Enhanced self-association of mucins possessing the T and Tn carbohydrate cancer antigens at the single-molecule level

Mucins are linear O-glycosylated glycoproteins involved in inflammation, cell adhesion, and tumorigenesis. Cancer-associated mucins often possess increased expression of the T (Galβ1,3GalNAcαThr/Ser) and Tn (GalNAcαThr/Ser) cancer antigens, which are diagnostic markers for several cancers, including colon cancer. We have used AFM based single-molecule forced unbinding under near physiological conditions to investigate the self-interactions between porcine submaxillary mucin (PSM) as well as between PSM analogs possessing various carbohydrates including the T- and Tn-antigen. Distributions of unbinding forces and corresponding force loading rates were determined for force loading rates from 0.18 nN/s to 39 nN/s, and processed to yield most probable unbinding forces f* and lifetimes of the interactions. Parameter f* varied in the range 27 to 50 pN at force loading rates of about 2 nN/s among the various mucins. All mucin samples investigated showed self-interaction, but the tendency was greatest for PSM displaying only the Tn-antigen (Tn-PSM) or a mixture of Tn-, T-antigen, and the trisaccharide Fucα1,2Galβ1,3GalNAc (Tri-PSM). Weaker self-interactions were observed for native PSM (Fd-PSM), which consists of a nearly equal mixture of the longer core 1 blood group A tetrasaccharide (GalNAcα1,3(Fucα1,2)Galβ1,3GalNAcαSer/Thr) and Tn-antigen. The data are consistent with the truncated Tn and T glycans enhancing self-interaction of the mucins. These carbohydrate cancer antigens may, thus, play an active role in the disease by constitutively activating mucin and mucin-type receptors by self-association on cells.

Potential mechanisms of the human polyomavirus JC in neural oncogenesis

The human polyomavirus JC (JCV) is a small DNA tumor virus and the etiologic agent of the progressive multifocal leukoencephalopathy. In progressive multifocal leukoencephalopathy, active JCV replication causes the lytic destruction of oligodendrocytes. The normal immune system prevents JCV replication and suppresses the virus into a state of latency so that expression of viral proteins cannot be detected. In a cellular context that is nonpermissive for viral replication, JCV can affect oncogenic transformation. For example, JCV is highly tumorigenic when inoculated into experimental animals, including rodents and monkeys. In these animal tumors, there is expression of early T-antigen but not of late capsid proteins, nor is there viral replication. Moreover, mice transgenic for JCV T-antigen alone develop tumors of neural tube origin. Detection of JCV genomic sequences and expression of viral T-antigen and agnoprotein suggest a possible association of this virus with a variety of human brain and non-CNS tumors. Here, we discuss the mechanisms involved in JCV oncogenesis, briefly review studies that do and do not support a causative role for this virus in human CNS tumors, and identify key issues for future research.

Inhibition of DNA synthesis by RB: effects on G1/S transition and S-phase progression

The retinoblastoma tumor suppressor protein, RB, is a negative regulator of cell proliferation. Growth inhibitory activity of RB is attenuated by phosphorylation. Mutation of a combination of phosphorylation sites leads to a constitutively active RB. In Rat-1 cells, the phosphorylation-site-mutated (PSM)-RB, but not wild-type RB, can inhibit S-phase entry. In PSM-RB-arrested G1 cells, normal levels of cyclin E and cyclin E-associated kinase activity were detected, but the expression of cyclin A was inhibited. The ectopic expression of cyclin E restored cyclin A expression and drove the PSM-RB expressing cells into S phase. Interestingly, Rat-1 cells coexpressing cyclin E and PSM-RB could not complete DNA replication. Microinjection of cells that have passed through the G1 restriction point with plasmids expressing PSM-RB also led to the inhibition of DNA synthesis. The S-phase inhibitory activity of PSM-RB could be attenuated by the coinjection of SV40 T-antigen, adenovirus E1A, or a high level of E2F-1 expression plasmids. However, the S-phase inhibitory activity of PSM-RB could not be overcome by the coinjection of cyclin E or cyclin A expression plasmids. These results reveal a novel role for RB in the inhibition of S-phase progression that is distinct from the inhibition of the G1/S transition, and suggest that continued phosphorylation of RB beyond G1/S is required for the completion of DNA replication.

Transformation by simian virus 40 induces virus-specific, related antigens in the surface membrane and nuclear envelope

Nucleus- and mitochondrion-free membranes from hamster lymphocytes transformed by simian virus 40 (SV40), GD248 cells, cause guinea pigs to produce immune sera that reveal the presence in GD248 plasma membranes and mitochondria of two types of glycoprotein that are not detected in membranes of normal lymphocytes [Schmidt-Ullrich, R., Thompson, W. S. & Wallach, D. F. H. (1977) Proc. Natl. Acad. Sci. USA 74, 643-647]. Indirect immune fluorescence of living, SV40-transformed T19 hamster reticulum cells, Balb/c 3T3 mouse fibroblasts, and W18 VA2 human fibroblasts, using the antisera against GD248 membrane, at 4 degrees produced a distinct cell surface fluorescence; however, above 20 degrees , staining at the nuclear perimeter, the SV40 U-antigen reaction, becomes equally prominent. In SV40-transformed cells that had been fixed in cold acetone, as well as in purified GD248 nuclei, thermostable U-antigen staining is dramatic, but there is no reaction for nuclear T-antigen. Rabbit antisera against T19 cells gave immunofluorescence reactions equivalent to those obtained with the antisera against GD248 cells. Normal guinea pig or rabbit sera and cells that had not been transformed by SV40 gave no reaction. Our sera from tumor-bearing hamsters gave only nuclear T-antigen fluorescence. The results indicate the presence of related, SV40-specific antigens in the surface membranes, nuclear envelope, and possibly other intracellular organelles of SV40-transformed cells.

Analysis of the events leading to SV40-induced chromosome replication and mitosis in primary mouse kidney cell cultures

Abortive infection with simian virus 40 in confluent, "contact-inhibited", mouse kidney cell cultures was studied. The sequential events, in individual cells, are tentatively represented by the simplified scheme: (a) transcription of early virus-specific (messenger) RNA; (b) appearance of T-antigen; (c) [psychrosensitive event(s)]; (d) chromosome replication; (e) normal or abnormal mitosis. No evidence for the replication of viral progeny DNA was obtained. The sequence of the events (a)-(d) is analogous to that observed in contact-inhibited mouse kidney tissue culture cells during lytic infection with polyoma virus.