Position-dependent nuclear accumulation of the retinoblastoma (RB) protein during in vitro myogenesis

Retinoblastoma protein and CCAAT/enhancer-binding protein beta are required for 1,25-dihydroxyvitamin D3-induced monocytic differentiation of HL60 cells

Derivatives of vitamin D (deltanoids) are well known to have the ability to induce differentiation of a variety of malignant cells, including human leukemia cells, but the signaling pathways that lead to such an outcome are unclear. In this study we investigated the role of the retinoblastoma protein (pRb) and the CCAAT/enhancer-binding protein (C/EBP) beta in 1,25-dihydroxyvitamin D(3) (1,25D(3))-induced monocytic differentiation of human leukemia HL60 cells. It was found that in this system, pRb is up-regulated within 12 h of exposure to the inducer, and the kinetics of its increase parallel the appearance of the early markers of differentiation, CD14 and monocyte-specific esterase. The increase in pRb expression was accompanied by a similar increase in C/EBPbeta protein, and these two proteins coimmunoprecipitated, suggesting formation of a complex. Oligonucleotides antisense to pRb or C/EBPbeta (but not to C/EBPalpha) or containing the C/EBP-binding sequence ("decoys"), all inhibited 1,25D(3)-induced differentiation. Inhibition of signaling by vitamin D receptor or by mitogen-activated protein kinase (MAPK) extracellular signal-regulated kinase and c-Jun-NH(2)-terminal kinase pathways using pharmacological inhibitors ZK159222, PD98059, or SP600125, respectively, inhibited pRb and C/EBPbeta expression and differentiation in a coordinate manner. In contrast, inhibition of the p38MAPK pathway by SB202190 potentiated differentiation and the up-regulation of pRb and C/EBPbeta. We suggest that 1,25D(3) may signal monocytic differentiation of HL60 cells in a vitamin D receptor-dependent manner that includes activation of extracellular signal-regulated kinase and c-Jun-NH(2)-terminal kinase MAPK pathways, which then up-regulate pRb and C/EBPbeta expression and in turn initiate the differentiation process.

Expression of the murine CB2 cannabinoid receptor using a recombinant Semliki Forest virus

A recombinant Semliki Forest virus (SFV) RNA construct, SFV1-mCB(2) RNA, was employed for the high-level expression of the murine CB(2) (mCB(2)) cannabinoid receptor in baby hamster kidney cells. Biosynthetic radiolabel incorporation studies in concert with urea-sodium dodecylsulfate-polyacrylamide gel electrophoresis (urea-SDS-PAGE) and western immunoblotting revealed that two major proteins of approximately 26 and 40kDa were produced by the construct. The 40kDa product, but not the 26kDa product, was glycosylated as determined by 2-deoxy-D-glucose incorporation and peptide-N-glycosidase F digestion analysis. Assessment of [3H]CP55940 ([3H]-(-)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-trans-4-(3-hydroxypropyl)cyclohexanol) binding data for membranes of cells transfected with SFV1-mCB(2) RNA indicated a K(d) of 0.35+/-0.04nM and a B(max) of 24.4+/-2.7pmol/mg. A rank order of binding affinities for cannabinoids, which paralleled that reported for native mCB(2) receptors, was observed. The CB(2) receptor-specific antagonist SR144528 (N-[(1S)-endo-1,3,3-trimethyl bicyclo[2.2.1]heptan-2-yl]-5-(4-chloro-3-methylphenyl)-1-(4-methylbenzyl)-pyrazole-3-carboxamide) blocked binding of [3H]CP55940, while the CB(1) receptor-specific antagonist SR141716A [N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide hydrochloride] had a minimal effect. These results indicate that the recombinant receptor expressed from SFV1-mCB(2) RNA exhibits properties, including ligand binding features, that are consistent with those for the native mCB(2) receptor. However, the presence of both 26 and 40kDa receptor species is consistent with alternative translation from two AUG start sites using the SFV1-mCB(2) RNA expression system.

Myonuclear domains in muscle adaptation and disease

Adult skeletal muscle fibers are among the few cell types that are truly multinucleated. Recently, evidence has accumulated supporting a role for the modulation of myonuclear number during muscle remodeling in response to injury, adaptation, and disease. These studies have demonstrated that muscle hypertrophy is associated with, and is dependent on, the addition of newly formed myonuclei via the fusion of myogenic cells to the adult myofiber, whereas muscle atrophy and disease appear to be associated with the loss of myonuclei, possibly through apoptotic-like mechanisms. Moreover, these studies also have demonstrated that myonuclear domain size, i. e., the amount of cytoplasm per myonucleus, is unchanged following the acute phase of hypertrophy but is reduced following atrophy. Together these data demonstrate that modulation of myonuclear number or myonuclear domain size (or both) is a mechanism contributing to the remodeling of adult skeletal muscle in response to alterations in the level of normal neuromuscular activity.

BTG1: a triiodothyronine target involved in the myogenic influence of the hormone

The product of the B-cell translocation gene 1 (BTG1), a member of an antiproliferative protein family including Tis-21/PC3 and Tob, is thought to play an important role in the regulation of cell cycle progression. We have shown in a previous work that triiodothyronine (T3) stimulates quail myoblast differentiation, partly through a cAMP-dependent mechanism involved in the stimulation of cell cycle withdrawal. Furthermore, we found that T3 or 8-Br-cAMP increases BTG1 nuclear accumulation in confluent myoblast cultures. In this study, we report that BTG1 is essentially expressed at cell confluence and in differentiated myotubes. Whereas neither T3 nor cAMP exerted a direct transcriptional control upon BTG1 expression, we found that AP-1 activity, a crucial target involved in the triiodothyronine myogenic influence, repressed BTG1 expression, thus probably explaining the low BTG1 expression level in proliferating myoblasts. In transient transfection studies, we demonstrated that an AP-1-like sequence located in the BTG1 promoter was involved in this negative regulation. Our present data also bring evidence that the stimulation of BTG1 nuclear accumulation by T3 or 8-Br-cAMP probably results from an increased nuclear import or retention in the nucleus. Lastly, BTG1 overexpression in quail myoblasts mimicked the T3 or 8-Br-cAMP myogenic influence: (i) inhibition of myoblast proliferation due to an increased rate of myoblast withdrawal from the cell cycle; and (ii) stimulation of terminal differentiation. These data suggest that BTG1 is probably involved in T3 and cAMP myogenic influences. In conclusion, BTG1 is a T3 target involved in the regulation of myoblast differentiation.

RB-reconstituted human retinoblastoma cells form RB-positive intraocular and intracerebral but not subcutaneous tumors in SCID mice

WERI-Rb27 human retinoblastoma cells were reconstituted with an intact RB gene by retrovirus-mediated gene transfer, in order to study the phenotypic effects of the protein in vitro and in vivo. Extensive morphological changes were observed, dominated by the formation of multinucleated giant cells. Six weeks after retroviral infection, the giant cells began to die and small cells emerged, resembling the parental non-reconstituted line. They expressed RB and continued to grow, although they showed an increased sensitivity to serum starvation. The original RB-negative cells grew progressively after subcutaneous inoculation into SCID mice, whereas the reconstituted cells failed to grow. RB-positive cells grew progressively in the corpus vitreum of the eye and in the brain, however. The RB-reconstituted cells grew more slowly and were less invasive than the parental cells and cells infected with a firefly luciferase (LUX) gene carrying retrovirus, used as controls. RB-reconstituted cells re-explanted from the intraocular and intracranial tumors continued to express full-length RB protein. RBeye2, an RB-positive cell line established from an eye tumor, was still unable to grow subcutaneously. The reduced tumorigenicity of the RB-reconstituted cells in the subcutaneous space may be due to the influence of locally acting growth-controlling signals or the absence of microenvironment-specific trophic factors. Alternatively, it may reflect the action of residual immune effectors in the SCID mice. If this is the case, these would have to be more effective at the subcutaneous site than in the eye or brain.

Disruption of retinoblastoma protein function by coexpression of its C pocket fragment

The growth suppression function of the retinoblastoma protein (RB) is mediated by its interaction with a variety of cellular proteins. RB contains at least two protein-binding pockets: the large A/B pocket, which interacts with E2F and the D-type cyclins, and the C pocket, which interacts with the nuclear c-Abl tyrosine kinase. The large A/B pocket and the C pocket are shown here to be functionally distinct and can be occupied simultaneously. A complex containing E2F, RB, and c-Abl is detected in vivo and can be assembled in vitro. We propose that the biological activity of RB not only depends on the inhibition of its targets but also on its ability to properly assemble specific protein complexes. Consistent with this hypothesis, a fragment of RB, SE delta, containing only the C pocket is shown to act as a dominant-negative inhibitor of RB function. SE delta does not have growth inhibitory activity of its own. When coexpressed with full-length RB, SE delta does not disrupt the RB-E2F or RB-D2 complexes nor does it affect the expression, phosphorylation, or nuclear tethering of the full-length RB. SE delta does compete with RB for binding to c-Abl and is fully capable of inhibiting the c-Abl tyrosine kinase. Thus, SE delta can inactivate RB while maintaining the inhibition of E2F and c-Abl. These results suggest that the inhibition of RB-binding proteins is not sufficient to suppress cell growth and that the assembly of RB-mediated protein complexes is also important for the promotion of cell-cycle arrest.