Although the detection of several components of the fibroblast growth factor (FGF) signaling pathway in human embryonic stem cells (hESCs) has been reported, the functionality of that pathway and effects on cell fate decisions are yet to be established. In this study we characterized expression of FGF-2, the prototypic member of the FGF family, and its receptors (FGFRs) in undifferentiated and differentiating hESCs; subsequently, we analyzed the effects of FGF-2 on hESCs, acting as both exogenous and endogenous factors. We have determined that undifferentiated hESCs are abundant in several molecular-mass isoforms of FGF-2 and that expression pattern of these isoforms remains unchanged under conditions that induce hESC differentiation. Significantly, FGF-2 is released by hESCs into the medium, suggesting an autocrine activity. Expression of FGFRs in undifferentiated hESCs follows a specific pattern, with FGFR1 being the most abundant species and other receptors showing lower expression in the following order: FGFR1 → FGFR3 → FGFR4 → FGFR2.Initiation of differentiation is accompanied by profound changes in FGFR expression, particularly the upregulation of FGFR1. When hESCs are exposed to exogenous FGF-2, extracellular signal-regulated kinases are phosphorylated and thereby activated. However, the presence or absence of exogenous FGF-2 does not significantly affect the proliferation of hESCs. Instead, increased concentration of exogenous FGF-2 leads to reduced outgrowth of hESC colonies with time in culture. Finally, the inhibitor of FGFRs, SU5402, was used to ascertain whether FGF-2 that is released by hESCs exerts its activities via autocrine pathways. Strikingly, the resultant inhibition of FGFR suppresses activation of downstream protein kinases and causes rapid cell differentiation, suggesting an involvement of autocrine FGF signals in the maintenance of proliferating hESCs in the undifferentiated state. In conclusion from our data, we propose that this endogenous FGF signaling pathway can be implicated in self-renewal or differentiation of hESCs.

Signals via fibroblast growth factor receptors (FGFRs) are involved in mesoderm induction events and may be also critical for early hematopoietic specification and proliferation of the hemangioblast. In vitro differentiated embryonic stem cells represent excellent system for the study of early hematopoietic commitment, particularly for understanding signals regulating the onset of hematopoietic differentiation. We have used human embryonic stem cells (hESCs) to study the expression of FGFR1, 2, 3, and 4 in undifferentiated cells and their differentiated progeny. Culturing hESCs i/ in high densities (protocol 1), ii/ without feeder layer of mouse embryonal fibroblasts and basic fibroblast growth factor (protocol 2), and iii/ in three-dimensional aggregates called embryoid bodies (protocol 3), was used to induce the differentiation. To achieve more directed and homogenous differentiation feeder-free hESCs were first subjected to the aggregation step (formation of embryoid bodies) that resembles the gastrulation process. This was followed by differentiation in monolayer in the presence of basic fibroblast growth factor (protocol 4). Such two-step differentiation protocol (5 + 10 days) was shown to activate ectodermal and mesodermal genes and form ectodermal and mesodermal cells (Schuldiner et al., PNAS97:11307, 2000). The gene expression levels for all FGFRs were determined by quantitative real-time RT-PCR. Real-time RT-PCR results were normalized by comparison to the expression of ABL gene. We revealed that undifferentiated hESCs that were cultured with feeder cells and in low density express all four FGFRs in the following pattern: FGFR1 is highly expressed and dominant; FGFR3 is also strongly expressed; FGFR4 shows lower expression; and FGFR2 is only weakly expressed. This expression pattern was changed when hESCs grew and started to differentiate in high densities (protocol 1) or have initiated differentiation either by feeder cells and basic fibroblast growth factor withdrawal or by aggregation step (protocol 2 and 3). Two-fold upregulation of FGFR1 and FGFR4, and downregulation of FGFR3 characterize such changed expression pattern. Notably, the expression levels for all four FGFRs were increased when hESCc were subjected to the two-step differentiation protocol (protocol 4). Compared to the undifferentiated hESCs, FGFR1 and 4 exhibited 7-fold increase, and FGFR2 and 3 were found to be upregulated more than twice. In summary our results show that the expression of FGFRs tightly follows changing culture conditions that may direct hESCs to differentiate. Furthermore, strong upregulation of FGFR1 and 4 in prospective hESC-derived mesodermal cells suggests their involvement in the earliest stages of hematopoiesis. This research was supported in part by the Grant Agency of the Czech Republic (301/03/1122), Ministry of Health (MZ 00065269705), Ministry of Education, Youth, and Sports (MSM 432100001, LN 00A065), and Academy of Sciences of the Czech Republic (AV 0Z5039906).

Objectives To assess the potential of second‐generation proteasome inhibition by carfilzomib and its combination with the human immunodeficiency virus (HIV) protease inhibitors (HIV‐PIs) lopinavir and nelfinavir in vitro for improved treatment of clear cell renal cell cancer (ccRCC). Materials and Methods Cytotoxicity, reactive oxygen species (ROS) production, and unfolded protein response (UPR) activation of proteasome inhibitors, HIV‐PIs, and their combination were assessed in three cell lines and primary cells derived from three ccRCC tumours by MTS assay, flow cytometry, quantitative reverse transcriptase‐polymerase chain reaction and western blot, respectively. Proteasome activity was determined by activity based probes. Flow cytometry was used to assess apoptosis by annexin V/propidium iodide assay and ATP‐binding cassette sub‐family B member 1 (ABCB1) activity by MitoTracker™ Green FM efflux assay (Thermo Fisher Scientific, MA, USA). Results Lopinavir and nelfinavir significantly increased the cytotoxic effect of carfilzomib in all cell lines and primary cells. ABCB1 efflux pump inhibition, induction of ROS production, and UPR pre‐activation by lopinavir were identified as underlying mechanisms of this strong synergistic effect. Combined treatment led to unresolved protein stress, increased activation of pro‐apoptotic UPR pathway, and a significant increase in apoptosis. Conclusion The combination of the proteasome inhibitor carfilzomib and the HIV‐PIs lopinavir and nelfinavir has a strong synergistic cytotoxic activity against ccRCCin vitro at therapeutically relevant drug concentrations. This effect is most likely explained by synergistic UPR triggering and ABCB1‐modulation caused by HIV‐PIs. Our findings suggest that combined treatment of second‐generation proteasome inhibitors and HIV‐PIs should be investigated in patients with metastatic RCC within a clinical trial.