Lymphocyte lineage-specific and developmental stage specific mechanisms suppress cyclin D3 expression in response to DNA double strand breaks

Abstract

Mammalian cells are thought to protect themselves and their host organisms from DNA double strand breaks (DSBs) through universal mechanisms that restrain cellular proliferation until DNA is repaired. The Cyclin D3 protein drives G1-to-S cell cycle progression and is required for proliferation of immature T and B cells and of mature B cells during a T cell-dependent immune response. We demonstrate that mouse thymocytes and pre-B cells, but not mature B cells, repress Cyclin D3 protein levels in response to DSBs. This response requires the ATM protein kinase that is activated by DSBs. Cyclin D3 protein loss in thymocytes coincides with decreased association of Cyclin D3 mRNA with the HuR RNA binding protein that ATM regulates. HuR inactivation reduces basal Cyclin D3 protein levels without affecting Cyclin D3 mRNA levels, indicating that thymocytes repress Cyclin D3 expression via ATM-dependent inhibition of Cyclin D3 mRNA translation. In contrast, ATM-dependent transcriptional repression of the Cyclin D3 gene represses Cyclin D3 protein levels in pre-B cells. Retrovirus-driven Cyclin D3 expression is resistant to transcriptional repression by DSBs; this prevents pre-B cells from suppressing Cyclin D3 protein levels and from inhibiting DNA synthesis to the normal extent following DSBs. Our data indicate that immature B and T cells use lymphocyte lineage-and developmental stage-specific mechanisms to inhibit Cyclin D3 protein levels and thereby help prevent cellular proliferation in response to DSBs. We discuss the relevance of these cellular context-dependent DSB response mechanisms in restraining proliferation, maintaining genomic integrity, and suppressing malignant transformation of lymphocytes.

Immature Lymphocytes Inhibit Rag1 and Rag2 Transcription and V(D)J Recombination in Response to DNA Double-Strand Breaks

“…Since we discovered that the DDR of immature lymphocytes frequently is abnormal in immortalized cell lines and in primary cells cultured ex vivo (41), we conducted initial experiments by exposing live mice to IR. To limit apoptosis of immature lymphocytes, we utilized EμBCL2 mice with transgenic expression of the anti-apoptotic BCL2 protein in all B and T lineage cells.…”

Cell cycle RNA regulons coordinating early lymphocyte development

“…By contrast, the deletion of Elavl1 using Mb1cre, which is active from the pro‐B cell stage onwards, resulted in only a two‐fold reduction in pre‐ and immature‐B cells in the bone marrow . Deletion of Elavl1 using Lck‐cre , which deletes in double negative thymocytes also had a milder effect on the numbers of immature thymocytes . These differences could reflect a critical role for HuR in B cell development prior to the pro‐B cell stage and in the earliest thymocytes.…”

“…HuR has been demonstrated to bind to cell cycle mRNAs in an activated T cell line, and the cell cycle is among pathways altered by loss of HuR in activated mature B cells . Additionally, HuR has been demonstrated to promote cell cycle progression in cancer cell lines and can interact with a number of transcripts that both positively and negatively regulate cell cycle progression including p27 and Cyclin D3, the key cyclin for the proliferative expansion of developing lymphocytes …”

Cyclin D3: To translate or not to translate

“…Work by DeMicco et al describes mechanisms that regulate cyclin D3 accumulation during B and T lymphocyte maturation andprovidecluestotheimportanceoflineage-specificpathways. 1 Significant progress has been made with regard to the elucidation of how extracellular cues regulate cyclin D protein accumulation and biological functions during the past 20 years; however, much of this has focused on cyclin D1 due to its frequent overexpression in human cancer. With our current view that all cyclins are not created alike, the need to explore the regulation of cyclins D2 and D3 has come to the forefront.…”

“…Regardless of the precise mechanism, reduced cyclin D3 is essential for inhibition of unscheduled S-phase entry. 1 Although individual D-type cyclins clearly exhibit overlapping functions through their combinatorial activation of either CDK4 or 6, accumulating evidence is revealing unique, cell type specific regulatory mechanisms. While one aspect of lineage-specific regulation of a given cycle likely reflects inherent signaling specificity within a given cell type, the dependence of such cells on a cyclin D3-dependent kinase suggests unique functional characteristics.…”