Bcl-2 retards cell cycle entry through p27(Kip1), pRB relative p130, and altered E2F regulation.

Independent of its antiapoptotic function, Bcl-2 can, through an undetermined mechanism, retard entry into the cell cycle. Cell cycle progression requires the phosphorylation by cyclin-dependent kinases (Cdks) of retinoblastoma protein (pRB) family members to free E2F transcription factors. We have explored whether retarded cycle entry is mediated by the Cdk inhibitor p27 or the pRB family. In quiescent fibroblasts, enforced Bcl-2 expression elevated levels of both p27 and the pRB relative p130. Bcl-2 still slowed G(1) progression in cells deficient in pRB but not in those lacking p27 or p130. Hence, pRB is not required, but both p27 and p130 are essential mediators. The ability of p130 to form repressive complexes with E2F4 is implicated, because the retardation by Bcl-2 was accentuated by coexpressed E2F4. A plausible relevant target of p130/E2F4 is the E2F1 gene, because Bcl-2 expression delayed E2F1 accumulation during G(1) progression and overexpression of E2F1 overrode the Bcl-2 inhibition. Hence, Bcl-2 appears to retard cell cycle entry by increasing p27 and p130 levels and maintaining repressive complexes of p130 with E2F4, perhaps to delay E2F1 expression.

Expression of cyclin D1, but not cyclins E and A, is related to progression in bilharzial bladder cancer.

The present study was conducted to analyze the alterations affecting cyclins D1, E, and A in bilharzial bladder cancer and to assess their potential clinical significance. A total of 125 cases were examined. Histopathological subtypes included 68 squamous cell carcinomas, 55 transitional cell carcinomas, and 2 adenocarcinomas. Immunohistochemical analyses were performed using a panel of well-characterized antibodies. The results were correlated with proliferative index, as assessed by Ki67 antigen expression. The cyclin D1-positive phenotype, defined as the identification of positive immunoreactivity in the nuclei of >/=20% of tumor cells, was found in 33 of 107 (31%) evaluable cases. A significant association was observed between the cyclin D1-positive phenotype and deep muscle invasion (P = 0.02), high tumor grade (P = 0.02), and Ki67 high proliferative index (P = 0.03). The cyclin E-positive phenotype, defined as per cyclin D1, was found in 79 of 106 (75%) evaluable cases. The cyclin A-positive phenotype, defined using the above criteria, was identified in 60 of 108 (56%) evaluable cases. No statistically significant association was found between cyclins E or A and clinicopathological parameters or proliferative index. However, there was a strong association between the expression of cyclin D1 and the coexpression of cyclins A and/or E (P = 0.05). Ki67 proliferative index was considered high when >/=20% of tumor cells displayed positive nuclear staining, a phenotype that was observed in 99 of 115 (86%) cases. These data support the hypothesis that cyclin D1 activation determines the evolution of a particular subset of aggressive bladder tumors. In addition, cyclins E and A seem to follow an unscheduled pattern of expression, based on the high frequency of identifying a positive phenotype for these cyclins and the lack of correlation between their expression and Ki67 high proliferative index. It may be postulated that the expression of G1 cyclin genes is deregulated in bilharzial bladder cancer, and that cyclin D1 acts as an oncogenic event in these neoplasms. Moreover, the moderate number of tumors displaying the cyclin D1-positive phenotype (31%) versus the high frequency observed for both cyclins E (75%) and A (56%), suggests a short G1 disbalanced by a long S phase and a rapid transversal of the cell cycle, as evidenced by a high Ki67 index observed in 86% of these cases. This imbalance in the cell cycle, together with alterations reported on the p53 pathway, might underline the accumulation of DNA damage and the aggressive clinical course of bilharzial bladder cancer.

CX3CR1+ interstitial dendritic cells form a contiguous network throughout the entire kidney.

Dendritic cells (DCs) interface innate and adaptive immunity in nonlymphoid organs; however, the exact distribution and types of DC within the kidney are not known. We utilized CX3CR1GFP/+ mice to characterize the anatomy and phenotype of tissue-resident CX3CR1+ DCs within normal kidney. Laser-scanning confocal microscopy revealed an extensive, contiguous network of stellate-shaped CX3CR1+ DCs throughout the interstitial and mesangial spaces of the entire kidney. Intravital microscopy of the superficial cortex showed stationary interstitial CX3CR1+ DCs that continually probe the surrounding tissue environment through dendrite extensions. Flow cytometry of renal CX3CR1+ DCs showed significant coexpression of CD11c and F4/80, high major histocompatibility complex class II and FcR expression, and immature costimulatory but competent phagocytic ability indicative of tissue-resident, immature DCs ready to respond to environment cues. Thus, within the renal parenchyma, there exists little immunological privilege from the surveillance provided by renal CX3CR1+ DCs, a major constituent of the heterogeneous mononuclear phagocyte system populating normal kidney.

The absence of p27Kip1, an inhibitor of G1 cyclin-dependent kinases, uncouples differentiation and growth arrest during the granulosa->luteal transition.

The involvement of cyclin-dependent kinase inhibitors in differentiation remains unclear: are the roles of cyclin-dependent kinase inhibitors restricted to cell cycle arrest; or also required for completion of the differentiation program; or both? Here, we report that differentiation of luteal cells can be uncoupled from growth arrest in p27-deficient mice. In these mice, female-specific infertility correlates with a failure of embryos to implant at embryonic day 4.5. We show by ovarian transplant and hormone reconstitution experiments that failure to regulate luteal cell estradiol is one physiological mechanism for infertility in these mice. This failure is not due to a failure of p27-deficient granulosa cells to differentiate after hormonal stimulation; P450scc, a marker for luteal progesterone biosynthesis, is expressed and granulosa cell-specific cyclin D2 expression is reduced. However, unlike their wild-type counterparts, p27-deficient luteal cells continue to proliferate for up to 3.5 days after hormonal stimulation. By day 5.5, however, these cells withdraw from the cell cycle, suggesting that p27 plays a role in the early events regulating withdrawal of cells from the cell cycle. We have further shown that in the absence of this timely withdrawal, estradiol regulation is perturbed, explaining in part how fertility is compromised at the level of implantation. These data support the interpretation of our previous observations on oligodendrocyte differentiation about a role for p27 in establishing the nonproliferative state, which in some cases (oligodendrocytes) is required for differentiation, whereas in other cases it is required for the proper functioning of a differentiated cell (luteal cell).

Formation of p27-CDK complexes during the human mitotic cell cycle.

In eukaryotic cells, the coordinated activation of different cyclin-dependent kinases regulates entry into S-phase. In vitro and in nonproliferating cells, p27 associates with and inhibits cyclin/cycin-dependent kinase (CDK) holoenzymes containing either CDK4, CDK6, or CDK2. Although many different types of proliferating cells contain p27 protein, neither the interactions of p27 with cyclin/CDK complexes nor the consequences of this interaction during the mitotic cycle have been fully explored. We report that, in MANCA cells, the amount of p27 is constant during the cell cycle. In addition, p27 associates with three different CDKs: CDK2, CDK4, and CDK6. Furthermore, the amount of p27 is significantly lower than the amount of cyclin D3 in these cells. The amount of CDK4 and CDK6 associated with p27 does not change in a cell cycle-dependent fashion; in contrast, the amount of CDK2 associated with p27 is lowest in early G1 cells and increases to a maximum in mid-G1 phase, reaching a steady-state level in late G1-phase cells. After mid-G1 phase, the amount of each p27/CDK complex remains constant through the remainder of the cell cycle. p27-immunoprecipitates contain an Rb-kinase activity. The substrate specificity, the expression pattern of this kinase, and the ability to deplete 50% of this kinase activity with a CDK6-specific antibody suggest that the CDK6 protein mediates, in part, the p27-associated Rb-kinase activity. In contrast, p27 complexes containing CDK2 are incapable of phosphorylating histone H1. These data are consistent with a model wherein cyclin D/CDK complexes sequester the CDK2-dependent kinase inhibitory activity of p27.