Crosstalk Between MDM2 and Akt Signaling Pathway in Oncogenesis.

MDM2, the human homologue of the Mouse Double Minute 2 gene product, has been shown to be over-expressed in many cancers and to induce tumorigenesis. The role of MDM2 in oncogenesis was thought to be p53 dependent. However recent years have shown MDM2 to be a key player in a complex network of interactions that affect cell cycle, apoptosis, and tumorigenesis in a p53 independent manner. Here we report a novel p53 independent role for the multidimensional protein MDM2; its ability to induce phosphorylation of Akt at serine 473 residue. Transient and stable over-expression of MDM2 in cultured cell lines induces Akt phosphorylation. Silencing of MDM2 expression in cancer cells that over express MDM2 inhibits Akt phosphorylation suggesting endogenous MDM2 participates in Akt phosphorylation. Stable up-regulation of MDM2 expression reduced sensitivity of cells to chemotherapeutic drugs such as Etoposide, Carboplatin or Paclitaxel. The domain of MDM2 responsible for drug resistance overlaps with the Akt phosphorylation domain. An inhibitor of Akt phosphorylation abrogated MDM2-mediated Akt phosphorylation and reduction of Etoposide sensitivity indicating that MDM2 reduces Etoposide sensitivity of cancer cells by activating the Akt phosphorylation. A PI-3 kinase inhibitor Wortmannin inhibits the ability of MDM2 to induce Akt phosphorylation and silencing of Rictor, a known kinase of Akt, does not hamper the ability of MDM2 to induce phosphorylation of Akt. MDM2-mediated Akt phosphorylation does not require p53, and the p53-interaction domain of MDM2 is dispensable for Akt phosphorylation. The presence of MDM2 enhances the Insulin like Growth Factor 1 mediated activation of Akt. Further cells harboring MDM2 show enhanced Interleukin 8( IL8) activation, which could be a possible mechanism of Akt activation. Downstream of Akt activation we show increased events that have been correlated with Akt activation like increased Bcl-2 levels increased processing of NF-κB2, and GSK3α/β phosphorylation among others. Our observation reveals a novel signaling function of MDM2 important for regulation of cell growth and chemotherapeutic sensitivity through Akt phosphorylation

The human RecQ helicases BLM and RECQL4 cooperate to preserve genome stability

Bacteria and yeast possess one RecQ helicase homolog whereas humans contain five RecQ helicases, all of which are important in preserving genome stability. Three of these, BLM, WRN and RECQL4, are mutated in human diseases manifesting in premature aging and cancer. We are interested in determining to which extent these RecQ helicases function cooperatively. Here, we report a novel physical and functional interaction between BLM and RECQL4. Both BLM and RECQL4 interact in vivo and in vitro. We have mapped the BLM interacting site to the N-terminus of RECQL4, comprising amino acids 361-478, and the region of BLM encompassing amino acids 1-902 interacts with RECQL4. RECQL4 specifically stimulates BLM helicase activity on DNA fork substrates in vitro. The in vivo interaction between RECQL4 and BLM is enhanced during the S-phase of the cell cycle, and after treatment with ionizing radiation. The retention of RECQL4 at DNA double-strand breaks is shortened in BLM-deficient cells. Further, depletion of RECQL4 in BLM-deficient cells leads to reduced proliferative capacity and an increased frequency of sister chromatid exchanges. Together, our results suggest that BLM and RECQL4 have coordinated activities that promote genome stabilit

RECQL5 cooperates with Topoisomerase II alpha in DNA decatenation and cell cycle progression

DNA decatenation mediated by Topoisomerase II is required to separate the interlinked sister chromatids post-replication. SGS1, a yeast homolog of the human RecQ family of helicases interacts with Topoisomerase II and plays a role in chromosome segregation, but this functional interaction has yet to be identified in higher organisms. Here, we report a physical and functional interaction of Topoisomerase IIα with RECQL5, one of five mammalian RecQ helicases, during DNA replication. Direct interaction of RECQL5 with Topoisomerase IIα stimulates the decatenation activity of Topoisomerase IIα. Consistent with these observations, RECQL5 co-localizes with Topoisomerase IIα during S-phase of the cell cycle. Moreover, cells with stable depletions of RECQL5 display a slow proliferation rate, a G2/M cell cycle arrest and late S-phase cycling defects. Metaphase spreads generated from RECQL5-depleted cells exhibit undercondensed and entangled chromosomes. Further, RECQL5-depleted cells activate a G2/M checkpoint and undergo apoptosis. These phenotypes are similar to those observed when Topoisomerase II catalytic activity is inhibited. These results reveal an important role for RECQL5 in the maintenance of genomic stability and a new insight into the decatenation process

Stereochemically Distinct Cyclotetrasiloxanes Containing 3‑Pyridyl Moieties and Their Functional Coordination Polymers

Synthesis of new cyclotetrasiloxane scaffolds containing peripherally functionalized 3-pyridyl moieties, [MeSiO­(CHCH³Py)]₄ (L¹) and [MeSiO­(CH₂CH₂³Py)]₄ (L²), and their reactivity studies with certain d¹⁰ metal ions are reported. The ligand L¹ is obtained by the Heck-coupling reaction of tetramethyl tetravinyl tetrasiloxane (D₄^vi) and 3-bromopyridine in the presence of the Pd(0) catalysts. The as-synthesized ligand L¹ shows the presence of three stereoisomers, <i>cis–trans–cis</i> (L^1A), <i>cis–cis–trans</i> (L^1B), and all-<i>trans</i> (L^1C), which are quantitatively separated by column chromatography. Subsequent reduction of L^1A, L^1B, and L^1C with triethylsilane in the presence of catalytic amounts of Pd/C leads to the formation of the ligands L^2A, L^2B, and L^2C with retention of stereochemistry due to the precursor moieties. Treatment of ZnI₂ with L^1A gives a one-dimensional coordination framework [(L^1A)₄(ZnI₂)₂]_∞, <b>1</b>. These 1D-chains are further connected by π–π stacking interactions between the pyridyl groups of the adjacent chains leading to the formation of a three-dimensional network with the topology of a PtS net. The reaction of silver nitrate with ligand L^1B gives a chain like one-dimensional cationic coordination polymer {[(L^1B)₄Ag₂]·2NO₃·H₂O·CH₃OH }_∞, <b>2</b>, consisting of two different kinds of 32-membered macrocycles. Treatment of the all-<i>trans</i> ligand L^2C with copper­(I) iodide salt results in the formation of a cubane-type Cu₄I₄ cluster MOF [(L^2C)₄Cu₄I₄]_∞, <b>3</b>, in a two-dimensional 4-connected uninodal sql/Shubnikov tetragonal plane net topology represented by the Schläfli symbol {4⁴.6²}. This MOF displays a thermochromic luminescence behavior due to Cu₄I₄ clusters showing an orange emission at 298 K and a blue emission at 77 K

The human RecQ helicases BLM and RECQL4 cooperate to preserve genome stability

Bacteria and yeast possess one RecQ helicase homolog whereas humans contain five RecQ helicases, all of which are important in preserving genome stability. Three of these, BLM, WRN and RECQL4, are mutated in human diseases manifesting in premature aging and cancer. We are interested in determining to which extent these RecQ helicases function cooperatively. Here, we report a novel physical and functional interaction between BLM and RECQL4. Both BLM and RECQL4 interact in vivo and in vitro. We have mapped the BLM interacting site to the N-terminus of RECQL4, comprising amino acids 361-478, and the region of BLM encompassing amino acids 1-902 interacts with RECQL4. RECQL4 specifically stimulates BLM helicase activity on DNA fork substrates in vitro. The in vivo interaction between RECQL4 and BLM is enhanced during the S-phase of the cell cycle, and after treatment with ionizing radiation. The retention of RECQL4 at DNA double-strand breaks is shortened in BLM-deficient cells. Further, depletion of RECQL4 in BLM-deficient cells leads to reduced proliferative capacity and an increased frequency of sister chromatid exchanges. Together, our results suggest that BLM and RECQL4 have coordinated activities that promote genome stabilit

Thermochromic and Mechanochromic Luminescence Umpolung in Isostructural Metal–Organic Frameworks Based on Cu₆I₆ Clusters

Two isostructural metal–organic framework (MOF) materials, namely, {[MeSi­(³Py)₃]₆(Cu₆I₆)}_<i>n</i> (<b>1</b>) and {[ MeSi­(³Qy)₃]₆(Cu₆I₆)}_<i>n</i> (<b>2</b>), featuring Cu₆I₆ clusters were synthesized from tridentate arylsilane ligands of the type MeSi­(³Py)₃ (³Py = 3-pyridyl) and MeSi­(³Qy)₃ (³Qy = 3-quinolyl), respectively. While the MOF <b>1</b> displays the usual thermochromism associated with traditional Cu₄I₄Py₄ clusters, the MOF <b>2</b> shows ³XLCT/³MLCT emission due to the Cu₆I₆ cluster core at both 298 and 77 K, albeit with some marginal variations in its emission wavelengths. Interestingly, an unusual reversal in the mechanochromic luminescent behavior was observed for these isostructural MOFs at 298 K wherein a pronounced blue-shifted high energy emission for <b>1</b> (from orange to yellowish-orange) and a red-shifted low-energy emission for <b>2</b> (from green to orange) were obtained upon grinding these samples. This is primarily due to the variations in their cuprophilic interactions as <b>1</b> displays shorter Cu···Cu distances (2.745(1) Å) in comparison with those present in <b>2</b> (3.148(0) Å). As a result, the ground sample of <b>2</b> exhibits a prominent red shift in luminescence owing to the reduction of its Cu···Cu distances to an unknown value closer to the sum of van der Waals radii between two Cu­(I) atoms (2.80 Å). However, the blue-shifted emission in <b>1</b> is presumably attributed to the rise in its lowest unoccupied molecular orbital energy levels caused by changes in the secondary packing forces. Furthermore, the absorption and emission characteristics of <b>1</b> and <b>2</b> were substantiated by time-dependent density functional theory calculations on their discrete-model compounds. In addition, the syntheses, reactivity studies, and photophysical properties of two one-dimensional MOFs, namely, {[MeSi­(³Qy)₃]₂(Cu₂I₂)}_<i>n</i> (<b>3</b>) and {[MeSi­(³Qy)₃]­(CuI)}_<i>n</i> (<b>4</b>), having dimeric Cu₂I₂ and monomeric CuI moieties, respectively, were examined