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Am J Physiol Heart Circ Physiol 295: H447-H453, 2008. First published June 13, 2008; doi:10.1152/ajpheart.00461.2008
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PERSPECTIVE

The physiological basis of intracrine stem cell regulation

Richard N. Re and Julia L. Cook

Ochsner Clinic Foundation, New Orleans, Louisiana

Submitted 1 May 2008 ; accepted in final form 11 June 2008

ABSTRACT

Intracrine peptides and proteins participate in the regulation of adult and pleuripotential embryonic-like stem cells. Included among these factors are VEGF, dynorphin, the readthrough form of acetylcholinesterase, Oct3/4, Pdx-1, Pax-6, and high-mobility group protein B1, among others. In some cases, the establishment of intracrine feedback loops can be shown to be relevant to this regulation, consistent with previously proposed principles of intracrine action. Here the role of intracrines in stem cell regulation is reviewed, with particular attention to the intracrine regulation of cardiac stem cells. The reprogramming of cells to restore the pleuripotent phenotype and the possible role of stem/progenitor cells in neoplasia are also discussed.

stem cells; differentiation

A LARGE NUMBER OF PEPTIDE growth factors, hormones, enzymes, DNA binding proteins, and other moieties have been shown to act as extracellular signaling agents, while at the same time apparently having the capability to act in cellular interiors: be it the interior of the cell that synthesized them, or of a target cell that internalized them. In some cases, one product of an intracrine gene acts in the intracellular space, while another is secreted, with or without subsequent internalization by target cells. We have termed these factors intracrines and have developed principles of intracrine action (43–52). A list of representative intracrines is found in Table 1. These considerations, in turn, let us propose a role for long-lived, intracellular, finite-gain, positive feedback loops in some forms of cellular differentiation. That is, cells supporting such intracellular intracrine loops could potentially secrete intracrines to affect nearby target cells. Internalization of intracrines by target cells could then lead to the establishment of long-lived intracellular loops in those target cells and result in an active form of cellular differentiation, that is, a change in state that persists long after the initial intracrine secretion has ceased (45, 46, 48, 50–52) (Fig. 1). Although this view of active differentiation was not widely appreciated when we initially proposed it, evidence has steadily accumulated to support this notion, including the recent demonstration that the transcription factor Pax5 sustains the B-lymphocyte phenotype through an active intracellular positive feedback loop (5, 36). These views regarding intracrine physiology and its role in hormonal responsiveness, differentiation, and memory are discussed in detail elsewhere (45, 46, 48, 50–52). Here we propose to examine the role of intracrines in stem cell biology in further detail and to illustrate these intracrine actions using the regulation of cardiomyocyte differentiation as an example. Important to this exercise is the realization that many homeodomain proteins are capable of being secreted and taken up by target cells, whereupon the available evidence suggests that some can act as transcription factors. Defined sequences in the homeodomain are associated with secretion and internalization, respectively, although it is not known if these sequences are unique. These homeodomain transcription factors arguably fit the definition of intracrine (extracellular signaling and intracellular action), and, given their role in cellular differentiation, it can be appreciated that they must be included in any evaluation of intracrine biology in differentiation (40).


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  		Table 1. Intracrines

 

Figure 1
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  		Fig. 1. Intracrine differentiation. Intracrine factors can establish positive, limited-gain, feedback loops in cells, leading to an active form of cellular differentiation. Secretion and internalization of intracrine factors by nearby cells establishes similar positive feedback loops in these cells, leading to tissue-wide differentiation, which persists after intracrine secretion has ceased.

 
Intracrines In Differentiation

Representative intracrine action in stem cell biology. A number of intracrine factors have been demonstrated to play a role in cellular and tissue differentiation. Some of these are listed in Table 2. As remarkable as the level of intracrine participation in differentiation is, the more interesting question is to what extent is intracrine functionality integral to the participation of these proteins in stem cell biology and, conversely, to what extent do they operate via cell membrane receptors and the generation of classical second messengers? In the case of VEGF, for example, there is clear evidence of intracrine action in the peptide's role in the differentiation of hematopoietic stem cells (HSCs) (as well as in the proliferation of certain neoplastic cells). HSCs normally express VEGF, but cells engineered to be VEGF–/– demonstrate reduced survival and colony formation. Small-molecule inhibitors of VEGF receptor kinase, which act in the intracellular space, mimic the effects of VEGF knockout, but anti-VEGF antibody administered to the culture medium does not. VEGF–/– cells can, however, be rescued by extracellular administration of VEGF. Thus a positive intracrine feedback loops is operative in these cells and is required for differentiation; extracellular VEGF appears to rescue cells after internalization and reconstitution of intracellular VEGF concentration in VEGF-deficient cells (9, 20, 30, 56). This situation finds both a parallel and a contrast in the action of erythropoietin, which similarly establishes an intracellular positive feedback loop driving HSC differentiation, as shown by the observation that antisense to erythropoietin blocks this action, but this inhibitory effect is usually not reversed by extracellular administration of erythropoietin (13, 33). However, in some situations, extracellular erythropoietin can rescue antisense-treated cells, even cells that, in the basal state, are unaffected by the application of anti-erythropoietin antibody but are affected by antisense. Thus erythropoietin utilizes an intracrine mode of action but does not appear, in every case, to be internalized from the extracellular space as some other intracrines are (13, 33). In contrast, the observation that the extracellular administration of VEGF can rescue cells administered antisense to VEGF is consistent with the expanded principles of intracrine differentiation that our laboratory has previously developed (30) (Fig. 1).


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  		Table 2. Representative intracrines involved in stem cell regulation

 
Similarly, the so-called "readthrough" form of acetylcholinesterase (AChE-R), a longer translational product generated by alternative splicing, is secreted by myeloid hematological progenitor cells, whereupon a unique COOH-terminal fragment [AChE-R peptide (ARP)] is cleaved from the protein in the extracellular space. This fragment, which lacks enzymatic activity, is subsequently internalized by target stem cells; ARP is found in 5–15% of circulating blood cells, bone marrow cells, and fetal human CD34+ cells, and promotes proliferation following upregulation of its own synthesis, that is, by forming a positive feedback loop. Administration of antisense to AChE-R prevents the proliferation. AChE, therefore, is an intracrine-AChE gene product that, depending on splicing, can function as extracellular signaling molecules or as moieties active in the intracellular space (1, 6, 7, 34, 39).

The more common synaptic AChE variant, AChE-S, is found in erythroleukemia K562 cells, in association with the transcriptional co-repressor COOH-terminal binding protein in cell nuclei; AChE-S reduces the repressive effect of COOH-terminal binding protein on the hematopietic transcription factor Ikaros (1). This is additional evidence that AChE is also an intracrine, which is active in differentiation.

It is clear that AChE-R/ARP generates a positive feedback loop, which regulates myelopoiesis, but does ARP, which is known to be internalized by target cells, act in the intracellular space after uptake, or is cell surface interaction sufficient with internalization an epiphenomenon? Recall that, unlike the case of VEGF, where the autocrine loop is entirely intracellular, ARP is generated in the extracellular space. In this regard, it should be noted that AChE-R is expressed in the normal brain after exposure to stress (it is upregulated by cortisol), and it enhances fear-conditioned memory (1, 7, 34, 39). As in the case of myelopoiesis, neuronal AChE-R is secreted, cleaved to produce ARP, which is then internalized by target cells to establish a positive feedback loop inhibited by antisense to AChE. Thus the brain and the HSC systems operate in similar fashions (7). In neurons, AChE-R interacts via its ARP terminus with the scaffold protein receptor for activated C kinase (RACK) 1 and through it with its target protein kinase C-βII, a factor known to be involved in fear conditioning (1). In addition, ARP is internalized by neurons through endocytosis and retrograde transport and, thereafter, enhances fear conditioning in an AChE antisense-sensitive manner (34). Thus extracellular as well as any intracellularly generated ARP appear to have the opportunity to interact with RACK1. Direct intraneural injection of synthetic ARP upregulates PKC-βII expression and leads to impairment of nerve conduction. Collectively, these observations strongly suggest that AChE through its ARP domain acts in an intracrine fashion; that is, it is an extracellular signaling molecule that also acts in the intracellular space and, in the case of HSCs, establishes a differentiation: regulating positive feedback loop. Moreover, as already noted, the evidence strongly suggests that ARP affects target cell differentiation after internalization and interaction with intracellular proteins such as RACK1.

Homeodomain transcription factors and DNA binding proteins. Homeodomain transcription factors are highly conserved and are frequently involved in the establishment of the architecture of the early embryo. The Hox family of homeodomain transcription factors, for example, is important in defining body segments. Homeodomain proteins are characterized by a 60-amino acid sequence called the homeodomain, which is highly conserved in all animals and is the DNA binding domain of the molecule (8, 21, 29, 35, 40, 59). In the 1990s, it was discovered that the homeodomain protein anntenapedia (so named because, when it is dysfunctional, legs replace the development of antennae in fruit flies) could be secreted by neurons and regulate neural morphogenesis. Indeed, the homeodomain sequence of this transcription factor was termed penetratin and has been used to produce fusion proteins capable of trafficking biologically active peptide moieties (as part of the fusion protein) into cells. Other homeoproteins, including the transcription factors Engrailed 1 and 2, were found to operate in a similar fashion and to be both secreted and internalized by target cells. The homeoprotein Hox 5 was shown to be internalized by neurons and to then traffic to nucleus. The nucleus is likely the major site of action of internalized homeoproteins, as indicated by the fact that the homeodomain of antennapedia, when administered to neurons, traffics to nucleus and downregulates a reporter gene driven by a HoxD9 promoter, which itself has binding sites for homeoproteins. The nuclear uptake of the antennapedia homeodomain is associated with neurite outgrowth. Interestingly, homeodomain protein secretion and uptake are accomplished by atypical mechanisms. Secretion is not energy dependent, and it occurs at low temperatures (8, 29), and uptake does not require a specific cell surface receptor. Further research has determined that two specific sequences in the Engrailed 1 and 2 homeodomains are necessary for secretion and uptake, respectively (8, 21, 29, 35, 40, 59).

These findings suggested that the secretion of homeodomain proteins in the embryo could provide a diffusion gradient of these factors so as to influence the development of cells (through the gene regulatory properties of the internalized transcription factors) in a topologically important manner. More recent studies of the role of the homeodomain protein Pax6 in zebra fish eye development confirm the validity of this paradigm in that antibody to Pax6 interferes with eye development, apparently by interfering with the intercellular transit of the protein. Although this study does not conclusively demonstrate that the internalization of the secreted homeoprotein is required for eye development, taken with earlier work, it supports this notion (21).

Collectively, these studies suggest that most (or perhaps all) homeodomain proteins, at least those having a complete homeodomain sequence, are intracrines. One might then ask if homeodomain proteins participate in positive feedback loops, and, indeed, there is considerable evidence that they do. This has potentially important implications because, if homeodomain factors function according to the intracrine differentiation paradigm we have outlined, they could have unexpected biological and even therapeutic efficacy. For example, glucagon-related protein is known to upregulate the homeoprotein gene Pdx1 in ductal cells and to thereby drive the cells along the β-islet cell differentiation pathway. This led us to suggest that, as an intracrine homeodomain protein, the external application of Pdx1 to target cells should produce the same effect (48). Some time later, it was shown by others that these cells internalized extracellularly administered Pdx1, trafficked it to nucleus, whereupon it upregulated its own transcription (positive feedback loop) and drove the cells toward the islet cell phenotype, as evidenced by the upregulation of insulin gene expression (35).

Thus there is strong evidence for the notion that homeodomain proteins function according to the principles of intracrine physiology. This functionality seems more reasonable when one notes that other DNA binding proteins/transcription factors also can traffic between cells and in some cases influence differentiation. For example, in plants, a variety of transcription factors traffic between cells through specialized pores termed plasmodesmata, whereupon they upregulate their synthesis in target cells, induce cell differentiation, and then traffic to nearby cells, thereby establishing a wave of differentiation in the plant (47, 48). Of note, some of these factors are homeoproteins, and in those cases the homeodomain sequences needed for secretion and internalization in mammalial cells are also needed for transit through the plasmodesmata (59).

It can also be noted in this regard that high-mobility group protein B1 (HMGB1) is an intracrine, and other DNA binding proteins may be as well (14, 28, 46–48, 51). HMGB1, a sequence-independent DNA binding protein, is involved in gene regulation through the alteration of nucleosome positioning. It can also be secreted and taken up by target cells via the receptor for advanced glycation end products; there is evidence that it can function after internalization. Circulating HMGB1, also called amphoterin, is an important mediator of septic shock. Thus HMGB1 is an intracrine DNA binding protein. Of note, HMGB1 also enhances stem cell trafficking to damaged myocardium and may, therefore, play a role in cellular differentiation (see below) (14, 46–48, 51).

The intracrine functionality of DNA binding proteins, especially the homeodomain proteins, could have important biological and therapeutic implications. It was recently reported independently by three groups that dermal fibroblasts could be reprogrammed into pleuripotent embryonal stem cell-like cells through the retroviral introduction of four genes: Oct3/4, Sox2, c-Myc, and Klf4 (26, 27, 37, 57, 58, 69). It is noteworthy that only short-term expression of these genes is required, because the transfected genes are silenced during reprogramming. Indeed, a recent study suggests that, in the process of downregulating exogenous gene transcription, the cells maintain a defined level of synthesis of these transcription factors, whether driven by the retroviral construct or the endogenous gene. This is consistent with the notion that positive, finite-gain, regulatory loops are operative in the cells during reprogramming. Because intracrine physiology indicates that long-lived positive feedback loops can develop after acute internalization of intracrines and because Oct3/4 is a POU/homeodomain (Pit1-Oct1/2-Unc86) protein and, therefore, by virtue of possessing a complete homeodomain, very likely capable of acting in an intracrine mode, it was natural to suggest that this factor, and perhaps one or more of the other three factors, need not be retrovirally introduced into target cells, but, rather, the transcription factor itself could be administered to the cells, be internalized, and then establish a differentiating intracrine loop (50). This view is supported by the observation that Oct3/4 acts synergistically with Sox2, thereby upregulating Oct 3/4, Sox2, and Nanong, the later a homeodomain transcription factor upregulated in stem cells but not required for reprogramming in these first reports. We also noted that Sox2 contains a high-mobility group motif and, therefore, suggested that it might function after extracellular administration (50). Thus it was our view that the reprogramming activity of Oct3/4, Sox2, and Nanong, if Nanong should be useful to improve reprogramming efficiency in one species or another, could be achieved by extracellular administration of these proteins followed by their internalization and intracrine functioning (50). If this were the case, efficiency problems associated with DNA uptake by transfection or transduction could be avoided, as could retroviral-induced oncogenesis. Indeed, we suggested that penetration fusion proteins could be employed to safely introduce the two remaining factors (50). Within months, additional published reports showed that Myc could be dispensed with and that Oct4, Sox2, Nanong, and the RNA binding protein Lin28 could produce reprogramming (72). Thus it seems likely that intracrine proteins can mediate long-term effects without the necessity of conventional gene transfer approaches. Finally, it may well be that transcription factors other than homeoproteins can be secreted and internalized and, therefore, function as intracrines in some circumstances. This may be an area worthy of additional exploration.

Cardiac stem cell differentiation. The role of dynorphin B in cardiac stem cell differentiation further exemplifies these principles (61–66). Dynorphin B is a product of the dynorphin gene and is a {kappa}-opioid receptor agonist. In addition to functioning in the nervous system, dynorphin B is synthesized and secreted by cardiac myocytes and acts at membrane receptors on target cells. Early on, dynorphin binding sites were identified on cell nuclei, and binding of dynorphin B to these receptors upregulates gene expression as well as synthesis of prodynorphin (61, 63). This then is consistent with the existence, in cardiac cells, of an intracrine feedback loop, which promotes differentiation and involves dynorphin.

Stem cells in culture can be directed along the cardiac lineage pathway by induced upregulation of dynorphin gene expression, as well as by direct application of this intracrine. This is exemplified by reports of studies in P19 and GTR1 stem cells. Embryonic pluripotent P19 cells express prodynorphin and secrete dynorphin. The administration of exogenous dynorphin to P19 cells upregulates cardiac lineage genes, such as GATA-4 and Nkx-2.5 (itself a homeodomain transcription factor), with subsequent upregulation of cardiac myosin heavy chain and light chain genes; rhythmically contracting colonies of cardiac myocyes result (61, 62, 64). GTR1 undifferentiated stem cells synthesize and secrete dynorphin. When the stem cell maintenance factor, leukemia inhibitory factor (an intracrine), is removed from cultures of GTR1 cells, differentiation along the cardiac lineage occurs and is associated with upregulation of prodynorphin and dynorphin B. Undifferentiated GTR1 cells possess nuclear {kappa}-opioid receptors, and the number of these nuclear receptors increases with differentiation into cardiac myocytes. This suggests a role for upregulation of nuclear dynorphin activity in the differentiation of these cells. Nuclear runoff studies demonstrate that exposure of the undifferentiated stem cell nuclei to dynorphin B results in increased GATA-4 and Nkx-2 transcription, as well as upregulation of prodynorphin expression, the latter observation consistent with the existence of a positive feedback loop (61–66).

Collectively, these findings suggest the participation of dynorphin intracrine positive feedback loops in the differentiation of stem cells along a cardiac lineage and further suggest that dynorphin loops interact with other transcription factors in this process, possibly through the upregulation of other intracrine loops. For example, the participation of the homeoprotein Nkx-2 raises the possibility that it too could function in an intracrine mode. It should also be noted that the intracrine, HMGB1, a sequence-independent DNA binding protein, which, when secreted, acts as an immune modulator after binding to the receptor for advanced glycation end products, can be internalized and is functionally active in target cells. In animal models of myocardial infarction, the application of HMGB1 to the infracted area results in proliferation and differentiation of endogenous cardiac C-kit+ stem cells, resulting in repair of myocardial damage (24). This suggests a role for intracrines in the regulation of tissue stem cells, as well as pleuripotent stem cells, and is consistent with, as we previously addressed, the role of Pdx-1 in islet cell differentiation. The positive feedback loop generated by the homeodomain transcription factor cdx-2 (it upregulates its own synthesis under regulation by the homeodomain transcription factor Oct1) in pancreatic islets and intestinal cells also supports the notion that intracrine transcription factor/DNA binding protein functionality plays a role in tissue-specific differentiation, including cardiac stem cell differentiation (15). In addition, the intracrine transforming growth factor-β induces the development of bone marrow-derived stem cells into cardiomyocytes (23). Finally, there is considerable evidence that the tissue regenerative effects of mesenchymal stem cells, after their trafficking to tissue injury, are at least partially the result of cytokine secretion and the upregulation of endogenous stem cells (38, 41). It can be speculated that the secretion of intracrine factors by these cells is responsible for the stimulation of endogenous stem cells in damaged tissues. For example, the ability of adipose-derived mesenchymal stem cells to exert a protective anti-apoptotic effect in ischemic cardiomyocytes derives in large part from the secretion of the intracrines insulin-like growth factor I and VEGF by the stem cells (54). Future experimentation will be required to determine if, as suggested here, insulin-like growth factor I and VEGF act in an intracrine fashion in mediating this effect.

New Directions

Tumor stem cells. If correct, the view of intracrine differentiation discussed here potentially has implications beyond those associated with regenerative medicine. For example, the traditional paradigm holds that neoplastic cells are pleuripotent; most, if not all, cancer cells have the potential to proliferate indefinitely and to form tumors, checked only by genomic instability and the level of angiogenic support. However, a growing body of evidence is challenging this view, because it appears that, in the case of at least some neoplasms, not all tumor cells can, in fact, form new tumors when explanted into immunodeficient mice (55, 60, 68). This observation has given rise to the notion that there exists tumor stem cells, slow-growing immortal subsets of cells that can form new tumors by producing rapidly dividing daughter cells with finite life spans. If this paradigm is correct, therapies directed at destroying rapidly dividing cells will miss slowly dividing stem cells, leading to failure of tumor eradication.

If cancer stem cells are the root cause of disease, one might expect these stem cells to derive from tissue-specific stem cells, as opposed to pleuripotent-like stem cells, with the result that tumors resemble, to some extent, the tissues in which they arise. If, as we believe, this is correct, intracrine participation in stem cell deregulation leading to cancer can likely occur, as suggested by the importance of such intracrines as Pdx-1, Cdx-2, Nkx-2, VEGF, dynorphin, and HMGB1, among others, in tissue stem cell regulation. It could be expected that the interruption of intracrine loops would have the effect of reducing stem cell proliferation and, therefore, be beneficial in the treatment of neoplasms.

While there is substantial evidence that tumor stem cells, likely derived from tissue stem cells, contribute to neoplasia, there remains support for the participation of pleuripotent stem cells as well. Variable numbers of cells expressing Oct3/4 immunoreactivity have been detected in a wide variety of canine neoplasms (68). Some tissue stem cells have also been reported to be Oct3/4 positive, including Oct3/4+ bone marrow stem cells that differentiate into cardiomyocytes (38). These data require critical analysis because, in many cases, the positive cells are found in extremely low numbers. Also, there exist two Oct3/4 isoforms, a long form, which localizes to nucleus and supports pleuripotent cells, as well as a shorter form, which is cytoplasmic in location and does not support pleuripotent stem cells. Thus the finding of Oct3/4 immunoreactivity in cells is not conclusive evidence for the involvement of this transcription factor in stem cell maintenance (17). However, the reports of Oct3/4 in canine cancer stem cells and in some tissue stem cells demonstrate Oct3/4 localization in nucleus, suggesting the presence of the active, long isoform. This then leads to the question: What is the role of the usually rare cells expressing Oct3/4 in human tumors? Is this simply a curiosity? Clearly this situation is not in accord with the intracrine principles outlined above, because these would suggest that local expression of Oct3/4 should lead to subsequent upregulation of this factor in nearby cells, thereby increasing total tissue expression. Also, in some tumors, the frequency of Oct3/4-positive cells seems too low for these cells to be the progenitors of the entire tumor mass.

A possible answer to these questions comes from the realization that tumor stem cells are abnormal: they do not generate normal organisms or tissues. It could well be that many tumor stem cells have lost the ability to upregulate some intracrines, such as Oct3/4 but, nonetheless, require these factors, after uptake from the extracellular space, to maintain their ability to serve as cancer progenitor cells. Thus the synthesis and secretion of intracrines, such as Oct 3/4, by a relatively small number of "feeder" stem cells could be sufficient to maintain stemness in these "conditional" stem/progenitor cells (Fig. 2). These conditional cancer stem cells, if they exist, would offer a novel therapeutic target. Neutralizing antibodies directed at the secreted transcription factors they require or small-molecule inhibitors of the internalization of those factors could block the proliferation of these cells.


Figure 2
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  		Fig. 2. Proposed mode of conditional stem cell function. Conditional stem/progenitor cells are proposed to require intracrine factors, such as secreted Oct3/4, to complete the intracellular positive feedback loops that maintain their stemness. A small number of complete feeder stem cells could, in this way, support a large number of conditional stem cells, leading to the generation of a large number of rapidly proliferating daughter cells.

 
Pharmaceutical and therapeutic implications. The pharmacological implications of intracrine physiology have been reviewed elsewhere (50). In the context of stem cell biology, intracrine involvement suggests the use of intracrine peptides for reprogramming of cells to a pleuripotential state, thereby obviating the need for retroviral transfection and possible secondary neoplastic transformation. As discussed above, the notion that the interruption of intracrine trafficking between tumor progenitor cells could block tumor proliferation is also of interest. The exploration of the role of factors like (1) intracrine HMGB1, which appears to direct stem cell trafficking to tissue; (2) the homeoprotein (and likely intracrine) HoxB4, which confers engraftment potential on stem cell-derived hematopoietic progeniture cells; and (3) the homeogene mMix, which regulates the transition of mesodermal progeniture cells to the hemangioblast and hematopoietic lineages, also appears to be worthy areas for investigation and possible pharmacological intervention (19, 22, 24). Indeed, it is becoming clear that the therapeutic effects of tissue stem cells are multifaceted and, in large part, result from actions other than their proliferation and direct differentiation into parenchymal cells. Among these other effects of tissue stem cells is the elaboration of a variety of cytokines and growth factors. The possibility of intracrine functionality playing a role in these processes seems worthy of exploration. Moreover, in some cases, interactions between tumor cells and stem cells can be complex, as evidenced by the interaction of VEGF secreting tumor cells with VEGF releasing (from extracellular sites) medullary suppressor progenitor cells, with the resulting incorporation of the suppressor cells into tumor vessels, as well as the downregulation of cytotoxic T cells (31, 32, 70, 71). The view expressed here suggests that, in many cases, these interactions are intracrine in nature and, therefore, potential therapeutic targets.

Conclusions

  1. Intracrine factors are intimately involved in the regulation of stem cell proliferation and differentiation.
  2. The principles of intracrine physiology, in many instances, explain/predict the actions of intracrine factors in stem cell regulation, and it may be that future investigation will reveal that other stem cell regulatory factors operate according to those principles.
  3. The participation of intracrines and intracrine physiology in stem cell regulation points to areas for the development of novel pharmaceuticals, including those directed at regenerative medicine and tumor cell biology.

GRANTS

This study was supported by the Ochsner Clinic Foundation and National Heart, Lung, and Blood Institute Grant R01 HL072795.

FOOTNOTES


Address for reprint requests and other correspondence: R. N. Re, Scientific Director, Ochsner Clinic Foundation, New Orleans, LA 70121 (e-mail: rre{at}ochsner.org)

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

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R. N. Re and J. L. Cook
Senescence, apoptosis, and stem cell biology: the rationale for an expanded view of intracrine action
Am J Physiol Heart Circ Physiol, September 1, 2009; 297(3): H893 - H901.
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