The small molecule 2-phenylethynesulfonamide induces covalent modification of p53.

p53 is a tumor suppressor protein which is either lost or inactivated in a large majority of tumors. The small molecule 2-phenylethynesulfonamide (PES) was originally identified as the inhibitor of p53 effects on the mitochondrial death pathway. In this report we demonstrate that p53 protein from PES-treated cells was detected in reduced mobility bands between molecular weights 95-220 kDa. Resolution of p53 aggregates on urea gel was unable to reduce the high molecular weight p53 aggregates, which were shown to be primarily located in the nucleus. Therefore, our data suggest that PES exerts its effects through covalent cross-linking and nuclear retention of p53.

Identification of a Hematopoietic Cell Dedifferentiation-Inducing Factor.

It has long been realized that hematopoietic cells may have the capacity to trans-differentiate into non-lymphohematopoietic cells under specific conditions. However, the mechanisms and the factors for hematopoietic cell trans-differentiation remain unknown. In an in vitro culture system, we found that using a conditioned medium from proliferating fibroblasts can induce a subset of hematopoietic cells to become adherent fibroblast-like cells (FLCs). FLCs are not fibroblasts nor other mesenchymal stromal cells, based on their expression of type-1 collagen, and other stromal cell marker genes. To identify the active factors in the conditioned medium, we cultured fibroblasts in a serum-free medium and collected it for further purification. Using the fractions from filter devices of different molecular weight cut-offs, and ammonium sulfate precipitation collected from the medium, we found the active fraction is a protein. We then purified this fraction by using fast protein liquid chromatography (FPLC) and identified it by mass spectrometer as macrophage colony-stimulating factor (M-CSF). The mechanisms of M-CSF-inducing trans-differentiation of hematopoietic cells seem to involve a tyrosine kinase signalling pathway and its known receptor. The FLCs express a number of stem cell markers including SSEA-1 and -3, OCT3/4, NANOG, and SOX2. Spontaneous and induced differentiation experiments confirmed that FLCs can be further differentiated into cell types of three germ layers. These data indicate that hematopoietic cells can be induced by M-CSF to dedifferentiate to multipotent stem cells. This study also provides a simple method to generate multipotent stem cells for clinical applications.

Regulation of ceramide generation during macrophage apoptosis by ASMase and de novo synthesis.

The survival of macrophages depends on the presence of specific cytokines that activate survival signaling events, as well as suppressing formation of apoptosis-inducing pathways. We have previously shown that macrophages deprived of macrophage colony stimulating factor (M-CSF) produce ceramide that contributes to apoptosis of these cells, a pathway that is suppressed by exposure to oxidized LDL. In this study we have examined macrophages derived from mice lacking acid sphingomyelinase (ASMase) to ask whether these events are altered due to the impaired ability of these cells to break down sphingomyelin and produce ceramide. We found that these cells do survive better than cells from wild type mice, but they still undergo cell death and some ceramide is formed. We show that the ceramide is being produced by a de novo synthetic pathway. Therefore, ceramide production in M-CSF-deprived macrophages arises from a combination of ASMase activity and de novo synthesis.

An essential role for MCL-1 in ATR-mediated CHK1 phosphorylation.

Here we report a novel role for myeloid cell leukemia 1 (Mcl-1), a Bcl-2 family member, in regulating phosphorylation and activation of DNA damage checkpoint kinase, Chk1. Increased expression of nuclear Mcl-1 and/or a previously reported short nuclear form of Mcl-1, snMcl-1, was observed in response to treatment with low concentrations of etoposide or low doses of UV irradiation. We showed that after etoposide treatment, Mcl-1 could coimmunoprecipitate with the regulatory kinase, Chk1. Chk1 is a known regulator of DNA damage response, and its phosphorylation is associated with activation of the kinase. Transient transfection with Mcl-1 resulted in an increase in the expression of phospho-Ser345 Chk1, in the absence of any evidence of DNA damage, and accumulation of cells in G2. Importantly, knockdown of Mcl-1 expression abolished Chk1 phosphorylation in response to DNA damage. Mcl-1 could induce Chk1 phosphorylation in ATM-negative (ataxia telangectasia mutated) cells, but this response was lost in ATR (AT mutated and Rad3 related)-defective cells. Low levels of UV treatment also caused transient increases in Mcl-1 levels and an ATR-dependent phosphorylation of Chk1. Together, our results strongly support an essential regulatory role for Mcl-1, perhaps acting as an adaptor protein, in controlling the ATR-mediated regulation of Chk1 phosphorylation.

A proteolytic fragment of Mcl-1 exhibits nuclear localization and regulates cell growth by interaction with Cdk1.

Mcl-1 (myeloid cell leukaemia-1) is a Bcl-2 family member with short-term pro-survival functions but whose other functions, demonstrated by embryonic lethality of knockout mice, do not involve apoptosis. In the present study, we show a cell-cycle-regulatory role of Mcl-1 involving a shortened form of the Mcl-1 polypeptide, primarily localized to the nucleus, which we call snMcl-1. snMcl-1 interacts with the cell-cycle-regulatory protein Cdk1 (cyclin-dependent kinase 1; also known as cdc2) in the nucleus, and Cdk1 bound to snMcl-1 was found to have a lower kinase activity. The interaction with Cdk1 occurs in the absence of its cyclin partners and is enhanced on treatment of cells with G2/M blocking agents, but not by G1/S blocking. The snMcl-1 polypeptide is present during S and G2 phases and is negligible in G1. Overexpression of human Mcl-1 in a murine myeloid progenitor cell line resulted in a lower rate of proliferation. Furthermore, Mcl-1-overexpressing cells had lower total Cdk1 kinase activity compared with parental cells, in both anti-Cdk1 and anti-cyclin B1 immunoprecipitates. The latter results suggest that binding to snMcl-1 alters the ability of Cdk1 to bind its conventional partner, cyclin B1. Given the important role of Cdk1 in progression through G2 and M phases, it is probable that the inhibition of Cdk1 activity accounts for the inhibitory effect of Mcl-1 on cell growth.

Identification of a novel phosphorylation site, Ser-170, as a regulator of bad pro-apoptotic activity.

Bad is a pro-apoptotic member of the Bcl-2 family of proteins that is thought to exert a death-promoting effect by heterodimerization with Bcl-X(L), nullifying its anti-apoptotic activity. Growth factors may promote cell survival at least partially through phosphorylation of Bad at one or more of Ser-112, -136, or -155. Our previous work showed that Bad is also phosphorylated in response to cytokines at another site, which we now identify as Ser-170. The functional role of this novel phosphorylation site was assessed by site-directed mutagenesis and analysis of the pro-apoptotic function of Bad in transiently transfected HEK293 and COS-7 cells or by stable expression in the cytokine-dependent cell line, MC/9. In general, mutation of Ser-170 to Ala results in a protein with increased ability to induce apoptosis, similar to the S112A mutant. Mutation of Ser-170 to Asp, mimicking a constitutively phosphorylated site, results in a protein that is virtually unable to induce apoptosis. Similarly, the S112A/S170D double mutant does not cause apoptosis in HEK293 and MC/9 cell lines. These data strongly suggest that phosphorylation of Bad at Ser-170 is a critical event in blocking the pro-apoptotic activity of Bad.