Bcl-2 Retards Cell Cycle Entry through p27^Kip1, pRB Relative p130, and Altered E2F Regulation

Abstract

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.In addition to its well-established function in controlling cell survival, the Bcl-2 family (1) has been found to influence the cell cycle. Although Bcl-2 and its prosurvival relatives do not affect the growth rate in proliferating cultures, they can both accelerate withdrawal from the cycle (46) and retard reentry (4,29,30,35). Conversely, a shortened G 1 is found in lymphocytes that lack Bcl-2 (29) or express the Bcl-2 antagonist Bax (4). Thus, Bcl-2 appears to have a physiological role in influencing the transition between the quiescent and cycling states. That this ability is separate from its role in cell survival (46) is most clearly shown by mutations of Bcl-2 that eliminate its cell cycle activity but spare its antiapoptotic function (17,45).The cell cycle control function of Bcl-2 has ramifications for cellular homeostasis. Cycling cells are often more vulnerable to apoptosis, perhaps because, under conditions unfavorable for proliferation, certain cell cycle effectors promote apoptosis (11). Hence, promoting quiescence under conditions of stress may provide Bcl-2 with an additional, albeit indirect, means to enhance cell survival (30,46). Interference with Bcl-2's cell cycle effect may also augment its oncogenic role (see Discussion).Progression through the cell cycle requires the action of cyclin-dependent kinases (Cdks) (38, 39). As cells enter the cycle, newly synthesized D-type cyclins associate with and activate their Cdk-4/6 catalytic partners in mid-to late G 1 phase, while cyclin E appears later in G 1 and activates its Cdk-2 kinase subunit near the G 1 /S boundary. Opposing their activity are Cdk inhibitors (Cki) of two classes: INK4 proteins, such as p16, specifically inhibit D-cyclin kinases, whereas Cip/Kip proteins, such as p21 and p27, also inhibit Cdk-2 (38, 39). Other key negative regulators include the best-known Cdk substrates...

Constitutive expression of BCL‐XL in the T lineage attenuates collagen‐induced arthritis in Bcl‐XL transgenic mice

“…Overexpression of Bcl-2 has been shown to reduce proliferation of T cells and delay cell cycle entry of polyclonal-stimulated T lymphocytes (16)(17)(18). To determine whether Bcl-x L also affects cell cycle progression in CII-specific T cells, draining lymph node cells from Bcl-x L /V␤8.3 and V␤8.3 control mice were isolated 10 days postimmunization, restimulated with CII at 50 g/ml for 3 days, and dual-stained with anti-CD3 and propidium iodide to assess T cell DNA content.…”

Bcl-2 activates a programme of premature senescence in human carcinoma cells

“…Since the p53 gene is not required for Bcl-2-mediated growth inhibition [7,11], we selected a p53-null lung carcinoma-cell line, H1299. According to its growthinhibitory activity, stable transfection of Bcl-2 resulted in a reduced cloning efficiency in H1299 cells, as well as in other tumour cell lines ( [7,12] and results not shown).…”

“…In particular, we analysed those shown to have a critical role in Bcl-2-dependent cell-cycle arrest in immortalized 3T3 murine fibroblasts [11]. In line with this data, induction of Bcl-2 led to an increase in the amount of p27 KIP1 protein in all clones ( Figure 4A), whereas the levels of expression of the other members of the CIP/KIP family, p21 CIP1 and p57 KIP2 , were unchanged ( Figure 4A, and results not shown).…”

“…Both p27 KIP1 and the pRb-related protein p130 have been reported to be essential mediators of the Bcl-2 cell-cycle inhibitory effect [11]. Therefore we also examined the expression of the p130 pocket protein.…”

“…In contrast, overexpression of pro-apoptotic members such as Bax or Bad in T cells of transgenic mice increases the number of cycling thymocytes, favouring entry into S-phase [6,10]. The ability of Bcl-2 to interfere with cell-cycle progression is not limited to lymphoid cells, but has also been observed in murine fibroblasts [7,11]. Overexpression of the bcl-2 gene in normal, quiescent cells retards entry into the cell cycle, but does not affect growth of cells that are continuously cycling [7,8].…”