Separation of simian virus 40 large-T-antigen-transforming and origin-binding functions from the ability to block differentiation

Advancements in culture technology of adipose-derived stromal/stem cells: implications for diabetes and its complications

Stem cell-based therapies exhibit considerable promise in the treatment of diabetes and its complications. Extensive research has been dedicated to elucidate the characteristics and potential applications of adipose-derived stromal/stem cells (ASCs). Three-dimensional (3D) culture, characterized by rapid advancements, holds promise for efficacious treatment of diabetes and its complications. Notably, 3D cultured ASCs manifest enhanced cellular properties and functions compared to traditional monolayer-culture. In this review, the factors influencing the biological functions of ASCs during culture are summarized. Additionally, the effects of 3D cultured techniques on cellular properties compared to two-dimensional culture is described. Furthermore, the therapeutic potential of 3D cultured ASCs in diabetes and its complications are discussed to provide insights for future research.

Matrix reverses immortalization-mediated stem cell fate determination

Primary cell culture in vitro suffers from cellular senescence. We hypothesized that expansion on decellularized extracellular matrix (dECM) deposited by simian virus 40 large T antigen (SV40LT) transduced autologous infrapatellar fat pad stem cells (IPFSCs) could rejuvenate high-passage IPFSCs in both proliferation and chondrogenic differentiation. In the study, we found that SV40LT transduced IPFSCs exhibited increased proliferation and adipogenic potential but decreased chondrogenic potential. Expansion on dECMs deposited by passage 5 IPFSCs yielded IPFSCs with dramatically increased proliferation and chondrogenic differentiation capacity; however, this enhanced capacity diminished if IPFSCs were grown on dECM deposited by passage 15 IPFSCs. Interestingly, expansion on dECM deposited by SV40LT transduced IPFSCs yielded IPFSCs with enhanced proliferation and chondrogenic capacity but decreased adipogenic potential, particularly for the dECM group derived from SV40LT transduced passage 15 cells. Our immunofluorescence staining and proteomics data identify matrix components such as basement membrane proteins as top candidates for matrix mediated IPFSC rejuvenation. Both cell proliferation and differentiation were endorsed by transcripts measured by RNASeq during the process. This study provides a promising model for in-depth investigation of the matrix protein influence on surrounding stem cell differentiation.

Immortalization of chicken preadipocytes by retroviral transduction of chicken TERT and TR

The chicken is an important agricultural animal and model for developmental biology, immunology and virology. Excess fat accumulation continues to be a serious problem for the chicken industry. However, chicken adipogenesis and obesity have not been well investigated, because no chicken preadipocyte cell lines have been generated thus far. Here, we successfully generated two immortalized chicken preadipocyte cell lines through transduction of either chicken telomerase reverse transcriptase (chTERT) alone or in combination with chicken telomerase RNA (chTR). Both of these cell lines have survived >100 population doublings in vitro, display high telomerase activity and have no sign of replicative senescence. Similar to primary chicken preadipocytes, these two cell lines display a fibroblast-like morphology, retain the capacity to differentiate into adipocytes, and do not display any signs of malignant transformation. Isoenzyme analysis and PCR-based analysis confirmed that these two cell lines are of chicken origin and are free from inter-species contamination. To our knowledge, this is the first report demonstrating the generation of immortal chicken cells by introduction of chTERT and chTR. Our established chicken preadipocyte cell lines show great promise as an in vitro model for the investigation of chicken adipogenesis, lipid metabolism, and obesity and its related diseases, and our results also provide clues for immortalizing other avian cell types.

Conditional immortalization of primary adipocyte precursor cells

The production of new adipocytes requires the differentiation of adipocyte precursor (AP) cells residing within the adipose tissue stromal-vascular compartment. The objective was to obtain an immortalized primary adipogenic cell line derived from FACS isolated committed APs using the conditional expression of SV40 T antigen. Adipocyte precursors were isolated from white adipose tissue (WAT) using FACS to remove non-adipogenic cell populations from mice expressing a conditionally regulated SV40 T antigen. APs were maintained by continuous culture and induced to undergo adipogenic differentiation. Adipogenesis, determined by Oil Red O staining, was assessed with each passage and compared to wildtype controls. Adipogenic capability was rapidly lost with increased passage number in committed APs with concurrent reduction in cell proliferation and expression of essential late adipogenic genes, including Pparγ and C/ebpα. Thus, FACS purified committed APs have limited capability to undergo expansion and subsequent adipogenic differentiation in vitro even if they are immortalized with the SV40 T antigen.

Review of the history and current status of cell-transplant approaches for the management of neuropathic pain

Treatment of sensory neuropathies, whether inherited or caused by trauma, the progress of diabetes, or other disease states, are among the most difficult problems in modern clinical practice. Cell therapy to release antinociceptive agents near the injured spinal cord would be the logical next step in the development of treatment modalities. But few clinical trials, especially for chronic pain, have tested the transplant of cells or a cell line to treat human disease. The history of the research and development of useful cell-transplant-based approaches offers an understanding of the advantages and problems associated with these technologies, but as an adjuvant or replacement for current pharmacological treatments, cell therapy is a likely near future clinical tool for improved health care.

Binding of p110 retinoblastoma protein inhibits nuclear import of simian virus SV40 large tumor antigen

Nuclear import of the simian virus 40 large tumor antigen (T-ag) is dependent on its nuclear localization signal (NLS) within amino acids 126-132 that is recognized by the importin alpha/beta1 heterodimer, as well as a protein kinase CK2 site at serine 112 upstream of the NLS, which enhances the interaction approximately 50-fold. Here we show for the first time that T-ag nuclear import is negatively regulated by N-terminal sequences (amino acids 102-110), which represent the binding site (BS) for the retinoblastoma (Rb) tumor suppressor protein (p110(Rb)). Quantitative confocal laser scanning microscopic analysis of the transport properties of T-ag constructs with or without Rb binding site mutations in living transfected cells or in a reconstituted nuclear transport system indicates that the presence of the RbBS significantly reduces nuclear accumulation of T-ag. A number of approaches, including the analysis of T-ag nuclear import in an isogenic cell pair with and without functional p110(Rb) implicate p110(Rb) binding as being responsible for the reduced nuclear accumulation, with the Ser(106) phosphorylation site within the RbBS appearing to enhance the inhibitory effect. Immunoprecipitation experiments confirmed association of T-ag and p110(Rb) and dependence thereof on negative charge at Ser(106). The involvement of p110(Rb) in modulating T-ag nuclear transport has implications for the regulation of nuclear import of other proteins from viruses of medical significance that interact with p110(Rb), and how this may relate to transformation.

Regulatory circuits controlling white versus brown adipocyte differentiation

Adipose tissue is a major endocrine organ that exerts a profound influence on whole-body homoeostasis. Two types of adipose tissue exist in mammals: WAT (white adipose tissue) and BAT (brown adipose tissue). WAT stores energy and is the largest energy reserve in mammals, whereas BAT, expressing UCP1 (uncoupling protein 1), can dissipate energy through adaptive thermogenesis. In rodents, ample evidence supports BAT as an organ counteracting obesity, whereas less is known about the presence and significance of BAT in humans. Despite the different functions of white and brown adipocytes, knowledge of factors differentially influencing the formation of white and brown fat cells is sparse. Here we summarize recent progress in the molecular understanding of white versus brown adipocyte differentiation, including novel insights into transcriptional and signal transduction pathways. Since expression of UCP1 is the hallmark of BAT and a key factor determining energy expenditure, we also review conditions associated with enhanced energy expenditure and UCP1 expression in WAT that may provide information on processes involved in brown adipocyte differentiation.

Interaction of the retinoblastoma protein (pRb) with the catalytic subunit of DNA polymerase delta (p125)

The retinoblastoma gene product (pRb) interacts with many cellular proteins to function in the control of cell division, differentiation, and apoptosis. Several pRb binding proteins complex with pRb through an amino acid sequence called the LXCXE motif. The catalytic subunit of DNA polymerase delta (p125) contains a LXCXE motif. To further study the biochemical function of this polymerase, we sought to determine if p125 interacts with pRb. Experiments using GST-pRb fusion proteins showed that p125 from breast epithelial (MCF10A) cell extracts associates with pRb. In addition, GST-p125 fusion proteins bound pRb from the same cell extracts. The pRb that associated with GST-p125 was largely unphosphorylated. Coimmunoprecipitation experiments using cell cycle synchronized cells revealed that p125 and pRb form a complex predominantly during G1 phase, the phase during which pRb is mostly unphosphorylated. In vitro phosphorylation of GST-pRb by the cyclin dependent kinases reduced the ability of p125 to associate with GST-pRh. Addition of the LXCXE containing protein SV40 large T antigen to GST-pRb blocks the ability of p125 to associate with pRb, suggesting that it may be through a LXCXE sequence by which p125 interacts with pRb. Finally, in vitro polymerase assays demonstrate that GST-pRb fusion protein stimulates DNA polymerase delta activity.

Novel mechanisms of E2F induction by BK virus large-T antigen: requirement of both the pRb-binding and the J domains

E2F activity is regulated in part by the retinoblastoma family of tumor suppressor proteins. Viral oncoproteins, such as simian virus 40 (SV40) large-T antigen (TAg), adenovirus E1A, and human papillomavirus E7, can disrupt the regulation of cellular proliferation by binding to pRb family members and dissociating E2F-pRb family protein complexes. BK virus (BKV), which infects a large percentage of the human population and has been associated with a variety of human tumors, encodes a TAg homologous to SV40 TAg. It has been shown that BKV TAg, when expressed at low levels, does not detectably bind to pRb family members, yet it induces a serum-independent phenotype and causes a decrease in the overall levels of pRb family proteins. The experiments presented in this report show that, despite the lack of TAg-pRb interactions, BKV TAg can induce transcriptionally active E2F and that this induction does in fact require an intact pRb-binding domain as well as an intact J domain. In addition, E2F-pRb family member complexes can be detected in both BKV and SV40 TAg-expressing cells. These results suggest the presence of alternate cellular mechanisms for the release of E2F in addition to the well-established model for TAg-pRb interactions. These results also emphasize a role for BKV TAg in the deregulation of cellular proliferation, which may ultimately contribute to neoplasia.

The human cytomegalovirus IE1-72 protein interacts with the cellular p107 protein and relieves p107-mediated transcriptional repression of an E2F-responsive promoter

The Rb-related p107 protein has been implicated as an important control element in proper cell cycle progression. The p107 protein is thought to restrict cellular proliferation in part through its interaction with the E2F family of transcription factors and is, therefore, a specific target for regulation by several DNA viruses. Here, we demonstrate that p107 protein levels are induced in a biphasic manner in human fibroblasts during productive infection by the human cytomegalovirus (HCMV). Expression patterns of p107 protein levels during HCMV infection of human embryonic lung cells (HELs) demonstrate a sustained induction from early to late times of infection. We also demonstrate that the HCMV immediate-early protein IE1-72 complexes in vivo with the p107 protein and that this interaction can be reconstituted in an in vitro system by using reticulocyte-translated protein. Our data demonstrate that the interaction between p107 and the IE1-72 protein occurs at times of infection that temporally match the second tier of p107 protein induction and the phosphorylation pattern of the IE1-72 protein. Furthermore, we show here that the ability of p107 to transcriptionally repress E2F-responsive promoters can be overcome by expression of the IE1-72 protein. This effect appears to be specific, since the IE1-72 protein is not capable of relieving Rb-mediated repression of an E2F-responsive promoter. Finally, our data demonstrate that HCMV infection can induce cellular proliferation in quiescent cells and that IE1-72 expression alone can, to a degree, drive a similar progression through the cell cycle. These data suggest that IE1-72-mediated transactivation of E2F-responsive promoters through alleviation of p107 transcriptional repression may play a key role in the cell cycle progression stimulated by HCMV infection.

Association of p300 and CBP with simian virus 40 large T antigen

p300 and the CREB-binding protein CBP are two large nuclear phosphoproteins that are structurally highly related. Both function, in part, as transcriptional adapters and are targeted by the adenovirus E1A oncoprotein. We show here that p300 and CBP interact with another transforming protein, the simian virus 40 large T antigen (T). This interaction depends on the integrity of a region of T which is critical for its transforming and mitogenic properties and includes its LXCXE Rb-binding motif. T interferes with normal p300 and CBP function on at least two different levels. The presence of T alters the phosphorylation states of both proteins and inhibits their transcriptional activities on certain promoters. Although E1A and T show little sequence similarity, they interact with the same domain of p300 and CBP, suggesting that this region exhibits considerable flexibility in accommodating diverse protein ligands.

The block of adipocyte differentiation by a C-terminally truncated, but not by full-length, simian virus 40 large tumor antigen is dependent on an intact retinoblastoma susceptibility protein family binding domain

Simian virus 40 (SV40) can promote cell transformation and suppress differentiation. It does this partly by targeting tumor suppressors such as p53 and members of the retinoblastoma susceptibility protein (Rb) family. This work concentrates on mechanisms by which SV40 large tumor antigen (SVLT) suppresses adipocyte differentiation. We created cell lines derived from murine 3T3-L1 preadipocytes expressing different versions of SV40 early-region sequences. SVLT-expressing cells failed to exhibit adipocyte morphology, to induce glycerophosphate dehydrogenase activity, and to induce differentiation-dependent mRNA for adipocyte P2. SVLT alone was sufficient, in the absence of SV40 small tumor antigen, to inhibit differentiation. A truncated SVLT containing only the N-terminal 121 amino acids (SVLT1-121) blocked differentiation, thus mapping at least one differentiation blocking function to the N-terminal region. K1 (Glu-107-->Lys) point mutants of SVLT, which are unable to bind to the Rb protein family or induce neoplastic transformation, are defective for blocking differentiation in the case of SVLT1-121 but retain the ability to block differentiation in the case of full-length SVLT. This finding demonstrates that Rb family proteins are important in regulating adipocyte differentiation but that other functions of full-length SVLT can block adipocyte differentiation independently of RB family binding and transformation.

Stabilization of the tumor suppressor p53 during cellular transformation by simian virus 40: influence of viral and cellular factors and biological consequences

To understand the process and biological significance of metabolic stabilization of p53 during simian virus 40 (SV40)-induced cellular transformation, we analyzed cellular and viral parameters involved in this process. We demonstrate that neither large T expression as such nor the cellular phenotype (normal versus transformed) markedly influence the stability of p53 complexed to large T in SV40 abortively infected BALB/c mouse fibroblasts. In contrast, metabolic stabilization of p53 is an active cellular event, specifically induced by SV40. The ability of SV40 to induce a cellular response leading to stabilization of p53 complexed to large T is independent from the cellular phenotype and greatly varies between different cells. However, metabolic stability was conferred only to p53 in complex with large T, whereas the free p53 in these cells remained metabolically unstable. Comparative analyses of cellular transformation in various cells differing in stability of p53 complexed to large T upon abortive infection with SV40 revealed a strong correlation between the ability of SV40 to induce metabolic stabilization and its transformation efficiency. Our data suggest that metabolic stabilization and the ensuing enhanced levels of p53 are important for initiation and/or maintenance of SV40 transformation.

The cell cycle and the retinoblastoma protein family

Tumor formation results from alterations in the control of normal cell proliferation. To further our understanding of the molecular mechanisms underlying the deregulation of cell proliferation much attention, over the past decade, has been focused on the function of proto-oncogenes. Cellular oncogenes are thought to be growth promoting. More recently, a class of genes known as tumor suppressors have come under intense study. Tumor suppressors are largely thought to restrain cell proliferation. The retinoblastoma protein (Rb) is one of a growing list of tumor suppressors. Concurrent with the study of tumor suppressor genes has been a rapid increase in our understanding of the cell cycle at the molecular level. Rb and a related protein p107 are involved in the processes of cell proliferation and differentiation. Each functionally interacts with and affects the activity of the transcription factor E2F as well as other transcription factors involved in cell proliferation and differentiation. Additionally, Rb and p107 are modified by, and/or form specific complexes with, several elements of the basic cell cycle machinery. Specifically, Rb and p107 interact with and are modified by various cyclins and cyclin dependent kinases (cdk), some of which have been shown to be essential for cell cycle progression and in some cases their deregulation has been implicated in the development of cancer. This review will attempt to convey our current functional and mechanistic understanding of the biological roles Rb and p107 play in proliferation, development and differentiation. A knowledge of the interplay between these positive and negative regulators of cell proliferation and differentiation, noted above, is central to our understanding of human cancer.

Expression and interactions of human adenovirus oncoproteins

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The regions of the retinoblastoma protein needed for binding to adenovirus E1A or SV40 large T antigen are common sites for mutations

The protein product of the retinoblastoma (RB) gene is thought to function in a pathway that restricts cell proliferation. Recently, transforming proteins from three different classes of DNA tumor viruses have been shown to form complexes with the RB protein. Genetic studies suggest that these interactions with the RB protein are important steps in transformation by these viruses. In order to understand better the function of the RB-viral oncoprotein complexes, we have mapped the regions of the RB protein that are necessary for these associations. Two non-contiguous regions of RB were found to be essential for complex formation with adenovirus E1A or SV40 large T antigen. These two regions are found between amino acids 393 and 572 and 646 and 772. Interestingly, these binding sites on RB overlap with the positions of naturally occurring, inactivating mutations of the RB gene. These results strongly suggest that these viral oncoproteins are targeting a protein domain that is an important site in the normal function of the RB protein.

Large T antigens of many polyomaviruses are able to form complexes with the retinoblastoma protein

Stable protein complexes between the large T antigens of mouse, monkey, baboon, or human polyomaviruses and the retinoblastoma protein were detected by an in vitro coimmunoprecipitation assay. All of the large T antigens tested were able to bind to both human and mouse retinoblastoma polypeptides, showing that these interactions have been conserved during evolution.

Complex formation of human papillomavirus E7 proteins with the retinoblastoma tumor suppressor gene product

The E7 proteins encoded by the human papillomaviruses (HPVs) associated with anogenital lesions share significant amino acid sequence homology. The E7 proteins of these different HPVs were assessed for their ability to form complexes with the retinoblastoma tumor suppressor gene product (p105-RB). Similar to the E7 protein of HPV-16, the E7 proteins of HPV-18, HBV-6b and HPV-11 were found to associate with p105-RB in vitro. The E7 proteins of HPV types associated with a high risk of malignant progression (HPV-16 and HPV-18) formed complexes with p105-RB with equal affinities. The E7 proteins encoded by HPV types 6b and 11, which are associated with clinical lesions with a lower risk for progression, bound to p105-RB with lower affinities. The E7 protein of the bovine papillomavirus type 1 (BPV-1), which does not share structural similarity in the amino terminal region with the HPV E7 proteins, was unable to form a detectable complex with p105-RB. The amino acid sequences of the HPV-16 E7 protein involved in complex formation with p105-RB in vitro have been mapped. Only a portion of the sequences that are conserved between the HPV E7 proteins and AdE1A were necessary for association with p105-RB. Furthermore, the HPV-16 E7-p105-RB complex was detected in an HPV-16-transformed human keratinocyte cell line.