Primary rat cells expressing a hybrid polyomavirus-simian virus 40 large T antigen have altered growth properties

Stem Cell Secretome and Its Effect on Cellular Mechanisms Relevant to Wound Healing

Stem cells introduced to site of injury primarily act via indirect paracrine effects rather than direct cell replacement of damaged cells. This gives rise to understanding the stem cell secretome. In this study, in vitro studies demonstrate that the secretome activates the PI3K/Akt or FAK/ERK1/2 signaling cascades and subsequently enhances the proliferative and migratory abilities of various types of skin cells, such as fibroblasts, keratinocytes, and vascular epithelial cells, ultimately accelerating wound contraction. Indeed, inhibition of these signaling pathways with synthetic inhibitors resulted in the disruption of secretome-induced beneficial effects on various skin cells. In addition, major components of the stem cell secretome (EGF, basic FGF, and HGF) may be responsible for the acceleration of wound contraction. Stimulatory effects of these three prominent factors on wound contraction are achieved through the upregulation of PI3K/Akt or FAK/ERK1/2 activity. Overall, we lay the rationale for using the stem cell secretome in promoting wound contraction. In vivo wound healing studies are warranted to test the significance of our in vitro findings.

Tiny T antigen: an autonomous polyomavirus T antigen amino-terminal domain

Three mRNAs from the murine polyomavirus early region encode the three well-characterized tumor antigens. We report the existence of a fourth alternatively spliced mRNA which encodes a fourth tumor antigen, tiny T antigen, which comprises the amino-terminal domain common to all of the T antigens but is extended by six unique amino acid residues. The amount of tiny T antigen in infected cells is small because of its short half-life. Tiny T antigen stimulates the ATPase activity of Hsc70, most likely because of its DnaJ-like motif. The common amino-terminal domain may interface with chaperone complexes to assist the T antigens in carrying out their diverse functions of replication, transcription, and transformation in the appropriate cellular compartments.

Functional implications of mutations within polyomavirus large T antigen Rb-binding domain: effects on pRb and p107 binding in vitro and immortalization activity in vivo

In this study, we have extensively modified the Rb-binding domain of polyomavirus large T antigen. Mutant polyomavirus large T antigens were tested for their ability to bind pRb and p107 in vitro and assayed for their capacity to immortalize primary rat embryo fibroblasts in vivo. Polyomavirus large T antigen bound pRb and p107 through a common region located between amino acids 141 to 158, containing the consensus Rb-binding sequence D/N-L-X-C-X-E. Substitution of any amino acid within the core Rb-binding sequence abolished pRb and p107 binding in vitro and immortalization activity in vivo. Substitution of amino acids outside the core Rb-binding sequence reduced pRb and p107 binding in vitro and decreased or abolished immortalization of rat embryo fibroblasts in vivo. Although duplication of the Rb-binding domain within the polyomavirus large T antigen results in a molecule that can bind at least twice as much pRb and p107 in vitro, this mutant displayed an essentially wild-type level of immortalization activity. More importantly, we found that the addition of acidic residues within the casein kinase II consensus phosphorylation region flanking the Rb-binding domain, or the deletion of amino acids 256 to 272, increased the immortalizing activity of the mutant polyomavirus large T antigen. These two mutants displayed a greater than wild-type level of pRb binding in vitro, while in contrast, a decreased affinity for p107 binding in vitro was observed. Together, these results indicate that while pRb binding appears to be an essential event for immortalization, there is no tight correlation between the frequency of immortalization and the absolute level of pRb binding in vitro, indicating that other large T antigen functions are important for cellular immortalization.