Down-Regulation of the Extracellular Matrix Protein SPARC in Vsrc- and Vjun-Transformed Chick Embryo ®Broblasts Contributes to Tumor Formation in Vivo
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Oncogene (2000) 19, 1772 ± 1782 ã 2000 Macmillan Publishers Ltd All rights reserved 0950 ± 9232/00 $15.00 www.nature.com/onc Down-regulation of the extracellular matrix protein SPARC in vSrc- and vJun-transformed chick embryo ®broblasts contributes to tumor formation in vivo Emmanuel Vial1 and Marc Castellazzi*,1 1Unite de Virologie Humaine, Institut National de la Sante et de la Recherche MeÂdicale (INSERM-U412), Ecole Normale SupeÂrieure, 46 alleÂe d'Italie, 69364 Lyon Cedex 07, France In vitro transformation of primary cultures of chick cytoskeleton and the extracellular matrix (Jove and embryo ®broblasts by the membrane-bound vSrc or the Hanafusa, 1987). nuclear vJun oncoproteins is correlated with a down- Transcription factor AP1 is a nuclear target of the regulation of the secreted glycoprotein SPARC (also vSrc-induced oncogenic pathway (Muramaki et al., called BM-40 or osteonectin). This protein is a non- 1997; Suzuki et al., 1994) and is thought to trigger structural component of the extracellular matrix that is part of the phenotypic spectrum induced by vSrc thought to regulate cell ± matrix interaction during (Bos et al., 1990; Castellazzi et al., 1990; Muramaki development, wound repair, and carcinogenesis. Its et al., 1997; Suzuki et al., 1994; van Dam et al., precise function remains unclear. To estimate the 1998). This transcription factor consists of a contribution of SPARC down-regulation to the major collection of homo- and heterodimers between aspects of the transformed phenotype, we have re- members of the Jun, Fos and ATF/CREB family expressed this protein from a self-replicating retrovirus (Karin et al., 1997). The vJun oncoprotein, a Rcas, designated R-SPARC, in the transformed cultures. mutated version of cellular cJun, initially isolated These R-SPARC-infected cultures display the following from retrovirus ASV17 (Maki et al., 1987), is main properties: (i) they accumulate the SPARC protein thought to transform by promoting aberrant tran- to a level identical to or only slightly higher than the scription of oncogenically relevant target genes level in normal chick embryo ®broblasts; (ii) they retain (Vogt, 1994). the main phenotypic properties characteristic of in vitro In the search for such target genes whose activity transformed cells, that is, altered morphology, capacity is modulated in vSrc- and/or vJun- transformed to grow in a reduced amount of serum, and capacity to CEFs, we became interested in a particular compo- develop colonies from single cells in agar; (iii) they nent of the extracellular matrix, called SPARC for display a clearly reduced capacity to develop local secreted protein, acidic and rich in cysteine (Mason ®brosarcomas in vivo. Taken together, these data et al., 1986a,b). This protein was initially designated strongly suggest that down-regulation of SPARC as osteonectin (Termine et al., 1981) and later as contributes to the transformed phenotype triggered by BM-40 for basement membrane-40 (Dziadek et al., vSrc and vJun in primary avian ®broblasts, by 1986; reviewed in Reed and Sage, 1996; Timpl and facilitating in vivo tumorigenesis. Oncogene (2000) 19, Aumailley, 1993). SPARC displays a high degree of 1772 ± 1782. sequence conservation from Caenorhabditis elegans to humans, indicating a basic functional role in animal Keywords: vSrc; vJun; AP1; SPARC; chick embryo tissues (Lankat-Buttgereit et al., 1998). Although ®broblasts; cell transformation; tumorigenesis SPARC level is highest in bone and in basement membranes, it is also expressed in a variety of cell types and associated with remodelling tissues and Introduction sites of high cellular turn-over. In vitro studies with SPARC peptides (Funk and Sage, 1993) and with Transformation of chick embryo ®broblasts (CEFs) primary cultures from SPARC null mice (Bradshaw as induced by the membrane-associated vSrc onco- et al., 1999) indicate that this protein regulates cell protein from Rous sarcoma virus, results in the proliferation. SPARC has been shown to be acquisition of four major, abnormal phenotypic repressed by cJun in rat embryo ®broblasts at the properties: (i) an alteration in cell morphology; (ii) mRNA and protein level (Mettouchi et al., 1994) a reduced serum requirement for proliferation; (iii) an and by vSrc in CEFs at the mRNA level (Young et anchorage-independent growth capacity; and (iv) in al., 1986), suggesting a role for this protein in cell vivo tumorigenicity. Numerous reports in this avian transformation. model have associated transformation with complex In the present study, we asked whether SPARC changes in gene expression, aecting among other down-regulation contributes to the transformed aspects the composition and organization of the phenotype established by vSrc or vJun in primary avian cells. Therefore CEFs stably transformed by these oncoproteins were generated. After having shown that SPARC was indeed repressed in these *Correspondence: M Castellazzi cultures, we analysed the consequence of its re- Received 4 October 1999; revised 3 December 1999; accepted 25 expression with respect to the major oncogenic January 2000 properties induced by these oncoproteins. Anti-tumoral effect of SPARC in chickens E Vial and M Castellazzi 1773 Results Steady-state level of SPARC mRNA and protein in CEFs stably transformed by vSrc and vJun Uninfected CEFs or CEF cultures chronically infected by R, R-vSrc, R-vJun, or R-vJun-m1 (a mutant of vJun that exhibits enhanced tumorigenicity in vivo; Huguier et al., 1998) were generated. Total RNA was subjected to Northern blot analysis with an avian SPARC radioactive probe (Figure 1). The level of SPARC mRNA was estimated in the transformed CEFs in comparison with the non-transformed CEFs (referred to as 100%). A reduction of 50% was found with R-vJun, and of 80% with R-vSrc. The strongest reduction (over 90%) was observed with R-vJun-m1. Reprobing of the same blot with GAPDH cDNA Figure 2 Schematic representation of the retroviral vector indicated the same loading for the dierent lanes. Rcas-envD-SPARC sig+ designated RD-SPARC in the text. Protein extracts from the same cultures were The SPARC sig+ gene has been cloned at the unique ClaI site subjected to Western blot analysis followed by present in the vector. sd: splice donor; sa: splice acceptor. immunodetection of the SPARC protein. As expected Retroviral genes gag, pol, and envD are shown from the mRNA levels, SPARC accumulation was reduced in the transformed CEFs (Figure 3a). With non-transformed CEFs arbitrarily set at 100%, vSrc- and vJun-m1-transformed CEFs showed almost un- could be resolved into two distinct, closely-migrating detectable SPARC levels, i.e. 510% and 55%, bands. Although the relative intensity of these bands respectively. These data show that transformation of varied from one extract to another, the upper band CEFs by either vJun or vSrc is accompanied by a clear- was usually more intense. Both bands seemed to cut reduction in the accumulation of the SPARC correspond to the mature form of SPARC, devoid of mRNA and protein. the peptide signal (see right lane `in vitro translated' SPARC sig+ and SPARC sig7 for comparison). As shown in Figure 3b, a treatment of the extracts with Re-expression of SPARC in vSrc- and vJun-transformed N-glycosidase shifted the slower-migrating upper band CEFs to a single, fast-migrating band. The two closely- An Rcas envD derivative carrying the entire coding migrating bands are therefore likely to correspond to sequence of SPARC with the N-terminal peptide two isoforms of mature SPARC with distinct signal, designated RD-SPARC, was constructed glycosylation levels. This explanation is consistent (Figure 2). CEFs infected by RD or RD-SPARC with the existence of potential N-linked glycosylation were generated and superinfected with R, R-vJun, R- sites at Asn 67 and Lys 95 in the avian protein vJun-m1, or R-vSrc. Protein cell extracts were (Bassuk et al., 1993). subjected to Western blot analysis and probed Detection of the SPARC protein was also performed successively with antibodies speci®c to SPARC, Jun, at the single cell level by immuno¯uorescence. In and Src. As shown in Figure 3a (upper panel), agreement with previous reports (Mettouchi et al., infection with RD-SPARC correlated with an en- 1994; Reed and Sage, 1996), SPARC could be detected hanced accumulation of the SPARC protein in all intracellularly in the cytoplasm. The staining was cultures, reaching a level identical to, or slightly homogeneously distributed within the cell population. (about twofold) higher than the level in normal CEFs. As expected, SPARC accumulation was strongly By short exposure (Figure 3a lower panel), SPARC reduced in m1- and Src-transformed CEFs, and returned to a wild-type level as a consequence of RD-SPARC superinfection (data not shown). In the Jun-transformed CEFs (Figure 3c, upper panel), accumulation of the vJun and vJun-m1 oncoproteins was clearly detected, reaching about ®vefold the level of the endogenous cJun. There was no signi®cant dierence in the presence or absence of exogenous SPARC. In the vSrc-transformed CEFs (Figure 3c, lower panel), a strong accumulation of the oncoprotein was observed, reaching over 20-fold the level of the endogenous cSrc. Again, there was no signi®cant dierence with or without exogenous SPARC. In these vSrc-transformed cells endogenous cJun accumulated as a consequence of the activation of Figure 1 Level of accumulation of SPARC mRNA from CEFs AP1 by vSrc (Suzuki et al., 1994). chronically infected by the retroviruses R, R-vJun, R-vJun-m1 Taken together, these data show that SPARC can be (designated m1), or R-vSrc by Northern blot analysis. Total RNA was probed with either the complete SPARC probe or a GAPDH eciently re-expressed in the RD-SPARC-infected, probe as indicated transformed CEFs, and accumulates to levels identical Oncogene Anti-tumoral effect of SPARC in chickens E Vial and M Castellazzi 1774 Figure 3 Level of accumulation of the SPARC (a and b), Jun and Src (c) proteins from CEFs chronically infected by the empty vector Rcas, or by Rcas vectors expressing vJun, vJun-m1 or vSrc with or without SPARC as indicated.