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Proc Natl Acad Sci U S A. Aug 24, 2010; 107(34): 15099–15104.
Published online Aug 9, 2010. doi:  10.1073/pnas.1010018107
PMCID: PMC2930573
Cell Biology

Small-molecule inducers of insulin expression in pancreatic α-cells

Abstract

High-content screening for small-molecule inducers of insulin expression identified the compound BRD7389, which caused α-cells to adopt several morphological and gene expression features of a β-cell state. Assay-performance profile analysis suggests kinase inhibition as a mechanism of action, and we show that biochemical and cellular inhibition of the RSK kinase family by BRD7389 is likely related to its ability induce a β-cell-like state. BRD7389 also increases the endocrine cell content and function of donor human pancreatic islets in culture.

Keywords: BRD7389, pancreatic islets, Rsk kinase, transdifferentiation, beta cells

Type 1 diabetes is an autoimmune disease characterized by the loss of insulin-producing β-cells in pancreatic islets of Langerhans. Islet transplantation into the liver can effectively cure the disease (1), but is not an ideal treatment due to limited donor material and immunological complications. An alternative approach, not yet feasible, is to create new β-cells (2), either by stepwise differentiation of undifferentiated stem or stem-like cells (3), or by transdifferentiation (4), the heritable change of cell identity to an insulin-producing (β-like) cell. The latter approach could result in a replacement source for the deficient cell type directly from patient material (either in vivo or ex vivo). Increasing β-cell mass by small-molecule drug-induced transdifferentiation is a speculative but exciting approach to treating diabetes—one that is significantly different from currently available small-molecule drugs that increase insulin secretion in existing β-cells and are therefore ineffective in the later stages of type 1 diabetes, in which most β-cell mass has been lost.

Cell-type specification in the pancreas is regulated by a set of master regulatory transcription factors that control the transition from one progenitor cell state to the next, ultimately yielding mature endocrine cell types in islets (5). Recently, it has been shown that misexpression of these master regulatory transcription factors causes direct transdifferentiation between cell types. For example, ectopic overexpression of a single transcription factor (Arx) is sufficient for in vivo conversion of β-cells to α-cells in the adult mouse pancreas (6). Similarly, viral delivery of three transcription factors (Pdx1, Ngn3, MafA) to an adult mouse pancreas causes the transdifferentiation of acinar cells to β-cells (7). Finally, in vivo conversion of α-cells to β-cells has recently been achieved in mature mouse α-cells by ectopic overexpression of Pax4 (8).

Results

Because a single gene is sufficient to induce transdifferentiation of α-cells to β-cells, we sought to determine whether a small molecule could have the same effect. Possible readouts for induction of a β-cell state include insulin production and insulin secretion. We chose to target the production of insulin protein because we imagined that this would be more feasible to achieve in the course of a 3-d small-molecule treatment than insulin secretion. To that end, we developed a high-content, cell-based assay to detect insulin protein expression in the mouse α-cell line αTC1. Normal mouse α-cells are insulin negative, but have the ability to adopt a β-cell phenotype after extreme β-cell loss (9). Similarly, the α-cell line we used spontaneously reexpressed small but detectable levels of insulin, despite being a subclone selected for low insulin protein (10). During assay development and optimization, we could show, by spiking in β-cells and by antibody competition, that our assay was sensitive enough to reliably detect insulin levels in as few as 3% of cells, and at 15-fold lower levels than in β-cells (Fig. S1).

We screened 30,710 compounds for induction of insulin production using this assay and identified a molecule, BRD7389 (Fig. 1A), that after 3-d treatment induced insulin expression in mouse α-cells. BRD7389 induced a dose-dependent up-regulation of Ins2 mRNA, peaking at ≈0.85 μM; 5-d treatment with BRD7389 resulted in greater induction of insulin gene expression, about 50-fold at 0.85 μM (Fig. 1B), which could not be further increased by longer treatments up to 21 d. This compound appears to be specific to α-cells, because a pancreatic ductal cell line (PANC-1) showed no induction, and a mouse β-cell line (βTC3) no further increase of insulin expression. In addition to insulin expression, BRD7389 significantly up-regulated expression of Pdx1 (Fig. 1C), a master regulatory transcription factor that specifies pancreatic progenitors and directly activates the insulin promoter (11). We also observed a dose-dependent increase in the expression of other β-cell markers, including Pax4, Iapp, and Npy, after a 5-d treatment with BRD7389 (Fig. S2).

Fig. 1.
BRD7389 stimulates insulin production in a mouse α-cell line. (A) Structure of BRD7389. Quantitative PCR analysis of (B) insulin (Ins2) and (C) Pdx1 expression following 3- and 5-d treatment with the indicated concentrations of BRD7389. Gene expression ...

Treatment with BRD7389 caused a stable change in cell shape from a fibroblast-like morphology, characteristic of α-cells, to a clustered state resembling β-cells in culture (Fig. 1 D–F, Left). Finally, we detected low levels of insulin protein in compound-treated α-cells by immunofluorescence (Fig. 1 D–F, Right). Relative to background fluorescence in DMSO-treated α-cells, insulin staining is induced 1.5-fold following 5-d treatment with BRD7389, compared with 4-fold higher levels in β-cells. Both insulin mRNA and protein levels are significantly increased from a basal α-cell state in compound-treated cells, but do no reach levels detected in mature β-cells. Therefore, although these cells have not achieved a β-cell state, they have adopted several features of β-cells.

To 0identify the mechanism of action of BRD7389, we used screening data in ChemBank (12) to compare assay performance of BRD7389 with 9,995 other small molecules in a total of 32 assays involving both BRD7389 and other compounds. This computational method looks for similarity of biological assay-performance profiles among a diverse set of compounds, including many known “bioactives.” We uncovered multiple connections of BRD7389 to known kinase inhibitors. Accordingly, we profiled this compound at 10 μM against a panel of 219 kinases, selected to represent a diverse subset of the human kinome (13). We observed significant inhibition of a number of kinases, including FLT3, DRAK2, and the RSK family (Fig. 2A and Table S1). To validate these profiling results, we obtained dose–response curves and determined half-maximal inhibitory concentration (IC50) values for BRD7389 and the most potently inhibited kinases (Fig. S3). The compound was most active against the entire RSK family of kinases, with IC50 values of 1.5 μM, 2.4 μM, and 1.2 μM for RSK1, RSK2, and RSK3, respectively (Fig. 2B). Therefore, we focused on investigating the role of RSK kinases in α-cells.

Fig. 2.
BRD7389 inhibits the RSK family of kinases. (A) Inhibition of the AGC family of kinases (figure modified from ref. 13). Biochemical inhibition of selected kinases by 10 μM BRD7389 was tested at an ATP concentration within 15 μM of the ...

In addition to measuring the biochemical in vitro inhibition of RSKs, we also determined the functional consequences of BRD7389 on Rsk activity in mouse α-cells. All Rsk kinases consist of two functional domains, which are activated through a series of consecutive phosphorylation events (14). Kinase activity was measured using pan- and phospho-specific antibodies to detect total and active Rsk protein in αTC1 cells. Western blot analysis revealed a 50% decrease in kinase activity, as measured by autophosphorylation of both N-terminal and C-terminal domains, at concentrations above 3.4 μM (Fig. 2 C and E). Phosphorylation of ribosomal protein S6 at serines 235 and 236, direct targets of the Rsk kinases (15), was reduced by a similar amount after compound treatment (Fig. 2 D and F). These findings confirm that BRD7389 has activity as an Rsk family kinase inhibitor in vitro and in cell culture.

We then sought to determine whether knockdown of Rsk family members would have an effect on insulin production in α-cells. We observed 2- to 4-fold increases in insulin expression upon RNAi of individual Rsk proteins, especially Rsk2 and Rsk3, but the effect is not as strong as compound treatment with BRD7389 (Fig. 2G). The knockdown efficiency was at least 50% for all constructs (Fig. 2H), and better knockdown did not correlate with stronger induction of insulin expression. Similar to compound treatment, which causes maximum induction of insulin expression at concentrations around the biochemical IC50 for Rsks, only partial knockdown of the enzymes seems optimal for insulin induction.

Though mouse α-cells are useful for screening, species differences and potential microenvironmental factors make testing compounds in human pancreatic cells essential. Using human donor-derived pancreatic islets, we tested BRD7389 in dissociated islet cells cultured on an extracellular matrix (16) designed to preserve the functional characteristics of β-cells. Though we did observe donor-to-donor variability in the response to BRD7389, some observations were shared among islets from donors with a low body-mass index (BMI) (Fig. 3 and Figs. S4S8). For example, 5-d treatment with BRD7389 enhanced glucose-stimulated insulin secretion (GSIS) in both high- (16.7 mM) and low-glucose (1.67 mM) conditions (Fig. 3A), as well as glucose-stimulated glucagon secretion (GSGS) in low-glucose (1.67 mM) conditions (Fig. 3B). Moreover, we detected a dose-dependent increase in the expression of endocrine hormones and transcription factors following 5-d compound treatment (Fig. 3C). Microscopy revealed that the total number of cells in culture decreased, with ≈50% of cells remaining at 3.4 μM BRD7389 (Fig. 3 D and E). Nonetheless, the β-cell population remained essentially unchanged, decreasing only slightly at higher compound concentrations, whereas the α-cell population decreased dramatically at high concentrations (Fig. 3D). Staining for cleaved caspase 3, an indicator of apoptosis, revealed an increase in the fraction of total cells undergoing apoptosis (Fig. S9). Whereas other cell types start undergoing apoptosis at 1.7 μM BRD7389, β-cells are only marginally affected at the highest concentration tested. These differences in viability and the resulting changes in the ratios of cell types are likely too small to account for the increases in expression of β-cell-specific genes, suggesting that treatment with BRD7389 either induces β-cell-like characteristics in non–β-cells, or enhances existing β-cell function in human pancreatic islet culture.

Fig. 3.
BRD7389 affects primary human islets. (A) Glucose-stimulated insulin secretion after 5-d treatment with the indicated concentration of BRD7389. Data represent mean ± SD of four replicates. Single asterisk indicates significant difference from ...

Discussion

In summary, we have identified a unique small molecule that up-regulates insulin expression, normally a defining property of pancreatic β-cells, in terminally differentiated α-cells. A mechanism potentially involving the inhibition of RSK kinases is supported by the increase in insulin expression following knockdown of individual RSK kinases. Our findings raise the possibility that BRD7389 functions by inhibiting multiple RSK family members simultaneously. Interestingly, previously described RSK inhibitors (17) FMK and BI-D1870 did not induce insulin expression in α-cells. These compounds inhibit not only RSK enzymes, but also members of several other kinase families (18). These data suggest that a tight specificity profile for different kinases might be necessary for optimal induction of insulin expression in α-cells. Therefore, a systematic evaluation of the entire kinome by both small-molecule and knockdown approaches will better define the roles of on- and off-target effects and may lead to the identification of conditions for complete transdifferentiation to β-cells.

BRD7389 also increases β-cell–specific gene expression in primary human islet cells. These experiments could in principle be confounded by differences in donor age, sex, BMI, and the purity and viability of islet batches. We found that differences in BMI appear to influence compound effects; there was an increase in endocrine hormone secretion in islets from lower BMI donors, whereas islets from high-BMI donors had attenuated responses. Interestingly, primary human islet cells tolerate higher concentrations of BRD7389 than the mouse α-cell line used here. Although we observed pronounced compound effects on endocrine cell numbers and function, it is not clear whether these effects are mediated through effects on α-cells or other cell types in this culture system. Future experiments involving in vivo β-cell ablation, lineage tracing in animal models, and purified human α-cells will help illuminate the effects of BRD7389 in greater detail.

These findings show the feasibility of identifying compounds that induce insulin expression in α-cells and suggest small-molecule approaches to increase β-cell mass by transdifferentiation in vivo.

Materials and Methods

Reagents.

Compound BRD7389 (kbsa-0113758) was obtained from Aurora Fine Chemicals Ltd. All other reagents were obtained from Sigma Aldrich unless otherwise stated. Primers were bought from Eurofins MWG Operon, except for Rsk2 and Rsk3 primers, which were ordered from Applied Biosystems. Antibodies used in this study were insulin (Sigma I8510), glucagon (Sigma G2654), RSK1/RSK2/RSK3 (32D7; Cell Signaling Technology, CST 9355), phospho-p90RSK (Thr359/Ser363; CST 9344), phospho-p90RSK (Thr573; CST 9346), S6 ribosomal protein (CST 2217), phospho-S6 ribosomal protein (Ser235/236; CST 2211), phospho-S6 ribosomal protein (Ser240/244; CST 2215), β-actin (Sigma A1978), and cleaved-caspase 3 (Abcam, ab13847). Fluorescently labeled secondary antibodies were purchased from Jackson ImmunoResearch. IRDye antibodies for Western blots were purchased from Odyssey.

Cell and Human Islet Culture.

Mouse pancreatic cell lines αTC1 and βTC3 were grown in low-glucose DMEM supplemented with 10% FBS, 50 U/mL penicillin, and 50 μg/mL streptomycin.

Human islets were obtained through the Islet Cell Resource Consortium (http://icr.coh.org/) and through the National Disease Research Interchange (http://www.ndriresource.org/). The purity and viability of human islets are reported to be 70–93% and 70–98%, respectively, and the average age of cadaveric donors was 40.7 ± 9.0 y (range 32–57 y; n = 6). Specific data on individual donors is reported in Table S2. Islets were washed with PBS and incubated in CMRL medium supplemented with 10% FBS, 2 mM glutamine, 100 U/mL penicillin, and 100 μg/mL streptomycin. Islets were gently dissociated into a cell suspension by incubating in Accutase (37 °C, 10 min), and seeded in 96-well plates containing extracellular matrix secreted by the HTB9 human bladder carcinoma cell line [adapted from Beattie et al. (16)].

Compound treatments for both cell lines and primary human islet cultures were performed as follows: cells were plated and allowed to adhere overnight, after which compound solutions in DMSO were added to achieve the indicated concentrations in 0.1% DMSO. For 5-d treatment, media was changed and new compound added on day 3.

High-Content Screening.

A total of 10,000 αTC1 cells per well were plated in 50 μL media in black, optical bottom, tissue-culture-treated 384-well plates (Corning) and allowed to attach overnight. Compounds (100 nL per well) were pin-transferred from concentrated DMSO stocks. Three days after the beginning of compound treatment, cells were fixed with 1% formaldehyde in PBS for 30 min at room temperature. Following one wash with PBS, cells were permeabilized by addition of 50 μL PBS-T (PBS supplemented with 0.1% Triton X-100) for 60 min at room temperature and blocked with 2% BSA/PBS-T for 60 min. Twenty microliters of primary antiinsulin antibody, diluted 1:4,000 in 2% BSA/PBS-T, was added per well and incubated overnight at 4 °C. Following two PBS-T washes, 20 μL Cy-2–labeled donkey-α-guinea pig antibody diluted 1:500 in 2% BSA, 10 μg/mL Hoechst 33342/PBS-T was added per well and incubated for 1 h at room temperature in the dark. After two washes with 50 μL PBS-T, plates were stored in PBS in the dark at 4 °C until analysis.

Images were acquired on an ImageXpress Micro automated microscope (Molecular Devices) using a 4× objective (binning 2, gain 2), with laser- and image-based focusing (offset −130 μm, range ±50 μm, step 25 μm). Images were exposed for 10 ms in the DAPI channel (Hoechst) and 500 ms in the GFP channel (insulin). Image analysis was performed using the cell-scoring module of MetaXpress software (Molecular Devices). All nuclei were detected with a minimum width of 1 pixel, maximum width of 3 pixels, and an intensity of 200 gray levels above background. Cytoplasmic regions around these nuclei were evaluated for Cy2 staining in the green GFP channel (minimum width of 5 pixels, maximum width of 30 pixels, intensity >200 gray levels above background, 10 μm minimum stained area). In total, 75,264 wells were screened, corresponding to 30,710 unique compounds in duplicate plus control wells. The compounds screened were selected from a number of sublibraries in the Broad Institute compound collection. The screening set was comprised of 1,920 molecules with previously annotated biological activity, purchased from commercial vendors Biomol International Inc., Calbiochem, EMD Biosciences, Microsource Discovery Systems Inc., Prestwick Chemical Inc., and Sigma-Aldrich; 1,280 purified natural products from Analyticon Discovery; 15,356 commercial drug-like compounds from ChemDiv Inc., Maybridge, and TimTec LLC; and 12,154 diversity-oriented synthetic (DOS) compounds generated at the Broad Institute. The commercial drug-like compounds were prefiltered by the suppliers to avoid undesired reactive functional groups and meet physical property filters based on Lipinski's rule of five. The DOS compounds consisted of a series of stereochemically diverse eight- and nine-membered macrocycles ranging in molecular mass from 307 to 727 Da, with an average molecular mass of 572 Da.

Compound purity and identity were determined by UPLC-MS (Waters). Purity was measured by UV absorbance at 210 nm. Identity was determined on a SQ mass spectrometer by positive electrospray ionization. Mobile phase A consisted of 0.1% ammonium hydroxide; mobile phase B consisted of 0.1% ammonium hydroxide in acetonitrile. The gradient ran from 5% to 95% mobile phase B over 0.8 min at 0.45 mL/min. An Acquity BEH C18, 1.7 μm, 1.0 × 50-mm column was used with column temperature maintained at 65 °C. Compounds were dissolved in DMSO at a nominal concentration of 1 mg/mL, and 0.25 μL of this solution was injected.

Hits were selected based on the intensity of staining in the Cy2 channel and the number of Cy2 positive cells, and counterscreened in the same assay without the use of primary antibody and with Cy3-labeled secondary antibody to remove inactive autofluorescent compounds.

In all subsequent immunofluorescence experiments, Cy3 and Cy5 secondary antibodies were used to avoid effects of compound autofluorescence in the Cy2 channel.

Gene Expression Analysis.

Following compound treatment, cells were lysed and RNA isolated using the RNeasy Mini Kit (Qiagen) according to the manufacturer's protocol. RNA was reverse transcribed with random primers using the High Capacity cDNA Reverse Transcription Kit with RNase inhibitor (Applied Biosystems).

Quantitative PCR was performed with Power SYBR Green PCR Master Mix (Applied Biosystems) on an Applied Biosystems 7900HT real-time PCR machine using primers in Table S3.

Kinase Profiling.

Kinase profiling and dose–response curves were performed at Millipore's KinaseProfiler according to the manufacturer's protocols. ATP concentrations were within 15 μM of the apparent KM for each enzyme.

Western Blot Analysis.

Cell extracts were generated by lysing cells in modified RIPA buffer containing 1% Nonidet P-40, 0.1% Na deoxycholate, 150 mM NaCl, 1 mM EDTA, and 50 mM Tris (pH 7.5), and supplemented with protease and phosphatase inhibitors. A total of 20 μg of each sample were run on E-Page 48 gels (Invitrogen) and transferred to PVDF membranes using Invitrogen iBlot technology. Each blot was simultaneously probed with indicated primary antibody (all at 1:1,000) and 1:10,000 β-actin antibody, following by incubation with 1:5,000 IRDye-labeled secondary antibody. Blots were scanned on LI-COR Odyssey Infrared Imaging System and analyzed using Odyssey software. Each specific band was normalized to the β-actin signal, and phosphorylation was plotted as a ratio of normalized phospho-specific to normalized pan-antibody signal.

RNAi Experiments.

Lentiviruses resulting in the expression of shRNAs against RSK family members were obtained from the RNAi Consortion (TRC) (19). The following hairpins were used: Rsk1 shRNA1: NM_009097.1-559s1c1, Rsk1 shRNA2: NM_009097.1-685s1c1, Rsk2 shRNA1: NM_148945.1-269s1c1, Rsk2 shRNA2: NM_148945.1-1345s1c1, Rsk2 shRNA3: NM_148945.1-1833s1c1, Rsk3 shRNA1: NM_011299.3-384s1c1, Rsk3 shRNA2: NM_011299.3-627s1c1, Rsk3 shRNA3: NM_011299.3-2351s1c1. Mouse αTC1 cells were plated in 96-well plates at 15,000 cells per well in 200 μL of DMEM. The next day, polybrene was added to each well (8 μg/mL), and cells were spin-infected with 8 μL virus at 2,250 rpm for 30 min at 30 °C. Media was changed the following day to fresh, low-glucose DMEM containing 1 μg/mL puromycin and cultured for 4 additional d. Cells were lysed in RLT buffer and mRNA extracted using Qiagen RNeasy 96 Kit.

Hormone Secretion in Human Islets.

Dissociated human islets cultured in 96-well plates were washed once with 100 μL per well of PBS and incubated for 1 h in 100 μL low-glucose (1.67 mM) KRB buffer (138 mM NaCl, 5.4 mM KCl, 2.6 mM MgCl2, 2.6 mM CaCl2, 5 mM NaHCO3, 0.1% BSA), and for an additional hour in either high-glucose (16.7 mM) or low-glucose KRB buffer. Supernatant from the first hour was used for glucose-stimulated glucagon secretion using ALPCO Glucagon (human, mouse, rat) ELISA (following manufacturer's protocol for 50 μL of sample). Supernatant from the second hour was used to measure glucose-stimulated insulin secretion using ALPCO Insulin ELISA (human).

Supplementary Material

Supporting Information:

Acknowledgments

We thank Andrew Stern, Michelle Palmer, Lynn Verplank, and the entire Chemical Biology Platform at the Broad Institute for helpful suggestions in assay development and with high-content screening; Thomas Nieland, Serena Silver, and David Root from the Broad RNAi platform for lentiviral knockdown constructs and advice for optimization of the infection protocol; Jack Taunton (University of California, San Francisco) for a sample of the RSK inhibitor FMK and advice on RSK biology; Yuan Yuan (Chemistry and Chemical Biology Department, Harvard University) for expression primers; Robert Gould and the entire CB/NT Diabetes Team for helpful discussion and advice; and Alejandro Wolf Yadlin (Chemistry and Chemical Biology Department, Harvard University) for performing Western blot quantification. Funding for this project was provided by the Juvenile Diabetes Research Foundation and National Institute for General Medical Sciences Grant GM38627 (to S.L.S.); National Institutes of Health Grant RL1-HG004671 for computational work toward target-hypothesis generation (to V.D. and P.A.C.); Ernst Schering Research Foundation and European Union FP7 Marie Curie Program Grant PIOF-GA-2008-221135 (to S.K.); an MCO training grant from Harvard University (to D.F.); and Type 1 Diabetes Pathfinder Award DP2-DK083048 from the National Institutes of Health–National Institute of Diabetes and Digestive and Kidney Diseases (to B.K.W.). S.L.S. is an Investigator at the Howard Hughes Medical Institute.

Footnotes

The authors declare no conflict of interest.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1010018107/-/DCSupplemental.

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