Osteopontin (OPN/SPP1) isoforms collectively enhance tumor cell invasion and dissemination in esophageal adenocarcinoma

Abstract

Esophageal adenocarcinoma (EAC) is often diagnosed at an advanced stage, thus understanding the molecular basis for EAC invasion and metastasis is critical. Here we report that SPP1/OPN was highly overexpressed in primary EACs and intracellularly localized to tumor cells. We further demonstrate that all known OPN isoforms (OPNa, b, c, 4 and 5) were frequently co-overexpressed in primary EACs. Distinct pro-invasion and dissemination phenotypes of isoform-specific OPNb and OPNc stable transfectants were observed. Expression of OPNb significantly enhanced cell migration and adhesion to laminin. In contrast, OPNc cells showed significantly decreased cell migration yet increased cell detachment. Enhanced invasion, both in vitro and in vivo, was observed for OPNb- but not OPNc-expressing cells. Inhibition of RGD integrins, one family of OPN receptors, attenuated OPNb cell migration, abrogated OPNb cell adhesion and significantly reduced OPNb cell clonogenic survival but did not affect OPNc phenotypes, indicating that OPNb but not OPNc acts through integrin-dependent signaling. Differential expression of vimentin, E-cadherin and β-catenin in OPN stable cells may account for the variation in cell adhesion and detachment between these isoforms. We conclude that while all OPN isoforms are frequently co-overexpressed in primary EACs, isoforms OPNb and OPNc enhance invasion and dissemination through collective yet distinct mechanisms.

Osteopontin-a splice variant is overexpressed in papillary thyroid carcinoma and modulates invasive behavior

“…Other reports have demonstrated that OPN-SV play an important role in tumor progression by regulating cell growth, adhesion, migration and tumor formation. As reported by Lin J and co-workers [37], OPNb overexpressing cells derived from esophageal adenocarcinoma also evoked enhanced cell proliferation, migration and invasion. Additionally, our group also previously demonstrated that OPNc activates invasion and adhesion properties, as well as metastatic potential and angiogenesis in a prostate and an ovarian carcinoma model [26, 38].…”

Embracing the complexity of matricellular proteins: the functional and clinical significance of splice variation

“…This study found that all five characterised isoforms of OPN, including OPN-4 and OPN-5, are co-overexpressed in primary oesophageal cancer (60). However, no other studies have yet examined the function of OPN-4 and OPN-5 or their potential role in disease.…”

“…OPN-5 was recently described by Lin et al (60) and is now the longest transcript identified. As mentioned above, this variant has a unique additional exon (exon 4) containing an alternative translation start site.…”

“…As show in Figure 1, a total of six splice variants were identified across the two databases, and in Table 2 we have integrated the IDs for these splice variants in ENSEMBL, Genbank and the common names used in the literature. Three splice variants of OPN have been known for some time, widely referred to as OPN-a, OPN-b and OPN-c in the literature, and have been relatively well characterised, while OPN-4 and OPN-5 were only very recently identified (60). Variant 201 listed in ENSEMBL has not been previously described and appears to affect exon 7 outside the splicing region, although this variant is listed as TSL-2 and remains to be confirmed.…”

“…In a similar study by Lin et al, stable expression of OPN-b in esophageal carcinoma (EAC) cells increased migration and enhanced invasion compared to cells expressing OPN-c. However, OPN-c expression dramatically reduced cell adhesion, and the authors suggest that OPN isoforms have distinct effects on cell behaviour but can work together to promote EAC progression (60). This is consistent with the clinical evidence described above suggesting that both OPN-b and OPN-c are overexpressed in some cancers, for example pancreatic (69,74), gastric (70), lung (74), and prostate (73), while OPN-c may be most strongly associated with other cancers, such as breast cancer (64,74,75) and ovarian cancer (76).…”