Recombinant, Replication-Defective Adenovirus Gene Transfer Vectors Induce Cell Cycle Dysregulation and Inappropriate Expression of Cyclin Proteins

First-generation adenovirus (Ad) vectors that had been rendered replication defective by removal of the E1 region of the viral genome (ΔE1) or lacking the Ad E3 region in addition to E1 sequences (ΔE1ΔE3) induced G₂ cell cycle arrest and inhibited traverse across G₁/S in primary and immortalized human bronchial epithelial cells. Cell cycle arrest was independent of the cDNA contained in the expression cassette and was associated with the inappropriate expression and increase in cyclin A, cyclin B1, cyclin D, and cyclin-dependent kinase p34^cdc2 protein levels. In some instances, infection with ΔE1 or ΔE1ΔE3 Ad vectors produced aneuploid DNA histogram patterns and induced polyploidization as a result of successive rounds of cell division without mitosis. Cell cycle arrest was absent in cells infected with a second-generation ΔE1Ad vector in which all of the early region E4 except the sixth open reading frame was also deleted. Consequently, E4 viral gene products present in ΔE1 or ΔE1ΔE3 Ad vectors induce G₂ growth arrest, which may pose new and unintended consequences for human gene transfer and gene therapy.     

E2F mediates sustained G2 arrest and down-regulation of Stathmin and AIM-1 expression in response to genotoxic stress

Exposure of cells to genotoxic agents results in activation of checkpoint pathways leading to cell cycle arrest. These arrest pathways allow repair of damaged DNA before its replication and segregation, thus preventing accumulation of mutations. The tumor suppressor retinoblastoma (RB) is required for the G(1)/S checkpoint function. In addition, regulation of the G(2) checkpoint by the tumor suppressor p53 is RB-dependent. However, the molecular mechanism underlying the involvement of RB and its related proteins p107 and p130 in the G(2) checkpoint is not fully understood. We show here that sustained G(2)/M arrest induced by the genotoxic agent doxorubicin is E2F-dependent and involves a decrease in expression of two mitotic regulators, Stathmin and AIM-1. Abrogation of E2F function by dominant negative E2F abolishes the doxorubicin-induced down-regulation of Stathmin and AIM-1 and leads to premature exit from G(2). Expression of the E7 papilloma virus protein, which dissociates complexes containing E2F and RB family members, also prevents the down-regulation of these mitotic genes and leads to premature exit from G(2) after genotoxic stress. Furthermore, genotoxic stress increases the levels of nuclear E2F-4 and p130 as well as their in vivo binding to the Stathmin promoter. Thus, functional complexes containing E2F and RB family members appear to be essential for repressing expression of critical mitotic regulators and maintaining the G(2)/M checkpoint.     

Reevesioside A,a Cardenolide Glycoside,Induces Anticancer Activity against Human Hormone-Refractory Prostate Cancers through Suppression of c-myc Expression and Induction of G1 Arrest of the Cell Cycle

In the past decade, there has been a profound increase in the number of studies revealing that cardenolide glycosides display inhibitory activity on the growth of human cancer cells. The use of potential cardenolide glycosides may be a worthwhile approach in anticancer research. Reevesioside A, a cardenolide glycoside isolated from the root of Reevesia formosana, displayed potent anti-proliferative activity against human hormone-refractory prostate cancers. A good correlation (r² = 0.98) between the expression of Na⁺/K⁺-ATPase α₃ subunit and anti-proliferative activity suggested the critical role of the α₃ subunit. Reevesioside A induced G1 arrest of the cell cycle and subsequent apoptosis in a thymidine block-mediated synchronization model. The data were supported by the down-regulation of several related cell cycle regulators, including cyclin D1, cyclin E and CDC25A. Reevesioside A also caused a profound decrease of RB phosphorylation, leading to an increased association between RB and E2F1 and the subsequent suppression of E2F1 activity. The protein and mRNA levels of c-myc, which can activate expression of many downstream cell cycle regulators, were dramatically inhibited by reevesioside A. Transient transfection of c-myc inhibited the down-regulation of both cyclin D1 and cyclin E protein expression to reevesioside A action, suggesting that c-myc functioned as an upstream regulator. Flow cytometric analysis of JC-1 staining demonstrated that reevesioside A also induced the significant loss of mitochondrial membrane potential. In summary, the data suggest that reevesioside A inhibits c-myc expression and down-regulates the expression of CDC25A, cyclin D1 and cyclin E, leading to a profound decrease of RB phosphorylation. G1 arrest is, therefore, induced through E2F1 suppression. Consequently, reevesioside A causes mitochondrial damage and an ultimate apoptosis in human hormone-refractory prostate cancer cells.     

Complexity of the mechanisms of initiation and maintenance of DNA damage-induced G2-phase arrest and subsequent G1-phase arrest: TP53-dependent and TP53-independent roles

Through a detailed study of cell cycle progression, protein expression, and kinase activity in gamma-irradiated synchronized cultures of human skin fibroblasts, distinct mechanisms of initiation and maintenance of G2-phase and subsequent G1-phase arrests have been elucidated. Normal and E6-expressing fibroblasts were used to examine the role of TP53 in these processes. While G2 arrest is correlated with decreased cyclin B1/CDC2 kinase activity, the mechanisms associated with initiation and maintenance of the arrest are quite different. Initiation of the transient arrest is TP53-independent and is due to inhibitory phosphorylation of CDC2 at Tyr15. Maintenance of the G2 arrest is dependent on TP53 and is due to decreased levels of cyclin B1 mRNA and a corresponding decline in cyclin B1 protein level. After transiently arresting in G2 phase, normal cells chronically arrest in the subsequent G1 phase while E6-expressing cells continue to cycle. The initiation of this TP53-dependent G1-phase arrest occurs despite the presence of substantial levels of cyclin D1/CDK4 and cyclin E/CDK2 kinase activities, hyperphosphoryated RB, and active E2F1. CDKN1A (also known as p21(WAF1/CIP1)) levels remain elevated during this period. Furthermore, CDKN1A-dependent inhibition of PCNA activity does not appear to be the mechanism for this early G1 arrest. Thus the inhibition of entry of irradiated cells into S phase does not appear to be related to DNA-bound PCNA complexed to CDKN1A. The mechanism of chronic G1 arrest involves the down-regulation of specific proteins with a resultant loss of cyclin E/CDK2 kinase activity.     

Loss of DNMT1o Disrupts Imprinted X Chromosome Inactivation and Accentuates Placental Defects in Females

The maintenance of key germline derived DNA methylation patterns during preimplantation development depends on stores of DNA cytosine methyltransferase-1o (DNMT1o) provided by the oocyte. Dnmt1o^mat−/− mouse embryos born to Dnmt1^Δ1o/Δ1o female mice lack DNMT1o protein and have disrupted genomic imprinting and associated phenotypic abnormalities. Here, we describe additional female-specific morphological abnormalities and DNA hypomethylation defects outside imprinted loci, restricted to extraembryonic tissue. Compared to male offspring, the placentae of female offspring of Dnmt1^Δ1o/Δ1o mothers displayed a higher incidence of genic and intergenic hypomethylation and more frequent and extreme placental dysmorphology. The majority of the affected loci were concentrated on the X chromosome and associated with aberrant biallelic expression, indicating that imprinted X-inactivation was perturbed. Hypomethylation of a key regulatory region of Xite within the X-inactivation center was present in female blastocysts shortly after the absence of methylation maintenance by DNMT1o at the 8-cell stage. The female preponderance of placental DNA hypomethylation associated with maternal DNMT1o deficiency provides evidence of additional roles beyond the maintenance of genomic imprints for DNA methylation events in the preimplantation embryo, including a role in imprinted X chromosome inactivation.     

Conversion of Androgen Receptor Signaling From a Growth Suppressor in Normal Prostate Epithelial Cells to an Oncogene in Prostate Cancer Cells Involves a Gain of Function in c-Myc Regulation

In normal prostate, androgen-dependent androgen receptor (AR) signaling within prostate stromal cells induces their secretion of paracrine factors, termed “andromedins” which stimulate growth of the epithelial cells. The present studies demonstrate that androgen-dependent andromedin-driven growth stimulation is counter-balanced by androgen-induced AR signaling within normal adult prostate epithelial cells resulting in terminal G₀ growth arrest coupled with terminal differentiation into ΔNp63-negative, PSA-expressing secretory luminal cells. This cell autonomous AR-driven terminal differentiation requires DNA-binding of the AR protein, is associated with decreases in c-Myc m-RNA and protein, are coupled with increases in p21, p27, and SKP-2 protein expression, and does not require functional p53. These changes result in down-regulation of Cyclin D₁ protein and RB phosphoryation. shRNA knockdown documents that neither RB, p21, p27 alone or in combination are required for such AR-induced G₀ growth arrest. Transgenic expression of a constitutive vector to prevent c-Myc down-regulation overrides AR-mediated growth arrest in normal prostate epithelial cells, which documents that AR-induced c-Myc down-regulation is critical in terminal growth arrest of normal prostate epithelial cells. In contrast, in prostate cancer cells, androgen-induced AR signaling paradoxically up-regulates c-Myc expression and stimulates growth as documented by inhibition of both of these responses following exposure to the AR antagonist, bicalutamide. These data document that AR signaling is converted from a growth suppressor in normal prostate epithelial cells to an oncogene in prostate cancer cells during prostatic carcinogenesis and that this conversion involves a gain of function for regulation of c-Myc expression.     

Abrogation of retinoblastoma protein function by c-Abl through tyrosine kinase-dependent and -independent mechanisms.

The decision to enter the cell division cycle is governed by the interplay between growth activators and growth inhibitors. The retinoblastoma protein (RB) is an example of a growth inhibitor whose main function appears to be the binding and inactivation of key cell cycle activators. One target of RB is a proto-oncoprotein, the c-Abl tyrosine kinase. RB binds to the ATP-binding lobe in the kinase domain and inhibits the nuclear pool of c-Abl in quiescent and G1 cells. Phosphorylation of RB at G1/S releases c-Abl, leading to the activation of this nuclear tyrosine kinase. In this report, we describe the construction of a mutant Abl, replacing the ATP-binding lobe of c-Abl with that of c-Src. The mutant protein AS2 is active as a tyrosine kinase and can phosphorylate Abl substrates, such as the C-terminal repeated domain of RNA polymerase II. AS2, however, does not bind to RB, and its activity is not inhibited by RB. As a result, the nuclear pool of AS2 is no longer cell cycle regulated. Excess AS2, but not its kinase-defective counterpart, can overcome RB-induced growth arrest in Saos-2 cells. Interestingly, wild-type c-Abl, in both its kinase-active and -inactive forms, can also overcome RB. Furthermore, overexpression of a kinase-defective c-Abl in rodent fibroblasts accelerates the transition from quiescence to S phase and cooperates with c-Myc to induce transformation. These effects, however, do not occur with the kinase-defective form of AS2. Thus, the growth-stimulating function of the kinase-defective c-Abl is dependent on the binding and the abrogation of RB function. That RB function can be abolished by the overproduction of one of its binding proteins is consistent with the hypothesis that RB induces cell cycle arrest by acting as a "molecular matchmaker" to assemble protein complexes. Exclusive engagement of RB by one of its many targets is incompatible with the biological function of this growth suppressor protein.     

Regulation of the Cell Cycle by Focal Adhesion Kinase

In this report, we have analyzed the potential role and mechanisms of integrin signaling through FAK in cell cycle regulation by using tetracycline-regulated expression of exogenous FAK and mutants. We have found that overexpression of wild-type FAK accelerated G1 to S phase transition. Conversely, overexpression of a dominant-negative FAK mutant ΔC14 inhibited cell cycle progression at G1 phase and this inhibition required the Y397 in ΔC14. Biochemical analyses indicated that FAK mutant ΔC14 was mislocalized and functioned as a dominant-negative mutant by competing with endogenous FAK in focal contacts for binding signaling molecules such as Src and Fyn, resulting in a decreases of Erk activation in cell adhesion. Consistent with this, we also observed inhibition of BrdU incorporation and Erk activation by FAK Y397F mutant and FRNK, but not FRNKΔC14, in transient transfection assays using primary human foreskin fibroblasts. Finally, we also found that ΔC14 blocked cyclin D1 upregulation and induced p21 expression, while wild-type FAK increased cyclin D1 expression and decreased p21 expression. Taken together, these results have identified FAK and its associated signaling pathways as a mediator of the cell cycle regulation by integrins.