BRCA1 targets G2/M cell cycle proteins for ubiquitination and proteasomal degradation

The BRCA1 tumor suppressor protein heterodimerizes with its partner protein, BARD1, via the RING domain present in both proteins. The heterodimer contains an E3 ubiquitin ligase activity and participates in multiple cellular functions such as cell cycle control, DNA repair and regulation of gene transcription, collectively aimed at maintaining genomic stability and tumor suppression. Yet, the precise role of BRCA1 E3 ligase in these cellular functions is poorly understood. We present data showing that BRCA1 ubiquitinates G2/M cell cycle proteins, cyclin B and Cdc25C, leading to their accelerated degradation via a mechanism that is independent of APC/C. BRCA1-dependent degradation of cyclin B and Cdc25C is reversed by proteasome inhibitors and is enhanced following DNA damage, which may represent a possible mechanism to prevent cyclin B and Cdc25C accumulation, a requirement for mitotic entry. Our data provide mechanistic insight into how BRCA1 E3 ligase activity regulates the G2/M cell cycle checkpoint and, thus, contributes to maintenance of genomic stability.

Organ-injury-induced reactivation of hemangioblastic precursor cells

Early in mammalian development, the stem cell leukemia (SCL/TAL1) gene and its distinct 3' enhancer (SCL 3'En) specify bipotential progenitor cells that give rise to blood and endothelium, thus termed hemangioblasts. We have previously detected a minor population of SCL (+) cells in the postnatal kidney. Here, we demonstrate that cells expressing the SCL 3'En in the adult kidney are comprised of CD45+CD31- hematopoietic cells, CD45-CD31+ endothelial cells and CD45-CD31- interstitial cells. Creation of bone marrow chimeras of SCL 3'En transgenic mice into wild-type hosts shows that all three types of SCL 3'En-expressing cells in the adult kidney can originate from the bone marrow. Ischemia/reperfusion injury to the adult kidney of SCL 3'En transgenic mice results in the intrarenal elevation of SCL and FLK1 mRNA levels and of cells expressing hem-endothelial progenitor markers (CD45, CD34, c-Kit and FLK1). Furthermore, analysis of SCL 3'En in the ischemic kidneys reveals an increase in the abundance of SCL 3'En-expressing cells, predominantly within the CD45 (+) hematopoietic fraction and to a lesser extent in the CD45 (-) fraction. Our results suggest organ-injury-induced reactivation of bone marrow-derived hemangioblasts and possible local angioblastic progenitors expressing SCL and SCL 3'En.