Butler AE, Cao-Minh L, Galasso R, et al. of G1/S molecules to the cytoplasm of the human -cell represents an unanticipated obstacle to therapeutic human -cell expansion. Both type 1 and type 2 diabetes ultimately result from -cell deficiency. Although -cell replacement in Chlorthalidone humans can reverse diabetes, the paucity of -cells available from adult or juvenile human cadaveric islets, or from hES cell or iPS cell sources, makes this approach untenable for -cell replacement therapy on a public health level. Accordingly, a major goal of diabetes research is usually to develop means to induce human -cell proliferation and growth, targeting either endogenous human -cells or -cells produced ex lover vivo. This desire to expand human -cells is usually complicated by the fact that while there are numerous models of -cell replication in TLN1 juvenile rodents, adult cadaveric human -cellsthe major source of -cells available for research and therapeutic manipulationare notoriously refractory to induction of replication: indeed, no growth factors, mitogens, or (patho)physiologic maneuvers (such as pregnancy, partial pancreatectomy, or obesity) are known that are able to induce high rates of adult human -cell proliferation (1C12). Equally perplexingly, we have little understanding as to why this is. This is particularly surprising because in contrast to the intractable quiescence of adult human -cells, fetal and neonatal human -cells can and do replicate transiently from ~5 months antepartum to ~6 months postpartum (13C15). Yet, even here replication is very low: in the 3% range (13C15). Chlorthalidone Further, we are only beginning to understand the physiological signals or mechanisms that activate and Chlorthalidone then inactivate this perinatal -cell proliferation. As one example, we have only recently learned that loss of the platelet-derived growth factor (PDGF) receptor- in adult human -cells, with the resultant loss of ability to activate mitogen-activated protein kinase and methylation (Ezh2) and downstream cell cycle (p16) machinery, may underlie the refractoriness of human -cells to proliferation (16). With the goal of understanding how best to encourage human -cells to replicate, we as well as others previously delineated the repertoire of G1/S regulatory proteins present in the adult human islet and have used this information to develop a working model of the human islet G1/S proteome (12,14C29), hoping that it might be Chlorthalidone useful in developing therapeutic approaches to manipulating human -cell proliferation. Since many, and perhaps most, G1/S molecules are regulated at the level of protein stability, rather than or in addition to transcription (24,26,29), we have focused in this G1/S model on immunoblots of whole human islets rather than exploring mRNA expression of these molecules. The Chlorthalidone G1/S model has confirmed useful in predicting approaches to driving human -cell proliferation in in vitro and in vivo systems. For example, the model accurately predicted that it should be possible to induce pRb phosphorylation (and thus its inactivation) and thereby markedly activate adult human -cell replication (10C15% as assessed using BrdU incorporation or Ki67 immunohistochemistry) by overexpression of combinations of G1/S cyclins and cdks such as the d-cyclins, cyclin E, or cdks 2, 4, or 6 both in cultured adult human -cells and in transplanted adult human -cells in vivo (21C23,26). Further, it is also possible to use cyclin/cdk combinations to induce human -cell proliferation not only constitutively or constantly but also using doxycycline-inducible delivery systems to transiently induce human -cell proliferation in a regulated, reversible fashion that mimics the transitory replication that occurs in embryonic and neonatal life (28). However, the human islet G1/S proteome model is not perfect. One major limitation is usually that it was derived from immunoblots of whole human islets. This is problematic because it is usually well-known that human islets are composed of many cells types in addition to -cells..