Abstract
Bladder cancer associated protein (Blcap) expression is commonly down-regulated in invasive bladder cancer, and may have prognostic value given that its expression is negatively correlated with patient survival. We have previously investigated the expression patterns and cellular localization of Blcap in bladder cancer, where we found that about 20% of the lesions examined displayed strong nuclear expression of Blcap, and that this phenotype was associated with overall poor disease outcome. Here we report on the analysis of possible functional associations between nuclear expression of Blcap and canonical signaling pathways. We performed serial immunohistochemistry (IHC) analysis of bladder tissue samples, with serial sections stained with phospho-specific antibodies recognizing key signaling intermediates, such as P-Stat3, P-Akt, and P-Erk1/2, among others, in an immunophenotyping approach we have established and reported previously. Using this approach, we found that nuclear localization of Blcap was associated with expression of P-Stat3. A parallel analysis, cytokine profiling of bladder tumor interstitial fluids of samples expressing (or not) Blcap, showed interleukin (IL)-6, IL-8, and monocyte chemotactic protein 1 (MCP-1) to be correlated with nuclear expression of Blcap, independently supporting a role for Stat3 signaling in localization of Blcap. Multiple indirect immunofluorescence analysis of tissue biopsies confirmed that Blcap co-localized with Stat3. Furthermore, we could also demonstrate, using an in situ proximity ligation assay that Blcap and Stat3 are in close physical proximity of each other in bladder tissue, and that Blcap physically interacts with Stat3 as determined by co-immunoprecipitation of these proteins. Our data indicates that Blcap is a novel Stat3 interaction partner and suggests a role for Blcap in the Stat3-mediated progression of precancerous lesions to invasive tumors of the bladder.
Highlights
Bladder Cancer Associated Protein (Blcap), is a small (10 kDa), highly conserved protein whose expression is lost in various cancers, such as cervical, bladder and renal cancer, as well as in human tongue carcinoma and osteosarcoma [1,2,3,4,5,6,7]
We found that expression of three cytokines, monocyte chemotactic protein 1 (MCP-1), IL-8, and IL6, followed nuclear expression of Blcap, and of all the signaling intermediates examined, only p-Signal transducer and activator of transcription 3 (Stat3) correlated with Blcap
We found that localization of Blcap was highly correlated to that of p-Stat3 (Figs 2 and 3), suggesting that nuclear presence of Blcap might be the result of a physical interation with Stat3
Summary
Bladder Cancer Associated Protein (Blcap), is a small (10 kDa), highly conserved protein whose expression is lost in various cancers, such as cervical, bladder and renal cancer, as well as in human tongue carcinoma and osteosarcoma [1,2,3,4,5,6,7]. Given that Blcap is reportedly a tumor suppressor, able to inhibit cell proliferation and induce apoptosis [4, 9], it was somewhat counterintuitive that some tumors expressed this protein at very high levels, and that overexpression conferred a worse prognosis. Another challenging observation we made, concerned the strong nuclear Blcap expression observed, because primary sequence analysis of Blcap using two different protein topology prediction methods indicated Blcap as being an integral transmembrane protein (total probability of N-in 0.087213 for TMMOD and 0.01091 for TMHMM), with two trans-membrane domains, TM20-38 and TM45-69, respectively [1]. To investigate the biological underpinnings of these observations, we set out to identify factors involved in Blcap overexpression and/or nuclear localization
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