HMGN1

Abstract: High mobility group N1 (HMGN1) protein is a member of nonhistone chromosomal proteins that binds more strongly with nucleosomes than with DNA. Here we report the identification of the sites of in vivo phosphorylation of HMGN1 isolated from the MCF-7 human breast cancer cells. Our results showed that four serine residues, i.e., Ser6, Ser85, Ser88, and Ser98, can be phosphorylated in this protein. To our knowledge, this is the first demonstration that each of the three serine residues in the acidic C-terminal region of human HMGN1 can be phosphorylated. The additional negative charge resulting from the phosphorylation of the C-terminal serine residues is expected to modulate the interaction between HMGN1 and other proteins, which may enhance transcription and facilitate other cellular functions. In addition, the phosphorylation of HMGN1 at Ser85, which precedes Pro86, might play an important role in cellular signaling.

Abstract: A method for detecting and/or monitoring a chronic inflammation condition associated with rheumatoid arthritis. The method comprises the steps of: (i) collecting a sample; (ii) detecting in the sample, one or more of a HMGN1 protein, a LCP-1 protein, a prtn3 protein, a sec22b protein, and a PYGL glycogen phosphorylase protein thereby producing a result; and (iii) correlating the result with a control. A test kit for diagnosing or monitoring a chronic inflammatory condition associated with rheumatoid arthritis. The test kit comprises an antibody for complexing with one of a HMGN1 protein, a LCP-1 protein, a prtn3 protein, a sec22b protein, and a PYGL glycogen phosphorylase protein. A composition for treatment of a chronic inflammation condition associated with rheumatoid arthritis, comprising a therapeutically effective amount of an exogenous agonist for decreasing cellular production of one or more of a HMGN1 protein, a LCP-1 protein, a prtn3 protein, a sec22b protein, and a PYGL glycogen phosphorylase protein.

Abstract: We report that loss of HMGN1, a nucleosome-binding protein that alters the compaction of the chromatin fiber, increases the cellular sensitivity to ionizing radiation and the tumor burden of mice. The mortality and tumor burden of ionizing radiation–treated Hmgn1−/− mice is higher than that of their Hmgn1+/+ littermates. Hmgn1−/− fibroblasts have an altered G2-M checkpoint activation and are hypersensitive to ionizing radiation. The ionizing radiation hypersensitivity and the aberrant G2-M checkpoint activation of Hmgn1−/− fibroblasts can be reverted by transfections with plasmids expressing wild-type HMGN1, but not with plasmids expressing mutant HMGN proteins that do not bind to chromatin. Transformed Hmgn1−/− fibroblasts grow in soft agar and produce tumors in nude mice with a significantly higher efficiency than Hmgn1+/+ fibroblasts, suggesting that loss of HMGN1 protein disrupts cellular events controlling proliferation and growth. Hmgn1−/− mice have a higher incidence of multiple malignant tumors and metastases than their Hmgn1+/+ littermates. We suggest that HMGN1 optimizes the cellular response to ionizing radiation and to other tumorigenic events; therefore, loss of this protein increases the tumor burden in mice.

Abstract: HMGN1 is a nuclear protein that binds to nucleosomes and alters the accessibility of regulatory factors to their chromatin targets. To elucidate its biological function and identify specific HMGN1 target genes, we generated Hmgn1–/– mice. DNA microarray analysis of Hmgn1+/+ and Hmgn1–/– embryonic fibroblasts identified N-cadherin as a potential HMGN1 gene target. RT-PCR and western blot analysis confirmed a linkage between HMGN1 expression and N-cadherin levels. In both transformed and primary mouse embryonic fibroblasts (MEFs), HMGN1 acted as negative regulator of N-cadherin expression. Likewise, the N-cadherin levels in early embryos of Hmgn1–/– mice were higher than those of their Hmgn1+/+ littermates. Loss of HMGN1 increased the adhesiveness, motility and aggregation potential of Hmgn1–/– MEFs, a phenotype consistent with increased levels of N-cadherin protein. Re-expression of wild-type HMGN1, but not of the mutant HMGN1 protein that does not bind to chromatin, in Hmgn1–/– MEFs, decreased the levels of N-cadherin and restored the Hmgn1+/+ phenotype. These studies demonstrate a role for HMGN1 in the regulation of specific gene expression. We suggest that in MEFs, and during early mouse development, the interaction of HMGN1 with chromatin down-regulates the expression of N-cadherin.