Jessica C. Langer

Constitutive Expression of the Cyclin-dependent Kinase Inhibitor p21 Is Transcriptionally Regulated by the Tumor Suppressor Protein p53

The tumor suppressor protein p53 has been implicated in the response of cells to DNA damage. Studies to date have demonstrated a role for p53 in the transcriptional activation of target genes in the cellular response to DNA damage that results in either growth arrest or apoptosis. In contrast, here is demonstrated a role for p53 in regulating the basal level of expression of the cyclin-dependent kinase inhibitor p21 in the absence of treatment with DNA-damaging agents. Wild-type p53-expressing MCF10F cells had detectable levels of p21 mRNA and protein, whereas the p53-negative Saos-2 cells did not. Saos-2 cells were infected with recombinant retrovirus to establish a proliferating pool of cells with a comparable constitutive level of expression of wild-type p53 protein to that seen in untreated MCF10F cells. Restoration of wild-type but not mutant p53 expression recovered a basal level of expression of p21 in these cells. Constitutive expression of luciferase reporter constructs containing the p21 promoter was inhibited by co-transfection with the human MDM2 protein or a dominant-negative p53 protein and was dependent on the presence of p53 response elements in the reporter constructs. Furthermore, p53 in nuclear extracts of untreated cells was capable of binding to DNA in a sequence-specific manner. These results implicate a role for p53 in regulating constitutive levels of expression of p21 and demonstrate that the p53 protein is capable of sequence-specific DNA binding and transcriptional activation in untreated, proliferating cells.

Quantitative Trait Analysis Reveals Transforming Growth Factor-β2 as a Positive Regulator of Early Hematopoietic Progenitor and Stem Cell Function

Elucidation of pathways involved in mouse strain–dependent variation in the hematopoietic stem cell (HSC) compartment may reveal novel mechanisms relevant in vivo. Here, we demonstrate genetically determined variation in the proliferation of lin−Sca1++kit+ (LSK) primitive hematopoietic progenitor cells in response to transforming growth factor-β (TGF-β) 2, the dose response of which was biphasic with a stimulatory effect at low concentrations. In contrast, the dose responses of TGF-β1 or -β3 were inhibitory and did not show mouse strain–dependent variation. A quantitative trait locus (QTL) for the effect of TGF-β2 was identified on chromosome 4 overlapping with a QTL regulating the frequency of LSK cells. These overlapping QTL were corroborated by the observation that the frequency of LSK cells is lower in adult Tgfb2 +/− mice than in wild-type littermates, indicating that TGF-β2 is a genetically determined positive regulator LSK number in vivo. Furthermore, adult Tgfb2 +/− mice have a defect in competitive repopulation potential that becomes more pronounced upon serial transplantation. In fetal TGF-β2–deficient HSCs, a defect only appears after serial reconstitution. These data suggest that TGF-β2 can act cell autonomously and is important for HSCs that have undergone replicative stress. Thus, TGF-β2 is a novel, genetically determined positive regulator of adult HSCs.

The positive regulatory effect of TGF-beta2 on primitive murine hemopoietic stem and progenitor cells is dependent on age, genetic background, and serum factors.

TGF-β is considered a negative regulator of hemopoietic stem and progenitor cells. We have previously shown that one TGF-β isoform, TGF-β2, is, in fact, a positive regulator of murine hemopoietic stem cell function in vivo. In vitro, TGF-β2, but not TGF-β1 and TGF-β3, had a biphasic dose response on the proliferation of purified lin-Sca1++kit+ (LSK) cells, with a stimulatory effect at low concentrations, which was subject to mouse strain-dependent variation. In this study we report that the stimulatory effect of TGF-β2 on the proliferation of LSK cells increases with age and after replicative stress in C57BL/6, but not in DBA/2, mice. The age-related changes in the TGF-β2 effect correlated with life span in BXD recombinant strains. The stimulatory effect of TGF-β2 on the proliferation of LSK cells requires one or more nonprotein, low m.w. factors present in fetal calf and mouse sera. The activity of this factor(s) in mouse serum increases with age. Taken together, our data suggest a role for TGF-β2 and as yet unknown serum factors in the aging of the hemopoietic stem cell compartment and possibly in organismal aging.

Adeno-associated virus gene transfer into renal cells: potential for in vivo gene delivery.

The human parvovirus adeno-associated virus (AAV), type 2, has a number of features that make it an attractive choice as a vector for gene delivery to the kidney. AAV vectors permit long-term gene expression in vivo by integration into the host genome, have potential for site-specific integration on chromosome 19, do not express viral genes or generate a cellular immune response, and demonstrate enhancement of gene expression by chemotherapeutic agents that are approved for use in vivo. These properties confer advantages to AAV over other viral and nonviral methods for gene transfer. Preliminary experiments in our laboratory suggest that AAV is able to transfer genes to both renal cells in culture and the kidney in vivo. Thus, AAV has the potential to be an important gene transfer vector for the kidney in vivo.