Cellular reprogramming for pancreatic β-cell regeneration: clinical potential of small molecule control

Abstract

Recent scientific breakthroughs in stem cell biology suggest that a sustainable treatment approach to cure diabetes mellitus (DM) can be achieved in the near future. However, the transplantation complexities and the difficulty in obtaining the stem cells from adult cells of pancreas, liver, bone morrow and other cells is a major concern. The epoch-making strategy of transcription-factor based cellular reprogramming suggest that these barriers could be overcome, and it is possible to reprogram any cells into functional β cells. Contemporary biological and analytical techniques help us to predict the key transcription factors needed for β-cell regeneration. These β cell-specific transcription factors could be modulated with diverse reprogramming protocols. Among cellular reprogramming strategies, small molecule approach gets proclaimed to have better clinical prospects because it does not involve genetic manipulation. Several small molecules targeting certain epigenetic enzymes and/or signaling pathways have been successful in helping to induce pancreatic β-cell specification. Recently, a synthetic DNA-based small molecule triggered targeted transcriptional activation of pancreas-related genes to suggest the possibility of achieving desired cellular phenotype in a precise mode. Here, we give a brief overview of treating DM by regenerating pancreatic β-cells from various cell sources. Through a comprehensive overview of the available transcription factors, small molecules and reprogramming strategies available for pancreatic β-cell regeneration, this review compiles the current progress made towards the generation of clinically relevant insulin-producing β-cells.

Figure 1

Transcription factor-based cellular reprogramming. (A) Modern experimental techniques like DNA chips, expression arrays and next generation sequencer (Shown in the arrow) facilitate us to gain insight into the human genome and identify novel genes/factors conferring to diseases and/or cell fate modulation (B) Enforced transcriptional activation of defined factors reprogram human somatic fibroblasts into different cell types like pluripotent stem cells [8], cardiac progenitors [9] and hepatocytes [10].

Figure 2

Approaches to iPS cell generation and the obstacles to their clinical translation. (A) In canonical approach (dark-gray arrow), retroviral derived iPS cells could differentiate into varied cell types. However, the risk of tumor formation and their culture in xeno-medium can inhibit the clinical translation of these cells. Alternative non-integrating approaches [30,31] in which the overexpression of defined reprograming factors in various ways (green arrowhead) generate iPS cells that circumvent tumor formation, and when cultured in xeno-free medium, can avoid the xeno contamination that hinders their clinical translation. Several bottlenecks, including epigenetic errors, low efficiency, and protocol optimization, are highlighted (blue cylinder). (B) Various cell sources (microscopic images) can be reset for a journey back in time to iPS cells (illustrated as the time machine icon) [30].

Figure 3

Insulin producing cells (IPCs) from various cell sources. (A) Overexpression of the defined exogenous transcription factors (Pdx1; Pdx1/VP16 (fusion protein) + NeuroD; Pdx1/VP16 (fusion protein) + Ngn3) in liver cells generate IPCs [53,54]. (B) IPCs could also be induced from pancreatic non-β cells such as acinar cells and α-cells by forced expression of Ngn3 + Pdx1 + Mafa and Pax4, respectively [58]. (C) Adipose tissue-derived stem cells could be differentiated into functional IPCs by transducing Pdx1 [59].

Figure 4

Small molecules and β cell induction. Liver epithelial stem-like white blood (WB) cells can be reprogrammed into insulin-producing cells (IPCs) by sequential protocol using small molecules [68]. In the first stage, WB cells are dedifferentiated with 5-aza-2’-deoxycytidine (5-AZA) and Trichostatin A (TSA). Then, retinoic acid (RA) and a mix of insulin, transferrin and selenite (ITS) are added to induce PDX1-positive pancreatic precursor cells. Nicotinamide promotes maturation of IPCs in the last stage.

Figure 5

SAHA-PIP: potential tool for selective transcriptional activation. (A) Chemical structure of SAHA-pyrrole-imidazole polyamide (SAHA-PIP) conjugate capable of site-specific acetylation in the promoter region of p16 tumor suppressor gene [71]. (B) SAHA-PIP `δ` triggers the core pluripotency gene network in mouse embryonic fibroblasts through site-specific epigenetic activation. In human dermal fibroblasts [74], (C) SAHA-PIP K and (D) SAHA-PIP A activates gene regulatory network associated with and germ cell and pancreas function such as glucose metabolism, respectively [12,75].