Biomedical subjects

Ana D Lopez

Publications and source records attributed to Ana D Lopez.

4 recordsLinked to original sources

PubMed2005

Activin A maintains pluripotency of human embryonic stem cells in the absence of feeder layers.

To date, all human embryonic stem cells (hESCs) available for research require unidentified soluble factors secreted from feeder layers to maintain the undifferentiated state and pluripotency. Activation of STAT3 by leukemia inhibitory factor is required to maintain "stemness" in mouse embryonic stem cells, but not in hESCs, suggesting the existence of alternate signaling pathways for self-renewal and pluripotency in human cells. Here we show that activin A is secreted by mouse embryonic feeder layers (mEFs) and that culture medium enriched with activin A is capable of maintaining hESCs in the undifferentiated state for >20 passages without the need for feeder layers, conditioned medium from mEFs, or STAT3 activation. hESCs retained both normal karyotype and markers of undifferentiated cells, including Oct-4, nanog, and TRA-1-60 and remained pluripotent, as shown by the in vivo formation of teratomas.

Activins↗

PubMed2004

Maintenance of pluripotency in human embryonic stem cells is STAT3 independent.

The preservation of "stemness" in mouse embryonic stem (mES) cells is maintained through a signal transduction pathway that requires the gp130 receptor, the interleukin-6 (IL-6) family of cytokines, and the Janus Kinase-signal transducer and activator (JAK/STAT) pathway. The factors and signaling pathways that regulate "stemness" in human embryonic stem (hES) cells remain to be elucidated. Here we report that STAT3 activation is not sufficient to block hES cell differentiation when the cells are grown on mouse feeder cells or when they are treated with conditioned media from feeder cells. Human ES cells differentiate in the presence of members of the IL-6 family of cytokines including leukemia inhibitory factor (LIF) and IL-6 or in the presence of the designer cytokine hyper-IL-6, which is a complex of soluble interleukin-6 receptor (IL-6R) and IL-6 with greatly enhanced bioactivity. Human ES cells express LIF, IL-6, and gp130 receptors, as well as the downstream signaling molecules. Stimulation of human and mouse ES cells with gp130 cytokines resulted in a robust phosphorylation of downstream ERK1, ERK2, and Akt kinases, as well as the STAT3 transcription factor. Loss of the pluripotency markers Nanog, Oct-4, and TRA-1-60 was observed in hES cells during gp130-dependent signaling, indicating that signaling through this pathway is insufficient to prevent the onset of differentiation. These data underscore a fundamental difference in requirements of murine versus hES cells. Furthermore, the data demonstrate the existence of an as-yet-unidentified factor in the conditioned media of mouse feeder layer cells that acts to maintain hES cell renewal in a STAT3-independent manner.

Animals↗

PubMed2002

Exendin 4 up-regulates expression of PDX 1 and hastens differentiation and maturation of human fetal pancreatic cells.

In addition to stimulating insulin secretion, glucagon-like peptide and its long-acting analog exendin 4 have been reported to increase beta-cell mass by both differentiation/neogenesis of precursor cells and enhanced replication of existing beta-cells. Here, we investigated the effect of exendin 4 in the growth and differentiation of beta-cells from undifferentiated precursors in islet-like cell clusters (ICCs) derived from human fetal pancreases. Our results show that the addition of exendin 4 to the culture media stimulates PDX 1 expression in ICCs as shown by immunofluorescence staining. The up-regulation of PDX 1 was not accompanied by changes in insulin expression because we did not find a significant difference in the number of insulin-positive cells in the exendin 4-treated ICCs, compared with controls. We also tested the effects of exendin 4 in the glucose-induced insulin secretion of human ICCs transplanted under the kidney capsule of athymic rats. In the exendin 4-treated rats (given ip during 10 d) 8 wk after the beginning of the treatment, insulin was released in response to glucose as detected by the measurement of circulating human C-peptide. In control (saline-treated) rats, the basal levels of human C-peptide did not change significantly after glucose stimulation. Thus, exendin 4 induces functional maturation of fetal beta-cells in response to glucose. In these rats, serial sections of the kidney-bearing grafts were examined histologically for insulin containing cells. We found a significant increase in beta-cell number, compared with the control rats. Overall, these results show that in vivo exendin 4 causes growth and differentiation of human fetal beta-cells from undifferentiated precursor cells. It also accelerates the functional maturation of fetal beta-cells as evidenced by their glucose-stimulated insulin secretion.

Animals↗

PubMed2002

A novel approach to increase human islet cell mass while preserving beta-cell function.

Human islet expansion in monolayer culture leads to loss of function and senescence. By maintaining the 3-D configuration of islets in fibrin gels, it is feasible to expand beta-cells in response to hepatocyte growth factor (HGF) while preserving physiologic glucose responsiveness both in vitro and in vivo after transplantation into nude mice. Islets were cultured free floating with or without growth factors and nicotinamide and in fibrin gels with the same conditions. Proliferation was observed only in islets cultured in fibrin gels and the cocktail; total insulin increased by threefold, with a concomitant increase in beta-cell mass by morphometry. Insulin release after glucose challenge was also preserved. Islets in fibrin gels gave rise in vivo to large grafts rich in insulin and glucagon, and grafts from free-floating islets were smaller with fewer endocrine cells. Circulating human C-peptide levels were higher than in the mice receiving free-floating islets. In summary, fibrin allows for HGF-mediated cell proliferation while preserving glucose responsiveness in an environment that preserves cell-cell contacts. Limited islet ex vivo expansion under these conditions may improve recipient-donor tissue ratios to equal the functional results of whole-organ transplants.

Animals↗