e2f1

E2F1 is downregulated by LSJL.

Constitutive E2F1 overexpression delays endochondral bone formation by inhibiting chondrocyte differentiation.

"endochondral bone formation is critically dependent on the retinoblastoma family members p107 and p130. Using both constitutive and conditional E2F1 transgenic mice, we show that ubiquitous transgene-driven expression of E2F1 during embryonic development results in a dwarf phenotype and significantly reduced postnatal viability. Overexpression of E2F1 disturbs chondrocyte maturation, resulting in delayed endochondral ossification, which is characterized by reduced hypertrophic zones and disorganized growth plates. Employing the chondrogenic cell line ATDC5, we investigated the effects of enforced E2F expression on the different phases of chondrocyte maturation that are normally required for endochondral ossification. Ectopic E2F1 expression strongly inhibits early- and late-phase differentiation of ATDC5 cells, accompanied by diminished cartilage nodule formation as well as decreased type II collagen, type X collagen, and aggrecan gene expression. In contrast, overexpression of E2F2 or E2F3a results in only a marginal delay of chondrocyte maturation, and increased E2F4 levels have no effect. These data are consistent with the notion that E2F1 is a regulator of chondrocyte differentiation."

"First, the relative size of the proliferating zone was increased in the growth plate [of E2F1 transgenic mice] compared to the wild-type control. Second, the maturing prehypertrophic and hypertrophic layers were significantly reduced in size. The reduced amount of hypertrophic chondrocytes was also evident in the growth plates of the humerus, ulna, and radius"

"loss of E2F1 does not appear to inhibit either proliferation or cellular differentiation during embryonic development, overexpression has been shown to prevent terminal differentiation in different cell lineages."

"Overexpression of the AP-1 transcription factor c-Fos, a positive regulator of cell proliferation, also inhibits chondrocyte differentiation in ATDC5 cells with markedly reduced α1 (II) collagen and aggrecan gene expression"

Transcriptional regulation of fibrillin-2 gene by E2F family members in chondrocyte differentiation.

"Mutation in fibrillin-2, a major structural component of extracellular microfibrils in connective tissue, results in the autosomal dominant disease congenital contractural arachnodactyly. This genetic disease is characterized by dolichostenomelia and arachnodactyly, in addition to contractures of the large joints and abnormal pinnae formation, thus indicating the significance of fibrillin-2 in chondrogenesis. In this study, we investigated the transcriptional regulation of fibrillin-2 in chondrogenic differentiation. Although mRNA expression of fibrillin-1, a highly homologous protein to fibrillin-2, remained almost unchanged during chondrogenesis of mouse ATDC5 cells, fibrillin-2 mRNA expression varied. Fibrillin-2 was highly expressed at the early stage and declined progressively during differentiation. The 5'-flanking region of the fibrillin-2 gene contains potential binding sites for E2F, Runx, AP-2, and Sox transcription factors. The promoter activity of fibrillin-2 decreased markedly following deletion and mutagenesis of the E2F binding site between -143 and -136 bp. Overexpression of E2F1 resulted in a marked increase in its promoter activity, whereas expression of other transcription factors including AP-2alpha and Runx2 had no effect. The increase in promoter activity by E2F1 was completely suppressed by the coexpression of E2F4. E2F2 and E2F3 had positive effects on the promoter activity. Although ATDC5 cells expressed transcripts for the E2F family genes at all stages of differentiation, the expression profiles differed. E2F1 expression remained almost unchanged, whereas E2F4 expression increased markedly at the late stage of differentiation."

"ATDC5 cells constitutively expressed E2F1 mRNA except for the decrease of E2F1 expression at a late stage. E2F2 and E2F3 mRNA levels increased slightly during early chondrogenic differentiation and then finally diminished. In contrast, E2F4 transcripts were not detected in undifferentiated cells, but increased markedly during chondrogenic differentiation. Moreover, the expression changes of E2F1 and E2F4 protein were coincident with that of each mRNA during differentiation. It is highly likely that the expression of fibrillin-2 is upregulated by transactivators (E2F1, E2F2, and E2F3) during the early phase of chondrogenesis and subsequently downregulated as a result of rapid increases in transrepressor (E2F4). This hypothesis was supported by our finding that forced expression of E2F4 inhibited the E2F1-mediated increase in fibrillin-2 promoter activity in a dose-dependent manner"

Anti-chondrosarcoma effects of PEDF mediated via molecules important to apoptosis, cell cycling, adhesion and invasion.

"Chondrosarcoma develops as a result of overgrowth of chondrocytes and overproduction of cartilage matrix. It is currently surgically treated, although non-invasive methods are being sought. In this report, pigment epithelium-derived factor (PEDF) induced apoptosis in the chondrosarcoma cell line - JJ012, with upregulation of Bax, Fas, caspase-3 and -6 and downregulation of Bcl-2. Cell cycling was also decreased with decreased expression of p38, p-Akt, p-Erk and JNK1 and increased expression of p73 and E2F1. Furthermore, PEDF increased adhesion of cells to collagen-I, with decreased expression of p-Fak, RhoA and cdc42. Invasion of cells through collagen-I was also reduced by PEDF, with decreased expression of uPAR, MMP-14 and increased expression of PAI-1. These findings seminally indicate that PEDF may have potential as an anti-cancer agent against chondrosarcoma."

"E2F1 has been found to be responsible for promoting the G1 to S-phase progression of the cell cycle, thereby promoting increased cell cycling. However, E2F1 also possesses the ability to induce apoptosis in a p53-dependent or –independent manner, mainly through p73, thereby giving E2F1 the ability to trigger both cell proliferation and apoptosis"


Loss of pRB and p107 disrupts cartilage development and promotes enchondroma formation.

"The pocket proteins pRB, p107 and p130 have established roles in regulating the cell cycle through the control of E2F activity. The pocket proteins regulate differentiation of a number of tissues in both cell cycle-dependent and -independent manners. Prior studies showed that mutation of p107 and p130 in the mouse leads to defects in cartilage development during endochondral ossification, the process by which long bones form. Despite evidence of a role for pRB in osteoblast differentiation, it is unknown whether it functions during cartilage development. Here, we show that mutation of Rb in the early mesenchyme of p107-mutant mice results in severe cartilage defects in the growth plates of long bones. This is attributable to inappropriate chondrocyte proliferation that persists after birth and leads to the formation of enchondromas[cartilage cysts] in the growth plates as early as 8 weeks of age. Genetic crosses show that development of these tumorigenic lesions is E2f3 dependent. These results reveal an overlapping role for pRB and p107 in cartilage development, endochondral ossification and enchondroma formation that reflects their coordination of cell-cycle exit at appropriate developmental stages"