Rb-dependent cellular senescence, multinucleation and susceptibility to oncogenic transformation through PKC scaffolding by SSeCKS/AKAP12

NIRF constitutes a nodal point in the cell cycle network and is a candidate tumor suppressor

In biological networks, a small number of "hub" proteins play critical roles in the network integrity and functions. The cell cycle network orchestrates versatile cellular functions through interactions between many signaling modules, whose defects impair diverse cellular processes, often leading to cancer. However, the network architecture and molecular basis that ensure proper coordination between distinct modules are unclear. Here, we show that the ubiquitin ligase NIRF (also known as UHRF2), which induces G1 arrest, interacts with multiple cell cycle proteins including cyclins (A2, B1, D1 and E1), p53 and pRB, and ubiquitinates cyclins D1 and E1. Consistent with its versatility, a bioinformatic network analysis demonstrated that NIRF is an intermodular hub protein that is responsible for the coordination of multiple network modules. Notably, intermodular hubs are frequently associated with oncogenesis. Indeed, we detected loss of heterozygosity of the NIRF gene in several kinds of tumors. When a cancer outlier profile analysis was applied to the Oncomine database, loss of the NIRF gene was found at statistically significant levels in diverse tumors. Importantly, a recurrent microdeletion targeting NIRF was observed in non-small cell lung carcinoma. Furthermore, NIRF is immediately adjacent to the single nucleotide polymorphism rs719725, which is reportedly associated with the risk of colorectal cancer. These observations suggest that NIRF occupies a prominent position within the cell cycle network, and is a strong candidate for a tumor suppressor whose aberration contributes to the pathogenesis of diverse malignancies.

Control of protein kinase C activity, phorbol ester-induced cytoskeletal remodeling, and cell survival signals by the scaffolding protein SSeCKS/GRAVIN/AKAP12

The product of the SSeCKS/GRAVIN/AKAP12 gene ("SSeCKS") is a major protein kinase (PK) C substrate that exhibits tumor- and metastasis-suppressing activity likely through its ability to scaffold multiple signaling mediators such as PKC, PKA, cyclins, calmodulin, and Src. Although SSeCKS and PKCα bind phosphatidylserine, we demonstrate that phosphatidylserine-independent binding of PKC by SSeCKS is facilitated by two homologous SSeCKS motifs, EG(I/V)(T/S)XWXSFK(K/R)(M/L)VTP(K/R)K(K/R)X(K/R)XXXEXXXE(E/D) (amino acids 592-620 and 741-769). SSeCKS binding to PKCα decreased kinase activity and was dependent on the two PKC-binding motifs. SSeCKS scaffolding of PKC was increased in confluent cell cultures, correlating with significantly increased SSeCKS protein levels and decreased PKCα activity, suggesting a role for SSeCKS in suppressing PKC activation during contact inhibition. SSeCKS-null mouse embryo fibroblasts displayed increased relative basal and phorbol ester (phorbol 12-myristate 13-acetate)-induced PKC activity but were defective in phorbol 12-myristate 13-acetate-induced actin cytoskeletal reorganization and cell shape change; these responses could be rescued by the forced expression of full-length SSeCKS but not by an SSeCKS variant deleted of its PKC-binding domains. Finally, the PKC binding sites in SSeCKS were required to restore cell rounding and/or decreased apoptosis in phorbol ester-treated LNCaP, LNCaP-C4-2, and MAT-LyLu prostate cancer cells. Thus, PKC-mediated remodeling of the actin cytoskeleton is likely regulated by the ability of SSeCKS to control PKC signaling and activity through a direct scaffolding function.

Expression and prognostic significance of a new tumor metastasis suppressor gene LASS2 in human bladder carcinoma

The LASS2 gene has been identified as a new tumor metastasis suppressor gene and has been seen to correlate with the degree of invasion and recurrence in carcinomas of prostate, breast, liver, ovarian, and pancreas. However, expression and prognostic significance of LASS2 in human bladder carcinoma are largely unknown. In this study, the protein expression of LASS2 in 80 patients with different stages was detected by immunohistochemical staining. The prognostic value of LASS2 in bladder cancers can also be assessed by a long-term follow-up investigation. The mRNA expression level of the LASS2 gene was examined using real-time quantitative PCR (qPCR) in human bladder carcinoma and paired non-tumor bladder tissues, which were obtained from 30 patients who underwent total cystectomy. We found that patients with LASS2-negative bladder cancer were linked to poor clinical prognosis. The expression of LASS2 mRNA was significantly correlated with clinical stage (P < 0.001), depth of tumor invasion (P < 0.001), and recurrence (P < 0.001). Thus, LASS2 expression may be correlated with the development and progression of human bladder carcinoma and may be a prognostic indicator for this carcinoma.

Protein kinase Cα signaling regulates inhibitor of DNA binding 1 in the intestinal epithelium

Increasing evidence supports a role for PKCα in growth arrest and tumor suppression in the intestinal epithelium. In contrast, the Id1 transcriptional repressor has pro-proliferative and tumorigenic properties in this tissue. Here, we identify Id1 as a novel target of PKCα signaling. Using a highly specific antibody and a combined morphological/biochemical approach, we establish that Id1 is a nuclear protein restricted to proliferating intestinal crypt cells. A relationship between PKCα and Id1 was supported by the demonstration that (a) down-regulation of Id1 at the crypt/villus junction coincides with PKCα activation, and (b) loss of PKCα in intestinal tumors is associated with increased levels of nuclear Id1. Manipulation of PKCα activity in IEC-18 nontransformed intestinal crypt cells determined that PKCα suppresses Id1 mRNA and protein via an Erk-dependent mechanism. PKCα, but not PKCδ, also inhibited Id1 expression in colon cancer cells. Id1 was found to regulate cyclin D1 levels in IEC-18 and colon cancer cells, pointing to a role for Id1 suppression in the antiproliferative/tumor suppressive activities of PKCα. Notably, Id1 expression was elevated in the intestinal epithelium of PKCα-knock-out mice, confirming that PKCα regulates Id1 in vivo. A wider role for PKCα in control of inhibitor of DNA binding factors is supported by its ability to down-regulate Id2 and Id3 in IEC-18 cells, although their suppression is more modest than that of Id1. This study provides the first demonstrated link between a specific PKC isozyme and inhibitor of DNA binding factors, and it points to a role for a PKCα → Erk ⊣ Id1 → cyclin D1 signaling axis in the maintenance of intestinal homeostasis.