EDITORIAL
JOP. J Pancreas (Online) 2011 May 6; 12(3):216-219.
HOX Genes in Pancreatic Development and Cancer
Sophie Gray1, Hardev S Pandha1, Agnieszka Michael1, Gary Middleton2, Richard Morgan1
1Postgraduate Medical School, Faculty of Health and Medical Sciences, University of Surrey; 2Royal Surrey County Hospital NHS Trust. Guildford, United Kingdom
Summary
The HOX genes are a family of homeodomain-containing transcription factors that determine cellular identity during development and which are subsequently re-expressed in many types of cancer. Some recent studies have shown that HOX genes may have key roles both in pancreatic development and in adult diseases of the pancreas, including cancer. In this review we consider recent advances in elucidating the role of HOX genes in these processes, how they may connect early developmental events to subsequent adult disease, and their potential both as diagnostic markers and therapeutic targets.
HOX Genes
The development and maintenance of cellular identity is vital in both embryonic and adult tissues for normal organ function. Key to this is the establishment of stable transcriptional states within the cell, a process in which transcription factors have a key role. One group of transcription factors of particular note in this regard are the HOX genes, a family of homeodomain-containing transcription factors that determine cellular and tissue identity by regulating specific transcriptional programmes [1, 2, 3]. Mammals have 39 HOX genes split between four groups of linked genes on different chromosomes, which are thought to have arisen from a series of duplication events. These groups are known as A, B, C and D, and the genes within each group are numbered from the 3’ most member (1) to the 5’ most member (13) [4]. Thus, for example the 3’ most member of the HOXA group is known as HOXA1 (Figure 1). Equivalently numbered genes in each group (e.g., HOXB4 and HOXD4) are referred to as paralogues and are thought to have arisen from a common ancestral gene as they represent the same position within the ancestral HOX cluster. Members of each group often share enhancer regions and are co-regulated, giving rise, in part, to coordinated temporal and spatial expression patterns in the developing embryo whereby the 3’ most HOX genes are expressed earlier and more anteriorly than their more 5’ neighbours [5]. In this manner the HOX genes give rise to a pattern of overlapping expression domains along the anteroposterior axis that is key to defining the identities of cells along it [5].
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Figure 1. The arrangement of HOX genes in the mammalian genome and their potential roles in pancreatic development and cancer. |
The HOX transcription factors have a relatively limited specificity for binding to DNA, but this is enhanced by the binding of co-factors such as PBX and MEIS that increase DNA binding specificity and also modify transcriptional regulation [6, 7, 8]. These co-factors, as well as the HOX genes themselves, are highly conserved between animal phyla. The HOX genes also show considerable similarities between each other, especially paralogues and neighbouring members of the same group [1, 2, 3]. This has resulted in a high degree of functional redundancy, particular in respect to early developmental events although later, more organ specific development is generally dependent on a smaller number of HOX genes [9, 10]. The expression domains of HOX genes that are established during development are generally preserved in the adult [11], and there are a number of examples where adult HOX expression is required for the continuation of correct cell identities. This is most apparent where cells are turning over quickly, for example in the proliferation and differentiation of blood cells [1] and in the renewal of the endometrium [12]. The importance of HOX genes in these processes indicates a role in the promotion of cell proliferation and survival, in addition to maintaining cellular identity, and indeed HOXB4 is crucial for the continued proliferation of hematopoietic stem cells [1]. It is perhaps not surprising then that HOX genes are frequently deregulated in cancer, where their primary function also seems to be in promoting proliferation whilst preventing apoptosis [13, 14, 15, 16, 17].
HOX Genes in Pancreas Development
The pancreas develops from endodermal cells in the future midgut region of the embryo, and is dependant on a number of inductive interactions between the endoderm and other tissues. The best characterised of these events is the development of the dorsal pancreatic primordia which is initiated in the endoderm by signalling from the overlying notochord through the secreted proteins activin and FGF-2. These signals are not spatially restricted to the notochord adjacent to the prepancreatic endoderm however, and the responsiveness of endodermal cells is presumably modified by pre-existing anteroposterior information in the endoderm. The HOX genes are key determinates of anteroposterior identity [2, 5, 18], and the expression patterns of HOX genes in the early endoderm suggests that HOXA4, HOXA5 and HOXB4 provide the spatial information needed to restrict the response to signals from the notochord [19]. Correspondingly, retinoic acid, which regulates HOX expression through binding to nuclear hormone receptors, is also known to have a key role in pancreatic development [20]. Furthermore retinoic acid is sufficient to drive embryonic stem cell differentiation towards functional insulin producing cells [21], and HOXA4, HOXA5, HOXB4 and HOXA1 (see below) are all activated directly by retinoic acid [22].
These early patterning events give rise to pancreatic progenitor cells which, in turn, are subdivided into exocrine and endocrine progenitors through the presence or absence of Notch signalling, respectively. In a number of early developmental processes Notch signalling is dependent on HOX transcription factors, together with the PBX co-factor [18, 23, 24], with the HOX1 paralogues (HOXA1 and HOXD1) mediating key transcriptional changes [18, 24]. Likewise HOXA1 expression is activated in pancreatic exocrine cells and is required for exocrine development, possibly by modulating TGF-beta signalling from the foregut mesoderm [25].
The endocrine progenitor cells are also defined by the expression of specific transcription factors, Pax-6 for alpha-cells and gamma-cells (glucagon and pancreatic polypeptide secreting, respectively), and Pax-4 for beta-cells and delta-cells (insulin and somatostatin secreting, respectively). Pax transcription factors are known to cross-regulate HOX target genes, at least in part through direct interactions with the latter [26], and also through the direct regulation of a number of HOX genes including HOXD4 [27]. Thus, HOX genes play a number of key roles in pancreatic development from the specification of early endodermal progenitor cells through to the determination of specific cellular subtypes within the maturing pancreas.
HOX Expression and Function in Pancreatic Cancer
The deregulation of HOX genes in cancer is now well established, although in general rather less is known about their function [28]. For a number of malignancies, including melanoma [14], myeloma [13], and ovarian [15], renal [17], lung [16] and, indeed, pancreatic cancer [29], it has been shown that the HOX genes of paralogue groups 1 through 9 can promote cell survival by blocking apoptosis. In this respect many of these 27 HOX genes have a redundant or at least a highly overlapping function [13, 17, 29]. Targeting the anti-apoptotic function of this group of genes has been achieved by antagonising the interaction between HOX proteins and the PBX co-factor. This approach exploits a highly conserved hexapeptide motif on PBX that is required for HOX binding [6, 7, 8]. The motif forms part of peptides such as HXR9 that act as competitive antagonists of HOX/PBX dimer formation can induce apoptosis both in vitro and in vivo, at least in part through the greatly elevated expression of cFos [13, 17, 29]. Disrupting HOX/PBX dimer formation in the pancreatic cancer derived cell line T3M4 also blocks cell proliferation [29], and reduces the expression of a number of cancer-related target genes by at least one order of magnitude. These include TMPRSS3, a transmembrane serine protease involved in tumour invasion and metastasis that is frequently over expressed in pancreatic cancer [30], and the S100 calcium binding protein P (S100P) which is involved in regulating the cell cycle and cell proliferation [31]. Interestingly HOXB2 and HOXA10 are also down regulated when HOX/PBX dimer formation is blocked, indicating a possible auto-regulatory pathway for these genes [29].
Other functions of HOX genes in pancreatic cancer may be more specific to particular members of the HOX family. Although to date there is only very limited data on HOX expression in normal pancreatic tissue of both the developing and adult pancreas, it would seem that generally these genes (HOXA1 [25], HOXA4 [19], HOXA5 [19], and HOXB4 [19]) are not up regulated in pancreatic cancer. Instead, other HOX genes are expressed, some of which have known oncogenic functions. Most notable amongst these is HOXB7 which has been shown to mediate epithelial to mesenchymal transition in breast cancer cells through the induction of bFGF [32], and to promote proliferation in oral cancer [33] and progression and metastasis in lung cancer [34]. HOXB7 is also over expressed in pancreatic cancer, both in primary pancreatic adenocarcinoma and in the pancreatic adenocarcinoma cells lines AsPc-1, BxPC-3, MiaPACA and PANC1 [35]. Two neighbouring genes of the HOXB cluster, HOXB5 and HOXB6, are also over-expressed in pancreatic cell lines as well as resected infiltrating pancreatic cancer tissue. Although the significance of this finding is unknown, it is noteworthy that forced over expression of HOXB6 in murine bone marrow is sufficient to immortalise a population of myelomonocytic precursor cells leading to acute myeloid leukemia [36]. Also up regulated in pancreatic cancer is HOXB2 [37], which was found to be present in 38% of pancreatic tumours. HOXB2 was shown to have prognostic value, as its expression is associated with nonresectable tumours, and when presented in resected tumours it is associated with poor survival. This finding may relate to the apparent ability of HOXB2 to promote metastasis in lung cancer, where its expression is also associated with a poor prognosis [38]. HOXB2 additionally promotes proliferation, as it is bound by the tumour suppressor protein p205 that is known to delay G2/M progression in dividing cells [39].
Other HOX genes are strongly down-regulated in pancreatic cancer. These include HOXD13, which is originally involved in the determination of the terminal digestive and urogenital tracts. HOXD13 expression is lower in tumour tissue compared to normal pancreatic parenchyma [40], and the absence of HOXD13 expression in tumours is associated with a significantly poorer prognosis, the 12-month survival rate for patients with HOXD13- tumours being 17.2% as compared to 79.8% for patients with HOXD13+ tumours. These findings imply that HOXD13 functions as a suppressor of metastasis, a characteristic shared by two further HOX genes, HOXB1 and HOXB3 [41]. Specific knockdown of either of these genes is sufficient to reduce the migration of pancreatic cancer cells in vitro and invasion and metastasis of pancreatic cancer in vivo, using zebrafish embryo xenotransplantation models. The same study also showed that HOXB1 and HOXB3 are suppressed by the microRNA miR-10a, the expression of which is significantly higher in metastatic pancreatic cancer [41].
HOX gene deregulation is also associated with pre-malignant pancreatic intraepithelial neoplasia (PanIN). HOXB2 was found to be expressed in 15% of early PanIN lesions [37], and as described above, is also associated with the fully malignant state, it is therefore possible that expression of HOXB2 in PanIN lesions may predict the development of cancer but this is yet to be shown conclusively. Also up regulated in PanIN is HOXA5 [42], which is of interest because other studies have suggested that its function is closer to a tumour suppressor rather than an oncogene as it activates the transcription of a number of key tumour suppressors, including p53 [43].
Future Directions
Although relatively little is still known of HOX expression and function in pancreatic cancer as compared to many other cancer types, it is already clear that HOX genes are of functional significance to the malignant phenotype. It is not therefore surprising to find that a number of HOX genes have prognostic value and may ultimately be of clinical relevance [38, 40]. More importantly still, HOX gene expression may prove to have diagnostic value for pancreatic cancer. The extremely poor survival rates for this disease are due in part to its predominantly late diagnosis; specific HOX genes expressed in, for example, circulating tumour cells, could potentially detect the presence of pancreatic cancer before the onset of symptoms. Additionally, a similar approach might also be of value in defining pancreatic cancer as a primary tumour in cases where the origin of metastasis is uncertain.
HOX genes are also potential therapeutic targets in pancreatic cancer. To date the only successful strategy for blocking HOX gene function is through interfering with the HOX/PBX interaction using a peptide mimic (HXR9) of the conserved PBX binding domain in HOX [13, 14, 15, 16, 17]. This is a novel strategy in pancreatic cancer, and is worthy of investigation as a possible therapeutic approach.
Key words HOXB7 protein, human; HOXD13 protein, human; HXR9 peptide; Pancreatic Neoplasms
Conflict of interest The authors have no potential conflict of interest
Correspondence
Richard
Morgan
Postgraduate Medical School
Faculty of Health and Medical Sciences
University of Surrey
Daphne Jackson Road
Guildford, GU2 7WG
United Kingdom
Phone: +44-1483.688.618
Fax: +44-1483.688.500
E-mail: r.morgan@surrey.ac.uk
References
1. Abramovich C, Humphries RK. Hox regulation of normal and leukemic hematopoietic stem cells. Curr Opin Hematol 2005; 12:210-6. [PMID 15867577]
2. Iimura T, Pourquie O. Hox genes in time and space during vertebrate body formation. Dev Growth Differ 2007; 49:265-75. [PMID 17501904]
3. Moens CB, Selleri L. Hox cofactors in vertebrate development. Dev Biol 2006; 291:193-206. [PMID 16515781]
4. Scott MP. A rational nomenclature for vertebrate homeobox (HOX) genes. Nucleic Acids Res 1993; 21:1687-8. [PMID 8098522]
5. Hoegg S, Meyer A. Hox clusters as models for vertebrate genome evolution. Trends Genet 2005; 21:421-4. [PMID 15967537]
6. Knoepfler PS, Calvo KR, Chen H, Antonarakis SE, Kamps MP. Meis1 and pKnox1 bind DNA cooperatively with Pbx1 utilizing an interaction surface disrupted in oncoprotein E2a-Pbx1. Proc Natl Acad Sci U S A 1997; 94:14553-8. [PMID 9405651]
7. Phelan ML, Sadoul R, Featherstone MS. Functional differences between HOX proteins conferred by two residues in the homeodomain N-terminal arm. Mol Cell Biol 1994; 14:5066-75. [PMID 7913516]
8. Piper DE, Batchelor AH, Chang CP, Cleary ML, Wolberger C. Structure of a HoxB1-Pbx1 heterodimer bound to DNA: role of the hexapeptide and a fourth homeodomain helix in complex formation. Cell 1999; 96:587-97. [PMID 10052460]
9. Di-Poi N, Koch U, Radtke F, Duboule D. Additive and global functions of HoxA cluster genes in mesoderm derivatives. Dev Biol 2010; 341:488-98. [PMID 20303345]
10. Lappin TR, Grier DG, Thompson A, Halliday HL. HOX genes: seductive science, mysterious mechanisms. Ulster Med J 2006; 75:23-31. [PMID 16457401]
11. Morgan R. Hox genes: a continuation of embryonic patterning? Trends Genet 2006; 22:67-9. [PMID 16325300]
12. Lim H, Ma L, Ma WG, Maas RL, Dey SK. Hoxa-10 regulates uterine stromal cell responsiveness to progesterone during implantation and decidualization in the mouse. Mol Endocrinol 1999; 13:1005-17. [PMID 10379898]
13. Daniels TR, Neacato II, Rodríguez JA, Pandha HS, Morgan R, Penichet ML. Disruption of HOX activity leads to cell death that can be enhanced by the interference of iron uptake in malignant B cells. Leukemia 2010; 24:1555-65. [PMID 20574452]
14. Morgan R, Pirard PM, Shears L, Sohal J, Pettengell R, Pandha HS. Antagonism of HOX/PBX dimer formation blocks the in vivo proliferation of melanoma. Cancer Res 2007; 67:5806-13. [PMID 17575148]
15. Morgan R, Plowright L, Harrington KJ, Michael A, Pandha HS. Targeting HOX and PBX transcription factors in ovarian cancer. BMC Cancer 2010;10:89. [PMID 20219106]
16. Plowright L, Harrington KJ, Pandha HS, Morgan R. HOX transcription factors are potential therapeutic targets in non-small-cell lung cancer (targeting HOX genes in lung cancer). Br J Cancer 2009; 100:470-5. [PMID 19156136]
17. Shears L, Plowright L, Harrington K, Pandha HS, Morgan R. Disrupting the interaction between HOX and PBX causes necrotic and apoptotic cell death in the renal cancer lines CaKi-2 and 769-P. J Urol 2008; 180:2196-201. [PMID 18804814]
18. Cordes R, Schuster-Gossler K, Serth K, Gossler A. Specification of vertebral identity is coupled to Notch signalling and the segmentation clock. Development 2004; 131:1221-33. [PMID 14960495]
19. Kawazoe Y, Sekimoto T, Araki M, Takagi K, Araki K, Yamamura K. Region-specific gastrointestinal Hox code during murine embryonal gut development. Dev Growth Differ 2002; 44:77-84. [PMID 11869294]
20. Tehrani Z, Lin S. Antagonistic interactions of hedgehog, Bmp and retinoic acid signals control zebrafish endocrine pancreas development. Development 2011; 138:631-40. [PMID 21228001]
21. Shi Y. Generation of functional insulin-producing cells from human embryonic stem cells in vitro. Methods Mol Biol 2010; 636:79-85. [PMID 20336517]
22. Langston AW, Gudas LJ. Retinoic acid and homeobox gene regulation. Curr Opin Genet Dev 1994; 4:550-5. [PMID 7950323]
23. Takács-Vellai K, Vellai T, Chen EB, Zhang Y, Guerry F, Stern MJ, Müller F. Transcriptional control of Notch signaling by a HOX and a PBX/EXD protein during vulval development in C. elegans. Dev Biol 2007; 302:661-9. [PMID 17084835]
24. Zakany J, Kmita M, Alarcon P, de la Pompa JL, Duboule D. Localized and transient transcription of Hox genes suggests a link between patterning and the segmentation clock. Cell 2001; 106:207-17. [PMID 11511348]
25. Lomberk GA, Imoto I, Gebelein B, Urrutia R, Cook TA. Conservation of the TGFbeta/Labial homeobox signaling loop in endoderm-derived cells between Drosophila and mammals. Pancreatology 2010; 10:74-84. [PMID 20339309]
26. Plaza S, Prince F, Adachi Y, Punzo C, Cribbs DL, Gehring WJ. Cross-regulatory protein-protein interactions between Hox and Pax transcription factors. Proc Natl Acad Sci U S A 2008; 105:13439-44. [PMID 18755899]
27. Nolte C, Rastegar M, Amores A, Bouchard M, Grote D, Maas R, et al. Stereospecificity and PAX6 function direct Hoxd4 neural enhancer activity along the antero-posterior axis. Dev Biol 2006; 299:582-93. [PMID 17010333]
28. Shah N, Sukumar S. The Hox genes and their roles in oncogenesis. Nat Rev Cancer 2010; 10:361-71. [PMID 20357775]
29. Aulisa L, Forraz N, McGuckin C, Hartgerink JD. Inhibition of cancer cell proliferation by designed peptide amphiphiles. Acta Biomater 2009; 5:842-53. [PMID 19249722]
30. Wallrapp C, Hahnel S, Muller-Pillasch F, Burghardt B, Iwamura T, Ruthenburger M, et al. A novel transmembrane serine protease (TMPRSS3) overexpressed in pancreatic cancer. Cancer Res 2000; 60:2602-6. [PMID 10825129]
31. Arumugam T, Simeone DM, Van Golen K, Logsdon CD. S100P promotes pancreatic cancer growth, survival, and invasion. Clin Cancer Res 2005; 11:5356-64. [PMID 16061848]
32. Wu X, Chen H, Parker B, Rubin E, Zhu T, Lee JS, et al. HOXB7, a homeodomain protein, is overexpressed in breast cancer and confers epithelial-mesenchymal transition. Cancer Res 2006; 66:9527-34. [PMID 17018609]
33. De Souza Setubal Destro MF, Bitu CC, Zecchin KG, Graner E, Lopes MA, Kowalski LP, et al. Overexpression of HOXB7 homeobox gene in oral cancer induces cellular proliferation and is associated with poor prognosis. Int J Oncol 2010; 36:141-9. [PMID 19956843]
34. Chen H, Lee JS, Liang X, Zhang H, Zhu T, Zhang Z, et al. Hoxb7 inhibits transgenic HER-2/neu-induced mouse mammary tumor onset but promotes progression and lung metastasis. Cancer Res 2008; 68:3637-44. [PMID 18463397]
35. Nguyen A, Yang N, Dawson D. HOXB7 overexpression promotes pancreatic adenocarcinoma growth and invasion. AACR Meeting Abstracts 2009; 100th AACR Annual Meeting:4267.
36. Fischbach NA, Rozenfeld S, Shen W, Fong S, Chrobak D, Ginzinger D, et al. HOXB6 overexpression in murine bone marrow immortalizes a myelomonocytic precursor in vitro and causes hematopoietic stem cell expansion and acute myeloid leukemia in vivo. Blood 2005; 105:1456-66. [PMID 15522959]
37. Segara D, Biankin AV, Kench JG, Langusch CC, Dawson AC, Skalicky DA, et al. Expression of HOXB2, a retinoic acid signaling target in pancreatic cancer and pancreatic intraepithelial neoplasia. Clin Cancer Res 2005; 11:3587-96. [PMID 15867264]
38. Inamura K, Togashi Y, Ninomiya H, Shimoji T, Noda T, Ishikawa Y. HOXB2, an adverse prognostic indicator for stage I lung adenocarcinomas, promotes invasion by transcriptional regulation of metastasis-related genes in HOP-62 non-small cell lung cancer cells. Anticancer Res 2008; 28:2121-7. [PMID 18751384]
39. Asefa B, Dermott JM, Kaldis P, Stefanisko K, Garfinkel DJ, Keller JR. p205, a potential tumor suppressor, inhibits cell proliferation via multiple pathways of cell cycle regulation. FEBS Lett 2006; 580:1205-14. [PMID 16458891]
40. Cantile M, Franco R, Tschan A, Baumhoer D, Zlobec I, Schiavo G, et al. HOX D13 expression across 79 tumor tissue types. Int J Cancer 2009; 125:1532-41. [PMID 19488988]
41. Weiss FU, Marques IJ, Woltering JM, Vlecken DH, Aghdassi A, Partecke LI, et al. Retinoic acid receptor antagonists inhibit miR-10a expression and block metastatic behavior of pancreatic cancer. Gastroenterology 2009; 137:2136-45. [PMID 19747919]
42. Prasad NB, Biankin AV, Fukushima N, Maitra A, Dhara S, Elkahloun AG, et al. Gene expression profiles in pancreatic intraepithelial neoplasia reflect the effects of Hedgehog signaling on pancreatic ductal epithelial cells. Cancer Res 2005; 65:1619-26. [PMID 15753353]
43. Raman V, Martensen SA, Reisman D, Evron E, Odenwald WF, Jaffee E, et al. Compromised HOXA5 function can limit p53 expression in human breast tumours. Nature 2000; 405:974-8. [PMID 10879542]