LTBP-3 promotes early metastatic events during cancer cell dissemination

Latent Transforming Growth Factor β (TGFβ) Binding Proteins (LTBPs) are important for the secretion, activation and function of mature TGFβ, especially so in cancer cell physiology. However, specific roles of the LTBPs remain understudied in the context of the primary tumor microenvironment. Herein, we investigated the role of LTBP-3 in the distinct processes involved in cancer metastasis. By using three human tumor cell lines of different tissue origin (epidermoid HEp-3 and prostate PC-3 carcinomas and HT-1080 fibrosarcoma) and several metastasis models conducted in both mammalian and avian settings, we show that LTBP-3 is involved in the early dissemination of primary cancer cells, namely in the intravasation step of the metastatic cascade. Knockdown of LTBP-3 in all tested cell lines led to significant inhibition of tumor cell intravasation, but did not affect primary tumor growth. LTBP-3 was dispensable in the late steps of carcinoma cell metastasis that follow tumor cell intravasation, including vascular arrest, extravasation and tissue colonization. However, LTBP-3 depletion diminished the angiogenesis-inducing potential of HEp-3 cells in vivo, which was restorable by exogenous delivery of LTBP-3 protein. A similar compensatory approach rescued the dampened intravasation of LTBP-3-deficient HEp-3 cells, suggesting that LTBP-3 regulates the induction of the intravasation-supporting angiogenic vasculature within developing primary tumors. Using our recently developed microtumor model, we confirmed that LTBP-3 loss resulted in the development of intratumoral vessels with an abnormal microarchitecture incompatible with efficient intravasation of HEp-3 carcinoma cells. Collectively, these findings demonstrate that LTBP-3 represents a novel oncotarget that has distinctive functions in the regulation of angiogenesis-dependent tumor cell intravasation, a critical process during early cancer dissemination. Our experimental data are also consistent with the survival prognostic value of LTBP3 expression in early stage head and neck squamous cell carcinomas, further indicating a specific role for LTBP-3 in cancer progression towards metastatic disease.