longitudinally. We successfully obtained weekly images on 16 mice for a total of 5 weeks on a 7-T multiple-mouse MRI. T2- and T1-weighted imaging with gadolinium enhancement of vascularity was used to detect tumor margins, tumor size, and growth. These experiments, using 3D whole brain images obtained in four mice at once, demonstrate the feasibility of obtaining repeat radiological images in intracranial tumor models and suggest that MRI should be incorporated as a research modality for the investigation of intracranial pathobiology.

Angiopoietin-1↗

PubMedDate not supplied

Peripheral nerve tumors: management strategies and molecular insights.

Because of its relative rarity and a wide variety of clinical manifestations, peripheral nerve tumors (PNTs) often present to specialists from widely different disciplines, thus often resulting in delayed diagnosis and a non-cohesive pattern of management. Critical appraisal of the history and physical examination followed by radiological investigations, by experienced medical personnel, ultimately suggests that the extremity mass is perhaps a PNT, rather than the wide variety and more common soft tissue tumors. Included in this appraisal is a search for a pre-disposition syndrome, the most common of which are neurofibromatosis-1 and -2 (NF1, NF2) and schwannomatosis, which may require life-long careful follow up. Intra-operative and post-operative management decisions in a multidisciplinary manner with knowledge of the biological, pathological and clinical behavior of the PNT, is of utmost importance. In the context of pre-disposition syndromes, where multiple tumors may exist along with other nervous system lesions, molecular biological insights and hopefully the biological therapies that stem from this knowledge are of interest. In this article the spectrum of PNTs and their management protocols, including the algorithm for treatment of malignant PNTs advocated by our institute, are presented and currently available genetic insights and probable role of experimental therapies are briefly reviewed.

Humans↗