Cdk6

Regulation of Rb family proteins by Cdk6/Ccnd1 in growth plates.

"As the growth plate lacks vasculature, apoptotic chondrocytes are not phagocytosed, and the fragmented DNA detected by TUNEL remains until they reach the bone marrow."

"transgenic mice overexpressing both Cdk6 and Ccnd1 show dwarfism. "

"overexpression of both Cdk6 and Ccnd1 [results] in the inactivation of all Rb family proteins by their phosphorylation. However, the phenotype of Cdk6/Ccnd1 double transgenic mice was different from those of p107/p130 or p107/Rb double knockout mice. BrdU-positive cells are increased, and chondrocyte differentiation is inhibited in p107/p130 and p107/Rb double knockout mice and Cdk6/Ccnd1 double transgenic mice. However, chondrocyte proliferation is enhanced without an increase in apoptosis in p107/p130 and p107/Rb double knockout mice, whereas chondrocyte proliferation is not enhanced, and apoptotic chondrocytes are greatly increased in Cdk6/Ccnd1 double transgenic mice. In Cdk6/Ccnd1 double transgenic mice, Rb but not p107 is highly phosphorylated, and mRNA and protein levels of p107 are increased."

"When Cdk6 and Ccnd1 are overexpressed, Rb, p107, and p130 should be phosphorylated, leading to the release of repressing E2fs (E2f4, 5) and activation of activating E2fs (E2f1–3), both of which enhance the transcription of E2f target genes including p107. Therefore, phosphorylation of Rb, p107, and p130 enhances the transcription of p107 by E2f and unphosphorylated p107 is continuously supplied. This is the reason why unphosphorylated p107 is dominant in Cdk6/Ccnd1 double transgenic mice."

e2f1 and e2f6 are downregulated by LSJL.

"Although p107 binds to repressing E2fs at the physiological condition, upregulation of p107 allows complex formation of activating E2fs with p107. Therefore, the increased amount of unphosphorylated p107 associates not only with repressing E2fs, but also with activating E2fs.  As p107 plays a major role in cell cycle regulation among Rb family proteins in chondrocytes, the association of p107 with repressing and activating E2fs would affect the expression of E2f target genes. It occurs in Cdk6/Ccnd1 double transgenic mice as shown by the downregulation of cyclin E, dhfr, cdc25a, and B-Myb. Further, chondrogenic ATDC5 cells overexpressing Cdk6 and Ccnd1 also had downregulated expression of these genes, and siRNA of p107 reversed the downregulated expression of these genes. Therefore, phosphorylation of Rb, p107, and p130 upregulates p107 expression, which dysregulates E2f target gene expression leading to p53-dependent apoptosis. p53 deletion in addition to Rb inactivation is still not sufficient for enhancement of chondrocyte proliferation, suggesting that p107 contributes to the low incidence of chondrosarcoma as an anti-oncogenic protein, because the frequency of chondrosarcoma is much less than those of sarcomas arising in soft tissues."