PROS1 Signaling: Revealing a Novel Biological Function and its Potential Therapeutic Targeting in Melanoma

Introduction: Melanoma is the most lethal form of skin cancer with a median overall survival of less than one year. Abnormal expression and activation of TYRO3, AXL and MERTK, collectively termed TAMs, has been implicated in proliferation, survival and drug resistance of many cancers, including melanoma. TAMs are activated by endogenous soluble ligands, Protein S (PROS1) and growth arrest-specific 6 (GAS6). The role of GAS6 in TAM activation is well documented, but the validity of PROS1 as a TAM agonist in melanoma was never tested. We hypothesize that PROS1 stimulates TAM oncogenic signaling in melanoma leading to increased proliferation, migration, invasiveness and survival.

Materials and methods: Numerous melanoma cell lines were screened for PROS1 protein expression levels. Stable PROS1 knockdown (PROS1-Kd) were generated for cell lines by shRNA technology and the impact of PROS1 inhibition was screened by several in-vitro assays including: (i) Proliferation (ii) Colony survival (iii) Anchorage-independent growth in soft agar. Further effects of PROS1-Kd were tested in-vivo. Control and PROS1-Kd cells were transplanted subcutaneously into host nude mice. Growth of xenograft tumors was measured and the impact of PROS1 silencing in various melanoma cell lines evaluated.

Results and discussion: Our preliminary results from several melanoma cell lines indicate different PROS1 expression levels, as well as altered downstream oncogenic signaling. Compared to their parental cell lines, PROS1-Kd cells showed decreased proliferation, colony survival and soft agar colony formation. These results were extended in-vivo, where reduction in tumor formation and volume was observed following PROS1 inhibition. Exogenous addition of PROS1 in naïve cells resulted in activation of the AKT and ERK pathways; which were inhibited by PROS1-kd. Activation of the relavant TAM receptor MERTK was inhibited following PROS1-Kd.

Conclusions: This is a novel study investigating the role of PROS1 in melanoma for the first time. Results indicate PROS1 functions to potentiate tumor cell phenotypes, and its inhibition suppresses oncogenic signaling in melanoma. Finally, our results also suggest PROS1 as a novel target for melanoma therapy.