Molecular Mechanisms Of Growth Control And Carcinogenesi

Dental & Craniofacial Research

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Abstract

Certain alterations of proteins involved in mitogenic signaling are known to exert profound effects on cellular behavior, including malignant transformation. Our overall objective is to explore the molecular bases of cancer, approaching this problem through the study of normal and aberrant functioning of molecules that participate in the transduction of proliferative signals. [unreadable] [unreadable] Molecular dissection of the pathway linking growth factor receptors to the nucleus: their role in normal cell growth and cancer.[unreadable] [unreadable] Emerging clinical and experimental evidence now indicates that nonsteroidal anti-inflammatory drugs, which inhibit two enzymes involved in prostaglandin biosynthesis, cyclooxygenase-1 (COX-1) and cyclooxygenase-2 (COX-2), can reduce cancer incidence. Indeed, a large body of evidence implicates the contribution of COX-2 and one of its pro-inflammatory metabolites, prostaglandin E2 (PGE2) in cancer progression. However, how COX-2 and PGE2 promote the aberrant growth of cancer cells remains poorly understood. In most cancer cells PGE2 stimulates cell proliferation through its GPCR, EP2. Thus, as part as an ongoing effort to investigate the role of GPCRs in cancer biology, we investigated in detail the molecular mechanism underlying the potent mitogenic effect of PGE2, using colon cancer cells as a model system. We found that EP2 promotes cell growth by activating the beta-catenin pathway through a novel signaling route that involves the activation of PI3K and AKT by free Gbeta-gamma subunits, and the direct association of Galpha with the RGS domain of Axin. These findings may provide a molecular framework for the future clinical evaluation of new chemopreventive strategies for many highly prevalent human cancers.[unreadable] [unreadable] GPCRs and polypeptide growth factors promote the re-initiation of DNA-synthesis through multiple intracellular signaling pathways that converge in the nucleus to control the expression of growth-promoting genes. Among them, the AP-1 family of transcription factors, including c-Fos and c-Jun family members, plays a key role. In a prior study, we have observed that PDGF stimulates the ERK-mediated phosphorylation of multiple residues in the carboxyl-terminal transactivation domain (TAD) of c-Fos, and that this phosphorylation is required to stimulate c-Fos- and AP-1-dependent transcription. In this reporting period, we have found that the prolyl-isomerase Pin1 binds c-Fos through specific phosphorylated residues within the c-Fos TAD, and that this interaction results in an enhanced transcriptional response of c-Fos to growth factors. These findings suggest that c-Fos represents a novel target for the isomerizing activity of Pin1, and support a role for Pin1 in the mechanism by which c-Fos regulates AP-1-dependent gene transcription upon phosphorylation by MAPKs. [unreadable] [unreadable] Molecular basis of developmental and tumor-induced angiogenesis. [unreadable] [unreadable] Tumor growth and metastasis requires the induction of angiogenesis, the growth and remodeling of new blood vessels from a pre-existing vascular network, to ensure the delivery of oxygen and nutrients to rapidly dividing transformed cells. Using a yeast two-hybrid approach to screen for molecules interacting with the PDZ-domain of PRG, we have previously identified the cytoplasmic tail of PlexinB2 as a candidate interacting molecule. Plexins are transmembrane receptors that transduce attractive and repulsive signals mediated by semaphorins during the development of the central nervous system. However, we have recently observed that Plexin B1 is highly expressed in endothelial cells, and that its ligand, Semaphorin 4D (Sema4D), is expressed in certain tumor cells, including squamous carcinomas. Indeed, we have recently shown that Sema4D can promote a potent chemotactic and pro-angiogenic response through Plexin-B1 in endothelial cells, and that this response requires the activation of the kinase Akt in a PI3K-dependent manner by Sema4D. The molecular dissection of the underlying mechanism revealed that binding of Sema4D to Plexin B1 results in the recruitment of a multimeric signaling complex that includes PYK2, Src, and PI3K to Plexin-B1, and the activation of Akt. Together, these recent findings support the emerging notion that plexins of the B family promote angiogenesis by initiating a complex array of signaling events. Of interest, while conducting a gene array analysis of oral cancer cells, we observed that Sema4D was highly expressed in head and neck squamous cell carcinomas (HNSCCs) at both the protein and message level. Immunohistochemical analysis of a large panel of HNSCC tissues revealed that Sema4D is also expressed in a cell surface pattern in invading islands of transformed epithelial cells, but not in normal and non-invasive dysplastic oral epithelium, a pattern that was also seen in prostate, colon, breast and lung cancer tissues. Furthermore, we observed that HNSCC cells can stimulate endothelial cell migration in a Sema4D-specific manner, and that knocking down Sema4D dramatically reduces the size and vascularity of HNSCC tumor xenografts in nude mice. Collectively, these findings provided the first evidence that expression of Sema4D is a frequently employed strategy by which a wide variety of carcinomas may promote angiogenesis, and therefore this family of axon guiding molecules may represent a new therapeutic target for the treatment of many highly prevalent human cancers. [unreadable] [unreadable] AIDS-associated Kaposi?s sarcoma: molecular mechanisms.[unreadable] [unreadable] Among the AIDS-associated malignancies, Kaposi?s sarcoma (KS) is the most common cancer arising in HIV-infected patients. The Human herpesvirus 8 (HHV-8) or KS associated herpesvirus (KSHV) is the infectious cause of KS. Oral transmission plays a key role in the spread of KS. As the most frequent oral neoplasm in AIDS patients, KS is an infectious disease of paramount concern for oral health. Multiple candidate genes bearing oncogenic potential have been identified in the KSHV genome. In prior studies, we have developed a high throughput in vivo endothelial specific retroviral gene transfer system, and used it to express candidate KSHV oncogenes in mice. Surprisingly, among the many KSHV genes tested, only one gene, a constitutively active GPCR, vGPCR, was able to promote the development of visible vascular tumors that strikingly resembled human KS lesions. Which mitogenic and survival pathways are utilized by vGPCR to induce tumorigenesis is under active investigation. Currently available evidence indicates that vGPCR stimulates the serine/threonine kinase Akt in vivo, and that the activation of Akt plays a key role in the direct and paracrine neoplasia induced by vGPCR. Indeed, Akt activation was found to be a hallmark of human KS. Recently, we focused our efforts on an atypical serine/threonine kinase, mTOR. This kinase is a downstream target of Akt that controls the synthesis of proteins required for cell proliferation, including p70-S6 kinase (p70S6K). We found that aberrant accumulation of the phosphorylated and active form of S6 is a frequent event in experimental and clinical Kaposi?s sarcoma lesions. By investigating the underlying mechanism leading to the phosphorylation of S6, we found that vGPCR stimulates the phosphorylation and inactivation of tuberin (TSC2), a tumor suppressor protein, thereby promoting the activation of mTOR. Moreover, mice haploinsufficient for TSC2 are predisposed to vascular sarcomas remarkably similar to KS. Interestingly, pharmacologic inhibition of mTOR with rapamycin prevented vGPCR sarcomagenesis, while over-activation of this pathway was sufficient to render endothelial cells oncogenic. These findings identified the Akt-mTOR pathway as a therapeutic target for Kaposi?s Sarcoma, and provided the rationale for the clinical evaluation of rapamycin and its analogs for the treatment of this AIDS-malignancy.