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A selective role for CDCA7 in lymphomagenesis
a common event in lymphoid tumors. CDCA7 protein levels were not higher in myeloid tumors cell lines than in LCLs, but its expression in these tumors should be com- pared with that in control cells of the same lineage before drawing definitive conclusions. In addition, we cannot rule out that CDCA7 expression may be enhanced in other cell lines or in primary tumors.
MYC and activator E2F transcription factors are expressed only in proliferating cells, and there is a correla- tion between expression of their target genes and the growth rate of cells in culture.36 The increase of CDCA7 mRNA levels in tumor samples relative to control tissues could therefore have been merely the consequence of the presence of more proliferating cells in the tumor tissue. We have shown however that CDCA7 mRNA and protein levels are sharply elevated in lymphoid tumor cell lines relative to immortal cell lines (LCLs) and we had previous- ly demonstrated that LCL and BL cell lines display indis- tinguishable proliferation rates and cell cycle profiles.30 These data suggest that the elevated expression of CDCA7 in lymphoid tumors is not simply the conse- quence of the presence of more proliferating cells relative to control tissues. Instead, our data support the notion that CDCA7 upregulation in these tumors is critical for anchor- age-independent growth and tumorigenesis. Indeed, we show that its silencing in lymphoid tumor cells markedly inhibits their anchorage-independent growth and their tumor formation capacity in immunodeficient mice. These data therefore point to CDCA7 as a potential target for therapeutic intervention in lymphoid tumors.
Previous reports provided contradictive evidence for the role of CDCA7 in malignant transformation. While one report showed that CDCA7 overexpression in an immortal fibroblast cell line impaired MYC-induced colony forma- tion,21 other reports proposed that forced CDCA7 expres- sion promoted transformation in vitro and in vivo.19,20 However, the experimental evidence supporting a trans- forming activity for CDCA7 was very weak. On the one hand, CDCA7 overexpression in a B-cell line or in a fibrob- last cell line increased only modestly their already existing capacity to form colonies in soft agar.19 Since both cell lines had a basal capacity to grow in this semisolid medium, it seems that these cells, instead of being merely immortal, were already transformed or at least partially transformed. On the other hand, only 4 out of 45 (8.88%) transgenic mice overexpressing CDCA7 in the B-cell compartment presented lymphoid malignancies at 1 year, whereas 3 out of 28 (10.71%) control littermates presented lymphomas.20 Together, these data did not support the notion that CDCA7 overexpression induces malignant transformation. In fact, we have shown herein that CDCA7 overexpression in non-transformed B cells fails to induce their growth in soft agar even in combination with constitutively active H- RAS. Our data therefore suggest that CDCA7 is necessary but not sufficient for lymphoid malignant transformation.
References
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Numerous genes that mediate tumor transformation and anchorage-independent growth are also essential for cell proliferation over rigid surfaces. This is the case, for instance, of E2F1, whose knock-down in BL cells inhibits tumor growth in vivo, colony formation in soft agar, and cell proliferation in liquid culture.30 The absence of anchor- age of non-tumor cells to a rigid surface does not permit their proliferation likely because they miss either the growth- or the survival-promoting signals provided by anchorage,37 whereas tumor cells might autonomously generate these signals.37 While normal epithelial cells and fibroblasts require attachment to a rigid surface for sur- vival, lymphocytes are viable in the blood and lymph. However, normal lymphocytes do not proliferate in the blood, but only in lymphoid organs where they might potentially interact with the extracellular matrix and with other cells within these organs. Thus, it seems likely that deprivation of growth-promoting signals provided by anchorage accounts for the inability of non-tumor lym- phoid cells to proliferate in soft agar. Aberrantly elevated CDCA7 levels in lymphoid tumor cells might correspond- ingly contribute to generate the growth-promoting signals provided by anchorage. It will be therefore of great inter- est to investigate the molecular mechanisms involved in CDCA7-promoted anchorage-independent growth.
Together, our findings identify CDCA7 as an important regulator of lymphoid tumor transformation. The inhibi- tion of tumor formation capacities of T-cell leukemia, DLBCL, and BL cells upon CDCA7 silencing point to CDCA7 as a candidate for therapeutic intervention in lymphoid tumors. Since CDCA7 knock-down in primary diploid fibroblasts does not inhibit their proliferation, it seems plausible that therapies aimed at inhibiting CDCA7 expression or function might significantly inhibit the growth of lymphoid tumors while likely not affecting the proliferation of normal cells.
Acknowledgments
The authors would like to thank D. Trono for plasmids; the CNIO Tumor Bank for providing some of the cases included in this study; and the Bioinformatics facility of Centro Nacional de Investigaciones Cardiovasculares (Madrid, Spain) for analysis of microarray data.
Funding
This work was supported by the Spanish Ministerio de Economía, Industria y Competitividad (MEIC) grants to MRC. (SAF2013-45258P and SAF2017-88881-R) and TIV. (SAF2014-52737P); and by Instituto de Salud Carlos III (CIBERNED) to TIV. The cost of this publication has been paid in part with FEDER funds. OK holds an FPI fellowship from MEIC (BES-2014-069236). The CNIC is supported by the Ministerio de Ciencia, Investigación y Universidades and the Pro CNIC Foundation, and is a Severo Ochoa Center of Excellence (SEV-2015-0505).
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