Tumor Necrosis Factor α Inhibits Cyclin A Expression and Retinoblastoma Hyperphosphorylation Triggered by Insulin-like Growth Factor-I Induction of New E2F-1 Synthesis

Abstract

Cyclin A is required for cell cycle S phase entry, and its overexpression contributes to tumorigenesis. Release of pre-existing E2Fs from inactive complexes of E2F and hypophosphorylated retinoblastoma (RB) is the prevailing dogma for E2F transcriptional activation of target genes such as cyclin A. Here we explored the hypothesis that new synthesis of E2F-1 is required for insulin-like growth factor-I (IGF-I) to induce cyclin A accumulation and RB hyperphosphorylation, events that are targeted by tumor necrosis factor ␣ (TNF␣) to arrest cell cycle progression. We first established that IGF-I increases expression of cyclin A, causes hyperphosphorylation of RB, and augments the mass of E2F-1 in a time-dependent manner. As expected, E2F-1 small interfering RNA blocks the ability of IGF-I to increase synthesis of E2F-1. Most important, this E2F-1 small interfering RNA also blocks the ability of IGF-I to increase cyclin A accumulation and to hyperphosphorylate RB. We next established that TNF␣ dose-dependently inhibits IGF-I-induced phosphorylation of both RB and histone H1 by cyclin A-dependent cyclin-dependent kinases. Cyclin-dependent kinase 2 (Cdk2) mediates this suppression because co-immunoprecipitation experiments revealed that TNF␣ reduces the amount of IGF-Iinduced cyclin A that binds Cdk2, leading to a reduction in Cdk2 enzymatic activity. TNF␣ antagonizes the ability of IGF-I to increase mass of both E2F-1 and cyclin A but not cyclin E or D1. The cytostatic property of TNF␣ is also shown by its ability to block IGF-I-stimulated luciferase activity of a cyclin A promoter reporter. Deletion of an E2F recognition site from this reporter eliminates the regulatory effects of both IGF-I and TNF␣ on cyclin A transcription, indicating the essential role of E2F-1 in mediating their cross-talk. Collectively, these results establish that TNF␣ targets IGF-I-induced E2F-1 synthesis, leading to inhibition of the subsequent accumulation in cyclin A, formation of cyclin A-Cdk2 complexes, hyperphosphorylation of RB, and cell cycle arrest.

Regulation of mammary parenchymal growth by the fat pad in prepubertal dairy heifers: role of inflammation-related proteins

“…Our results suggest that at least a portion of this effect of TNFa is due to the inhibition of IGF-I actions. Our results are also consistent with those showing that TNFa inhibited basal and IGF-I-mediated proliferation in human mammary epithelial cells (Shen et al 2004). Unlike others, however, we found that high doses of IL-6 and IL-1b (50 and 100 ng/ml respectively) did not affect basal or IGF-I-stimulated proliferation of bovine mammary cells.…”

HES-1 inhibits 17β-estradiol and heregulin-β1-mediated upregulation of E2F-1

“…Therefore, inhibition of E2F-1 transcription caused by treatment with atRA should lead to a profound decrease in the level of E2F-1 as observed in Figure 2b. A recent study has shown that inhibition of E2F-1 transcription by siRNA in breast cancer cells blocks Rb-phosphorylation and interference of this feed-forward loop would be an efficient barrier to neoplastic cell cycling (Shen et al, 2004). So far, we have studied two different E 2 -responsive breast cancer cell lines and our results indicate that, in both cases, HES-1 has to be downregulated for E 2 to stimulate cellular proliferation (Strom et al, 2000;Muller et al, 2002).…”

PBK/TOPK promotes tumour cell proliferation through p38 MAPK activity and regulation of the DNA damage response

“…A recent report has shown that PBK transcription is regulated by E2F and cAMP-response element binding protein (CREB) (Nandi and Rapoport, 2006). CREB is activated by IGF-I through the PI3K and ERK pathways (Maldonado et al, 2005); whereas IGF-I can also induce E2F expression (Shen et al, 2004). It is likely that E2F and CREB could be the transcription factors responsible for IGF-I-induced PBK expression.…”