LRP1b shows restricted expression in human tissues and binds to several extracellular ligands, including fibrinogen and apoE-carrying lipoproteins.

OBJECTIVE: To investigate low-density lipoprotein receptor-related protein 1b (LRP1b) expression in human tissues and to identify circulating ligands of LRP1b. METHODS AND RESULTS: Using two independent RT-PCR assays, LRP1b mRNA was detected in human brain, thyroid gland, skeletal muscle, and to a lesser amount in testis but absent in other tissues, including heart, kidney, liver, lung, and placenta. Circulating ligands were purified from human plasma by affinity chromatography using FLAG-tagged recombinant LRP1b ectodomains and identified by mass spectrometry. Using this technique, several potential ligands (fibrinogen, clusterin, vitronectin, histidine rich glycoprotein, serum amyloid P-component, and immunoglobulins) were identified. Direct binding of LRP1b ectodomains to fibrinogen was verified by co-immunoprecipitation. ApoE-carrying lipoproteins were shown to bind to LRP1b ectodomains in a lipoprotein binding assay. Furthermore, binding as well as internalization of very low density lipoproteins by cells expressing an LRP1b minireceptor was demonstrated. DISCUSSION: LRP1b expression in humans appears to be confined to few tissues, which could point out to specialized functions of LRP1b in certain organs. Most of the newly identified LRP1b ligands are well-known factors in blood coagulation and lipoprotein metabolism, suggesting a possible role of LRP1b in atherosclerosis.

Irreversible pan-ErbB tyrosine kinase inhibitor CI-1033 induces caspase-independent apoptosis in colorectal cancer DiFi cell line.

The epidermal growth factor receptor (EGFR) is overexpressed in the majority of colorectal carcinomas and represents a target for therapeutic interventions with signal transduction inhibitors. We investigated the ability of CI-1033 to induce apoptosis and inhibition of proliferation in the colorectal cancer cell lines DiFi and Caco-2, which both express high levels of EGFR. While in Caco-2 cells CI-1033 treatment at a concentration 0.1 microM for 72 hours demonstrated only antiproliferative (53.7 +/- 4.3%) but no pro-apoptotic effects, treatment of DiFi cells resulted in a reduced proliferation rate (31.4 +/- 3.1%) and in apoptosis (44.2 +/- 8.9%). In order to define proteins involved in the regulation of apoptosis, we aimed to determine differences in the proteome profile of both cell lines before and after treatment with CI-1033. Cellular proteins were analyzed by 2-D gel electrophoresis followed by computational image analysis and mass spectrometry. Our data show that DiFi cells differ from Caco-2 cells in nine significantly upregulated proteins, and their potential role in apoptosis is described. We demonstrate that induction of apoptosis was triggered via caspase-independent pathways. Overexpression of leukocyte elastase inhibitor (LEI) and translocation of cathepsin D to the cytosol accompanied by upregulation of other defined proteins resulted in Bax-independent AIF translocation from mitochondria into the nucleus and apoptosis. Definition of these proteins can pave the way for functional studies and contribute to a better understanding of the effects of CI-1033 and the pathways of caspase-independent cell death.

Hyperphosphorylation of histone H2A.X and dephosphorylation of histone H1 subtypes in the course of apoptosis.

Chromatin condensation paralleled by DNA fragmentation is one of the most important nuclear events occurring during apoptosis. Histone modifications, and in particular phosphorylation, have been suggested to affect chromatin function and structure during both cell cycle and cell death. We report here that phosphate incorporation into all H1 subtypes decreased rapidly after induction of apoptosis, evidently causing a strong reduction in phosphorylated forms of main H1 histone subtypes. H1 dephosphorylation is accompanied by chromatin condensation preceding the onset of typical chromatin oligonucleosomal fragmentation, whereas H2A.X hyperphosphorylation is strongly correlated to apoptotic chromatin fragmentation. Using various kinase inhibitors we were able to exclude some of the possible kinases which can be involved directly or indirectly in phosphorylation of histone H2A.X. Neither DNA-dependent protein kinase, protein kinase A, protein kinase G, nor the kinases driven by the mitogen-activated protein kinase (MAP) pathway appear to be responsible for H2A.X phosphorylation. The protein kinase C activator phorbol 12-myristate 13-acetate (PMA), however, markedly reduced the induction of apoptosis in TNFalpha-treated cells with a simultaneous change in the phosphorylation pattern of histone H2A.X. Hyperphosphorylation of H2A.X in apoptotic cells depends indirectly on activation of caspases and nuclear scaffold proteases as shown in zVAD-(OMe)-fmk- or zAPF-cmk-treated cells, whereas the dephosphorylation of H1 subtypes seems to be influenced solely by caspase inhibitors. Together, these results illustrate that H1 dephosphorylation and H2A.X hyperphosphorylation are necessary steps on the apoptotic pathway.

The N-terminally acetylated form of mammalian histone H1(o), but not that of avian histone H5, increases with age.

We report here on the HPCE separation of two chicken H5 histones, which do not show the heterogeneity (Gln/Arg) at residue 15 first found by Greenaway and Murray [Greenaway and Murray (1971) Nat. New Biol. 229, 233-238]. The two subfractions obtained were identified using reversed-phase HPLC, hydrophilic interaction HPLC, Edman degradation, and MALDI-MS analysis. We found that the two H5 subcomponents differ only by an acetylated (designated H5a) and an unacetylated N-terminus (H5b). In contrast to the N-terminally acetylated form of rat kidney histone H1(o), which increased by about 40% with aging of the animal, the corresponding form of chicken H5 did not: the ratio N-terminally acetylated: unacetylated remained constant (30:70) when histone H5 was extracted from erythrocytes of newly hatched chickens and from adult chickens, respectively. The HPCE technique used in this investigation represents a quick and convenient method for analyzing N-terminally acetylated proteins in the presence of unacetylated forms.

Age-dependent deamidation of H1(0) histones in chromatin of mammalian tissues.

The composition of the H1(o) histone subfractions was examined in different rat and mouse tissues. Using reverse-phase HPLC and hydrophilic-interaction liquid chromatography we have found that the relative proportions of all four forms of H1(o) differ from tissue to tissue and from species to species. In principle, we observed an age-dependent increase in the amount of both the N-terminally acetylated (H1(o)a Asn-3 and H1(o)a Asp-3) and the deamidated forms of H1(o) (H1(o)a Asp-3 and H1(o)b Asp-3). Compared with the proportion of N-terminally acetylated H1(o) forms in liver, kidney and brain of rats and mice 20 days of age, we found an increase in these H1(o) subfractions of up to 30% in the corresponding organs of 300-day-old animals. The proportion of deamidated H1(o) forms was 1.6- to 4-fold higher in the livers and 8- to 12-fold higher in the brains of 300-day-old mice and rats, respectively, than in 20-day-old animals. The tissue-specific nature of the ratio of H1(o) subfractions suggests that the different forms of histone H1(o) have specific individual functions. The possible biological significance of age-related accumulation of N-terminal acetylated and deamidated histone H1(o) forms is discussed in the light of our results.

The microheterogeneity of the mammalian H1(0) histone. Evidence for an age-dependent deamidation.

Histone H1(0) is known to consist of two subfractions named H1(0)a and H1(0)b. The present work was performed with the aim of elucidating the nature of these two subfractions. By using reversed-phase high performance liquid chromatography in combination with hydrophilic interaction liquid chromatography, we fractionated human histone H1(0) into even four subfractions. Hydrophilic interaction liquid chromatographic analysis of the peptide fragments obtained after cleavage with cyanogen bromide and digestion with chymotrypsin suggested that the four H1(0) subfractions differ only in their small N-terminal end of the H1(0) molecule (30 residues). Edman degradation of the N-terminal H1(0) peptide fragments and mass spectra analysis have indicated that human histone H1(0) consists of intact histones H1(0) (named H1(0) Asn-3) and deamidated H1(0) forms (H1(0) Asp-3) having an aspartic acid residue at position 3 instead of asparagine. Moreover, both H1(0) Asn-3 and H1(0) Asp-3 are blocked (H1(0)a Asn-3, H1(0)a Asp-3) and unblocked (H1(0)b Asn-3, H1(0)b Asp-3) on their N terminus. Acid-urea gel electrophoretic analysis has shown that the histone subfraction, in the literature originally named H1(0)a, actually consists of a mixture of H1(0)a Asn-3 and H1(0)a Asp-3, whereas H1(0)b consists of H1(0)b Asn-3 and H1(0)b Asp-3. Furthermore, we found that hydrophilic interaction liquid chromatography separates rat and mouse histone H1(0) just like human H1(0) into four subfractions. Hydrophilic interaction liquid chromatographic analysis of brain and liver histone H1(0) from rats of different ages revealed an age-dependent increase of both the N-terminally acetylated and the deamidated forms of H1(0). In addition, we found that the relative proportions of the four forms of H1(0) histones differ from tissue to tissue.

Substrate and sequential site specificity of cytoplasmic histone acetyltransferases of maize and rat liver.

The cytoplasmic B-type histone acetyltransferase was purified to apparent homogeneity from maize embryos. We established a novel protocol for easy large-scale preparation of acetylated core histone species, using preparative acetic acid-urea-Triton PAGE. The pure maize histone acetyltransferase B was highly specific for histone H4 under various assay conditions, modifying H4 up to the di-acetylated isoform. Only non-acetylated H4 isoform was accepted as substrate, whereas mono-acetylated H4 could not be further acetylated. The enzyme selectively acetylated lysines 12 and 5 in a sequential manner. The same results were obtained with a partially purified cytoplasmic histone acetyltransferase of rat liver. Protein sequencing results were supported by immunological characterization of acetylated H4 subspecies with site-specific H4-acetyllysine antibodies.

Application of hydrophilic-interaction liquid chromatography to the separation of phosphorylated H1 histones.

A new two-step high-performance liquid chromatography (HPLC) procedure has been developed to separate modified histone H1 subtypes. Reversed-phase (RP) HPLC followed by hydrophilic-interaction liquid chromatography (HILIC) was used for analytical and semi-preparative scale fractionation of multi-phosphorylated H1 histone subtypes into their non-phosphorylated and distinct phosphorylated forms. The HILIC system utilizes the weak cation-exchange column PolyCAT A and an increasing sodium perchlorate gradient in a methanephosphonic acid-triethylamine buffer (pH 3.0) in the presence of 70% (v/v) acetonitrile. The identity and purity of the individual histone subfractions obtained was assayed by capillary electrophoretic analysis. The results demonstrate that application of the combined RP-HPLC-HILIC procedure to the analysis and isolation of modified H1 histone subtypes provides an innovative and important alternative to traditional separation techniques that will be extremely useful in studying the biological function of histone phosphorylation.

Separation of acetylated core histones by hydrophilic-interaction liquid chromatography.

Hydrophilic-interaction liquid chromatography (HILIC) has recently been introduced as a highly efficient chromatographic technique for the separation of a wide range of solutes. The present work was performed with the aim of evaluating the potential utility of HILIC for the separation of postranslationally acetylated histones. The protein fractionations were generally achieved by using a weak cation-exchange column and an increasing sodium perchlorate gradient system in the presence of acetonitrile (70%, v/v) at pH 3.0. In combination with reversed-phase high-performance liquid chromatography (RP-HPLC) we have successfully separated various H2A variants and posttranslationally acetylated forms of H2A variants and H4 proteins in very pure form. An unambiguous assignment of the histone fractions obtained was performed using high-performance capillary and acid-urea-Triton gel electrophoresis. Our results demonstrate that for the analysis and isolation of modified core histone variants HILIC provides a new and important alternative to traditional separation techniques and will be useful in studying the biological function of histone acetylation.

Effect of buffer composition on the migration order and separation of histone H1 subtypes.

The effects of different buffer concentrations and compositions on the elution order and separation of H1 histone subtypes and their phosphorylated modifications isolated from several species was studied using high-performance capillary electrophoresis (CE). Various cations and anions were tested in an untreated silica capillary and low pH buffers, in the presence of the dynamic coating agent hydroxypropylmethyl cellulose. It was found that the cations and anions of buffers have a remarkable influence on both the efficiency and the selectivity of protein separations. A triethylammonium methanephosphonate system proved efficacious for the separation of rat histone subtype H1c from H1e and a perchlorate/triethylammonium phosphate system for the analysis of chicken and mouse linker histones. CE provides an attractive alternative to high-performance liquid chromatography and conventional gel electrophoresis.

Application of high-performance capillary electrophoresis to the analysis of H1 histones.

High-performance capillary electrophoresis for the separation of rat testis H1 histone variants and their phosphorylated modifications is described. The influence of buffer pH, hydroxypropylmethyl cellulose, and buffer concentration has been investigated. Under optimized conditions (500 mM phosphate buffer, pH 2, 0.03% hydroxypropylmethyl cellulose) using an uncoated capillary, eight H1 histone subfractions, including two H1(0) histones and H1t and their phosphorylated modifications, are resolved. Application of capillary electrophoresis to the separation of H1 histones provides an important new alternative to high-performance liquid chromatography (HPLC) and traditional gel electrophoresis.

Separation of phosphorylated histone H1 variants by high-performance capillary electrophoresis.

High-performance capillary electrophoresis (HPCE) was used to separate successfully distinct phosphorylated derivatives of individual histone H1 variants. With an untreated capillary (50 cm x 75 microns I.D.) the electrophoresis was performed in about 15 min. Inconvenient interactions of these highly basic proteins with the capillary wall were eliminated by using 0.1 M sodium phosphate buffer (pH 2.0) containing 0.03% hydroxypropylmethylcellulose. Under these experimental conditions the histone H1 variants H1b and H1c obtained from mitotic enriched NIH 3T3 fibroblasts and isolated by reversed-phase high-performance liquid chromatography were clearly separated in their non-phosphorylated and different phosphorylated forms. This result was confirmed by acid-urea gel electrophoresis, comparison with non-phosphorylated histones H1b and H1c, isolated from quiescent NIH 3T3 cells, and incubation of multi-phosphorylated histone H1b with alkaline phosphatase and subsequent acid-urea and capillary electrophoresis. The results illustrate that the application of HPCE to the analysis of histone modifications provides a new alternative to traditional gel electrophoresis.