Id proteins in cell growth and tumorigenesis.

Since the gene encoding Id1 was cloned in 1990, Id proteins have been implicated in regulating a variety of cellular processes, including cellular growth, senescence, differentiation, apoptosis, angiogenesis, and neoplastic transformation. The development of knockout and transgenic animal models for many members of the Id gene family has been particularly useful in sorting out the biologic relevance of these genes and their expression during normal development, malignant transformation, and tumor progression. Here we review the current understanding of Id gene function, the biologic consequences of Id gene expression, and the implications for Id gene regulation of cell growth and tumorigenesis.

Disruption of Id1 reveals major differences in angiogenesis between transplanted and autochthonous tumors.

Id genes regulate tumor angiogenesis and loss of Id1 inhibits tumor xenograft growth in mice. Here we evaluate the role of Id1 in a more clinically relevant tumor model system using a two-step chemical carcinogenesis protocol. Remarkably, we find that Id1-/- mice are more susceptible to skin tumorigenesis compared to their wild-type counterparts. Cutaneous neoplasms in Id1-/- mice show increased proliferation without alterations in tumor angiogenesis; however, Id1-/- mice possess 50% fewer cutaneous gammadelta T cells than their wild-type counterparts due to an intrinsic migration defect associated with loss of expression of the chemokine receptor, CXCR4. We suggest that there are important differences between the mechanisms of angiogenesis in transplanted and autochthonous tumors and that these findings will have significant implications for the potential utility of antiangiogenic therapies in cancer.

Id1 regulates angiogenesis through transcriptional repression of thrombospondin-1.

Id proteins are helix-loop-helix transcription factors that regulate tumor angiogenesis. In order to identify downstream effectors of Id1 involved in the regulation of angiogenesis, we performed PCR-select subtractive hybridization on wild-type and Id1 knockout mouse embryo fibroblasts (MEFs). Here we demonstrate that thrombospondin-1 (TSP-1), a potent inhibitor of angiogenesis, is a target of transcriptional repression by Id1. We also show that Id1-null MEFs secrete an inhibitor of endothelial cell migration, which is completely inactivated by depletion of TSP-1. Furthermore, in vivo studies revealed decreased neovascularization in matrigel assays in Id1-null mice compared to their wild-type littermates. This decrease was completely reversed by a TSP-1 neutralizing antibody. We conclude that TSP-1 is a major target for Id1 effects on angiogenesis.

A central role for transcription factor C/EBP-beta in regulating CD1d gene expression in human keratinocytes.

CD1d is a nonclassical Ag-presenting molecule that presents glycolipid Ags to NKT cells that are involved in immune defense and tumor rejection. It also plays a role in immunoregulatory functions in the epidermis. The mechanisms controlling the expression of CD1d are not well understood. Therefore, we cloned the CD1d gene promoter and characterized its activities in primary human keratinocytes and other cell lines of epithelial origin. We found that a CCAAT box in the CD1d promoter is required for its expression in keratinocytes. We show here that transcription factor C/EBP-beta binds to the CCAAT box in the CD1d promoter in vitro and in vivo. Consistent with these observations, deletion of the gene encoding for C/EBP-beta caused a loss of CD1d expression. The in vivo regulation of CD1d has significant implications for the pathologic mechanisms of certain immunologic skin diseases in which NKT cells play a role, such as allergic contact dermatitis and psoriasis. Together, these data show a central role for C/EBP-beta in regulating CD1d transcription.

A colony color method identifies the vulnerability of mitochondria to oxidative damage.

Mitochondrial dysfunction is a profound feature of cancer cells and is also known to cause several mitochondrial diseases. Mutations in mitochondrial DNA (mtDNA) have been reported frequently in these diseases. Although many environmental agents are known to cause damage to mitochondria, rapid methods need to be developed for testing agents that cause mitochondrial dysfunction and are involved in the development of mitochondrial and other diseases. Using Saccharomyces cerevisiae, we describe the development of a colorimetric method that identifies both physical and chemical agents that cause mitochondrial dysfunction and mutation of the mitochondrial genome. This method utilizes the previously reported ade2 mutant of S.cerevisiae that produces red colonies. However, when they lose mitochondrial function the colonies turn white. This colorimetric method has helped quantify the vulnerability of mtDNA to oxidative agents. Our study reveals that the oxidative agent adriamycin causes both mutation and extensive damage to mtDNA, which leads to loss of mtDNA. Our study also reveals that the lost mtDNA fragments migrate to the nucleus and integrate into the nuclear genome. Furthermore, our analysis reveals that loss of mtDNA leads to resistance to oxidative agents. The method described in this paper should aid in the rapid identification of environmental and other agents that cause mitochondrial dysfunction and mutagenesis, agents that may be involved in the development of mitochondrial and other diseases.