Tissue-specific deoxyribonuclease I-hypersensitive sites in the vicinity of the immunoglobulin C lambda cluster of man

In vivo and in vitro analysis of the human tissue-type plasminogen activator gene promoter in neuroblastomal cell lines: evidence for a functional upstream kappaB element

Besides its well-established role in wound healing and fibrinolysis, tissue-type plasminogen activator (t-PA) has been shown to contribute to cognitive processes and memory formation within the central nervous system, and to promote glutamate receptor-mediated excitotoxicity. The t-PA gene is expressed and regulated in neuronal cells but the regulatory transcriptional processes directing this expression are still poorly characterized. We have used DNase I-hypersensitivity mapping and in vivo foot printing to identify putative regulatory elements and transcription factor binding sites in two human neuroblastomal (KELLY and SK-N-SH) and one human glioblastomal (SNB-19) cell lines. Hypersensitive sites were found in the proximal promoter region of all cell lines, and within the first exon for KELLY and SNB-19 cells. Mapping of methylation-protected residues in vivo detected a cluster of protected residues corresponding to a cAMP response element (CRE) and Sp1 sites in the proximal promoter previously shown to be essential for basal expression in other cell types. Protected residues were also found at other sites, notably a kappaB element at position bp -3081 to -3072 that was partly protected in KELLY and SNB-19 cells. Analysis of transfected reporter constructs in KELLY and SNB-19 cells confirmed that this particular element is functionally significant in the transactivation of the t-PA promoter in both cell types. This study defines, by in vivo and in vitro methods, a previously undescribed kappaB site in the t-PA gene promoter that influences t-PA expression in neuronal cells.

Regulation of human Ig lambda light chain gene expression by NF-kappa B

The human Iglambda enhancer consists of three separated sequence elements that we identified previously by mapping DNase I-hypersensitive regions (HSS) downstream of the C region of the Iglambda L chain genes (HSS-1, HSS-2, and HSS-3). It has been shown by several laboratories that expression of the H chain genes as well as the kappa genes, but not the lambda genes, is dependent on constitutive NF-kappaB proteins present in the nucleus. In this study we show by band-shift experiments, in vivo footprinting, and transient transfection assays that all three hypersensitive sites of the human Iglambda enhancer contain functional NF-kappaB sites that act synergistically on expression. We further show that the chicken lambda enhancer also contains a functional NF-kappaB site but the mouse lambda enhancer contains a mutated, nonfunctional NF-kappaB site that is responsible for its low enhancer activity. It is possible that the inactivating mutation in the mouse Iglambda enhancer was compensated for by an expansion of the Igkappa L chain locus, followed by a contraction of the Iglambda locus in this species.

Hepatocyte nuclear factor 3 relieves chromatin-mediated repression of the alpha-fetoprotein gene

The alpha-fetoprotein gene (AFP) is tightly regulated at the tissue-specific level, with expression confined to endoderm-derived cells. We have reconstituted AFP transcription on chromatin-assembled DNA templates in vitro. Our studies show that chromatin assembly is essential for hepatic-specific expression of the AFP gene. While nucleosome-free AFP DNA is robustly transcribed in vitro by both cervical (HeLa) and hepatocellular (HepG2) carcinoma extracts, the general transcription factors and transactivators present in HeLa extract cannot relieve chromatin-mediated repression of AFP. In contrast, preincubation with either HepG2 extract or HeLa extract supplemented with recombinant hepatocyte nuclear factor 3 alpha (HNF3alpha), a hepatic-enriched factor expressed very early during liver development, is sufficient to confer transcriptional activation on a chromatin-repressed AFP template. Transient transfection studies illustrate that HNF3alpha can activate AFP expression in a non-liver cellular environment, confirming a pivotal role for HNF3alpha in establishing hepatic-specific gene expression. Restriction enzyme accessibility assays reveal that HNF3alpha promotes the assembly of an open chromatin structure at the AFP promoter. Combined, these functional and structural data suggest that chromatin assembly establishes a barrier to block inappropriate expression of AFP in non-hepatic tissues and that tissue-specific factors, such as HNF3alpha, are required to alleviate the chromatin-mediated repression.

A human immunoglobulin lambda locus is similarly well expressed in mice and humans

Transgenic mice carrying a 380-kb region of the human immunoglobulin (Ig) lambda light (L) chain locus in germline configuration were created. The introduced translocus on a yeast artificial chromosome (YAC) accommodates the most proximal Iglambda variable region (V) gene cluster, including 15 Vlambda genes that contribute to >60% of lambda L chains in humans, all Jlambda-Clambda segments, and the 3' enhancer. HuIglambdaYAC mice were bred with animals in which mouse Igkappa production was silenced by gene targeting. In the kappa-/- background, human Iglambda was expressed by approximately 84% of splenic B cells. A striking result was that human Iglambda was also produced at high levels in mice with normal kappa locus. Analysis of bone marrow cells showed that human Iglambda and mouse Igkappa were expressed at similar levels throughout B cell development, suggesting that the Iglambda translocus and the endogenous kappa locus rearrange independently and with equal efficiency at the same developmental stage. This is further supported by the finding that in hybridomas expressing human Iglambda the endogenous L chain loci were in germline configuration. The presence of somatic hypermutation in the human Vlambda genes indicated that the Iglambda-expressing cells function normally. The finding that human lambda genes can be utilized with similar efficiency in mice and humans implies that L chain expression is critically dependent on the configuration of the locus.

Deregulation of the proto-oncogene c-myc through t(8;22) translocation in Burkitt's lymphoma

In Burkitt's lymphoma (BL) cells the proto-oncogene c-myc is juxtaposed to one of the immunoglobulin (Ig) loci on chromosomes 2, 14, or 22. The c-myc gene becomes transcriptionally activated as a consequence of the chromosomal translocation and shows preferential usage of promoter P1 over P2, a phenomenon referred to as promoter shift. In order to define the responsible regulatory elements within the Ig lambda locus, we studied the effect of the human Ig lambda enhancer (HuE lambda) on c-myc expression after stable transfection into BL cells. A 12 kb genomic fragment encompassing HuE lambda, but not HuE lambda alone, strongly activated c-myc expression and induced the promoter shift. To identify additional elements involved in c-myc deregulation, we mapped DNaseI hypersensitive sites within the 12 kb lambda fragment on the construct. Besides one hypersensitive site corresponding to HuE lambda, three additional sites were detected. Two of these elements displayed enhancer activity after transient transfection. The third element did not activate c-myc transcription, but was required for full c-myc activation and promoter shift. Deletion analyses of the c-myc promoter identified the immediate promoter region as sufficient for activation by the Ig lambda. locus, but also revealed that induction of the promoter shift requires additional upstream elements.

The immunoglobulin lambda light chain enhancer consists of three modules which synergize in activation of transcription

V(D)J rearrangement, high level expression and somatic hypermutation of assembled Ig genes is tightly controlled by a number of regulatory sequence elements located in the vicinity of the J-, (D)-, and C-gene segments. During B cell maturation these elements become accessible to binding of trans-acting factors, reflecting the opening of the chromatin structure through an as yet unidentified mechanism. The mapping of regions of an altered chromatin structure (DNase I hypersensitivity) therefore is a powerful approach in identifying regulatory sequence elements. We here show that the human Ig lambda enhancer consists of three modules previously identified by us as DNase I-hypersensitive sites HSS-1, -2, and -3. The three sequence elements synergize in transcriptional activation of a reporter gene and together constitute a powerful tissue-specific enhancer which is a much stronger transcriptional activator than the kappa enhancers alone or in combination. We further show that the accessibility of the kappa and lambda enhancer elements for DNase I in the chromatin of a pre-B cell line (207) correlates with the transcriptional enhancer activities of kappa and lambda enhancer constructs. This finding is in support of an ordered model for Ig light chain activation (kappa before lambda).