Phosphoinositide 3-kinase beta controls replication factor C assembly and function

PI3Kβ-A Versatile Transducer for GPCR, RTK, and Small GTPase Signaling

The phosphoinositide 3-kinase (PI3K) family includes eight distinct catalytic subunits and seven regulatory subunits. Only two PI3Ks are directly regulated downstream from G protein-coupled receptors (GPCRs): the class I enzymes PI3Kβ and PI3Kγ. Both enzymes produce phosphatidylinositol 3,4,5-trisposphate in vivo and are regulated by both heterotrimeric G proteins and small GTPases from the Ras or Rho families. However, PI3Kβ is also regulated by direct interactions with receptor tyrosine kinases (RTKs) and their tyrosine phosphorylated substrates, and similar to the class II and III PI3Ks, it binds activated Rab5. The unusually complex regulation of PI3Kβ by small and trimeric G proteins and RTKs leads to a rich landscape of signaling responses at the cellular and organismic levels. This review focuses first on the regulation of PI3Kβ activity in vitro and in cells, and then summarizes the biology of PI3Kβ signaling in distinct tissues and in human disease.

Investigation of crucial genes and microRNAs in conventional osteosarcoma using gene expression profiling analysis

The present study aimed to screen potential genes associated with conventional osteosarcoma (OS) and obtain further information on the pathogenesis of this disease. The microarray dataset GSE14359 was downloaded from the Gene Expression Omnibus. A total of 10 conventional OS samples and two non‑neoplastic primary osteoblast samples in the dataset were selected to identify the differentially expressed genes (DEGs) using the Linear Models for Microarray Data package. The potential functions of the DEGs were predicted using Gene Ontology (GO) and pathway enrichment analyses. Protein‑protein interaction (PPI) data were also obtained using the Search Tool for the Retrieval of Interacting Genes database, and the PPI network was visualized using Cytoscape. Module analysis was then performed using the Molecular Complex Detection module. Additionally, the potential microRNAs (miRNAs) for the upregulated DEGs in the most significant pathway were predicted using the miRDB database, and the regulatory network for the miRNAs‑DEGs was visualized in Cytoscape. In total, 317 upregulated and 670 downregulated DEGs were screened. Certain DEGs, including cyclin‑dependent kinase 1 (CDK1), mitotic arrest deficient 2 like 1 (MAD2L1) and BUB1 mitotic checkpoint serine/threonine‑protein kinase (BUB1), were significantly enriched in the cell cycle phase and oocyte meiosis pathway. DEGs, including replication factor C subunit 2 (RFC2), RFC3, RFC4 and RFC5, were significantly enriched in DNA replication and interacted with each other. RFC4 also interacted with other DEGs, including CDK1, MAD2L1, NDC80 kinetochore complex and BUB1. In addition, RFC4, RFC3 and RFC5 were targeted by miRNA (miR)‑802, miR‑224‑3p and miR‑522‑3p. The DEGs encoding RFC may be important for the development of conventional OS, and their expression may be regulated by a number of miRNAs, including miR‑802, miR‑224‑3p and miR‑522‑3p.

Nuclear PI3K signaling in cell growth and tumorigenesis

The PI3K/Akt signaling pathway is a major driving force in a variety of cellular functions. Dysregulation of this pathway has been implicated in many human diseases including cancer. While the activity of the cytoplasmic PI3K/Akt pathway has been extensively studied, the functions of these molecules and their effector proteins within the nucleus are poorly understood. Harboring key cellular processes such as DNA replication and repair as well as nascent messenger RNA transcription, the nucleus provides a unique compartmental environment for protein–protein and protein–DNA/RNA interactions required for cell survival, growth, and proliferation. Here we summarize recent advances made toward elucidating the nuclear PI3K/Akt signaling cascade and its key components within the nucleus as they pertain to cell growth and tumorigenesis. This review covers the spatial and temporal localization of the major nuclear kinases having PI3K activities and the counteracting phosphatases as well as the role of nuclear PI3K/Akt signaling in mRNA processing and exportation, DNA replication and repair, ribosome biogenesis, cell survival, and tumorigenesis.

Nuclear phosphoinositides and their impact on nuclear functions

Polyphosphoinositides (PPIn) are important lipid molecules whose levels are de-regulated in human diseases such as cancer, neurodegenerative disorders and metabolic syndromes. PPIn are synthesized and degraded by an array of kinases, phosphatases and lipases which are localized to various subcellular compartments and are subject to regulation in response to both extra- and intracellular cues. Changes in the activities of enzymes that metabolize PPIn lead to changes in the profiles of PPIn in various subcellular compartments. Understanding how subcellular PPIn are regulated and how they affect downstream signaling is critical to understanding their roles in human diseases. PPIn are present in the nucleus, and their levels are changed in response to various stimuli, suggesting that they may serve to regulate specific nuclear functions. However, the lack of nuclear downstream targets has hindered the definition of which pathways nuclear PPIn affect. Over recent years, targeted and global proteomic studies have identified a plethora of potential PPIn-interacting proteins involved in many aspects of transcription, chromatin remodelling and mRNA maturation, suggesting that PPIn signalling within the nucleus represents a largely unexplored novel layer of complexity in the regulation of nuclear functions.

Alternative replication factor C protein, Elg1, maintains chromosome stability by regulating PCNA levels on chromatin

Proliferating cell nuclear antigen (PCNA) is loaded on chromatin upon initiation of the S phase and acts as a platform for a large number of proteins involved in chromosome duplication at the replication fork. As duplication is completed, PCNA dissociates from chromatin, and thus, chromatin-bound PCNA levels are regulated during the cell cycle. Although the mechanism of PCNA loading has been extensively investigated, the unloading mechanism has remained unclear. Here, we show that Elg1, an alternative replication factor C protein, is required for the regulation of chromatin-bound PCNA levels. When Elg1 was depleted by small interfering RNA, chromatin-bound PCNA levels were extremely increased during the S phase. The number of PCNA foci, regions in the nucleus normally representing DNA replication sites, was increased and PCNA remained on chromatin after DNA replication. Various chromatin-associated protein levels on chromatin were affected, and chromatin loop size was increased. During mitosis, cells with aberrant chromosomes and lagging chromosomes were frequently detected. Our findings suggest that Elg1 has an important role in maintaining chromosome integrity by regulating PCNA levels on chromatin, thereby acting as a PCNA unloading factor.