Abstract
Recent studies demonstrated that factors derived from embryonic stem cells inhibit the tumorigenicity of a variety of cancer cell lines. Embryonic stem cell-secreted Lefty, an inhibitor of Nodal-signalling pathway, was implicated in reprogramming cancer cells. Whether adult stem cells exhibited similar properties has not been explored. The aim of the present study was to investigate whether the conditioned medium (CM) derived from adult stem cells influence in vitro and in vivo tumor growth by a Nodal-dependent pathway. In particular we compared the anti-tumor effect of CM from human liver stem cells (HLSC) with that of bone marrow-derived mesenchymal stem cells (MSC). We found that HLSC-CM inhibited the in vitro growth and promoted apoptosis in HepG2 cells that expressed a deregulated Nodal pathway. The effect of HLSC-CM was related to the presence of Lefty A in the CM of HLSC. Silencing Lefty A in HLSC or Lefty A blockade with a blocking peptide abrogated the anti-proliferative and pro-apoptotic effect of HLSC-CM. Moreover, the administration of human recombinant Lefty A protein mimicked the effect of HLSC-CM indicating that Nodal pathway is critical for the growth of HepG2. At variance of HLSC, bone marrow-derived MSC did not express and release Lefty A and the MSC-CM did not exhibited an anti-tumor activity in vitro, but rather stimulated proliferation of HepG2. In addition, the intra-tumor administration of HLSC-CM was able to inhibit the in vivo growth of HepG2 hepatoma cells implanted subcutaneously in SCID mice. At variance, HLSC-CM derived from Lefty A silenced HLSC was unable to inhibit tumor growth. In conclusion, the results of present study suggest that Lefty A may account for the tumor suppressive activity of HLSC as a result of an inhibition of the Nodal-signalling pathway by a mechanism similar to that described for embryonic stem cells.