Numerous studies conducted in a diversity of adult tissues have shown that certain stem cells are characterized by the expression of a protein known as the ABCG2 transporter (where ABC is ATP- binding cassette). In the adult pancreas, although various multipotent progenitors have been proposed, the ABCG2 marker has only been detected in the so-called 'side population' (a primitive haematopoietic cell population with a multipotential capacity). In the present study we sought to identify new ABCG2+ pancreatic cell populations and to explore whether they exhibit the properties of progenitor cells. We isolated and expanded mitoxantrone-resistant cells from pancreata of lactating rats by drug selection. These cells were characterized and maintained in different stages of differentiation using several media 'cocktails' plus Matrigel (BD Biosciences). Differentiation was assessed by RT-PCR (reverse transcription-PCR), immunocytochemistry, electron microscopy and ELISA. The expanded cell population demonstrated a phenotype of PaSCs (pancreatic stellate cells). Spontaneous cell clusters occurred during cell expansion and they showed weak expression of the transcription factor Pdx1 (pancreatic and duodenal homeobox 1). Moreover, the presence of inductive factors in the Matrigel plus exendin-4 led to an increase in Pdx1 and endocrine genes, such as insulin, islet amyloid polypeptide, glucagon, the glucose transporter GLUT2, chromogranin A and the convertases PC1/3 and PC2 were also detected. Immunocytochemical analysis showed co-localization of insulin and C-peptide, whereas ultrastructural studies revealed the presence of granules. Insulin secretion from cell clusters was detected in the cell culture medium. We identified a population of PaSCs that express the ABCG2+ transporter and have the capacity to transdifferentiate into insulin-producing cells. Although the potential therapeutic application remains to be tested, PaSCs could represent a future option for insulin replacement in diabetes research.

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We have previously reported the identification of a pancreata mitoxantrone-resistant cell population which expressed the ABCG2 transporter with a pancreatic stellate cells phenotype (PaSC) and ability of secreting insulin after inducing their differentiation. The characterization of the secretome of this cell population by two-dimensional electrophoresis (2D) coupled with mass spectrometry MALDI-TOF was able to identify seventy-six protein spots involved in different cellular processes: development/differentiation, proteases, immune response, and other. Moreover, Ingenuity Pathway Analysis displayed several significant networks and TGFβ1 molecule was identified as a central node of one of them. The effect of this active molecule secreted in the conditioned medium was investigated in ductal cell line (ARIP). The results showed that the conditioned medium inhibited their proliferation without affecting their cell viability. Additionally, they showed an upregulation of PDX1 and downregulation of CK19. The rate of ARIP cell proliferation was recovered, but no effects on the gene expression were observed after using TGFβ1-neutralising antibody. Proteins associated with cell growth, development and differentiation such as PEDF, LIF, and Wnt5b, identified in the secretome, could be involved in the observed transcription changes. These finding may suggest a new paracrine action of PaSCs involved in the proliferation and differentiation pathways not yet identified.