The interaction between Hedgehog/Patched and Ras signaling in Rhabdomyosarcoma

Abstract

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma in children. In children, the most common histological subtypes are embryonal RMS (ERMS) and alveolar RMS (ARMS). One feature of human RMS is aberrant Hedgehog (HH) signaling. In mice, RMS formation occurs due to mutations in the Hh receptor Patched (Ptch), which equally activate Hh signaling. Furthermore approximately 60 % of ERMS show active rat sarcoma (RAS) signaling. RAS and HH signaling are known to interact in several cancer types like lung, colon and pancreas. However an interdependency of these pathways in RMS is unknown. The aim of this thesis was to analyze whether RAS and HH signaling influence each other and cooperate in the pathogenesis of RMS. For this purpose oncogenic NRAS, KRAS and HRAS (collectively named oncRAS) were overexpressed in human RMS cell lines and in Ptch mutant mice. Indeed, oncRas signaling seems to cooperate with Hh signaling in RMS. This includes changes in the expression of Hh target genes and differences in RMS growth in Ptch mutant mice. Interestingly, the cooperation between Ras and Hh depends on the Ras isoform. Thus, the data of the genetic approach shows that expression of oncKRas and oncHRas in RMS decrease the tumor latency time and increase the tumor incidence, whereas oncNRas does not. Surprisingly, oncNRas decreases the tumor multiplicity in Ptch mutant mice and suppresses the proliferation of the tumors and thus improves the prognosis. Furthermore, the in vitro data show that oncRAS suppresses HH signaling activity in human RMS cell lines. However the mode of action differs depending on the RMS subtype. While in the ERMS cell line RUCH-2 the RAS–mediated inhibition is caused by the MEK/ERK axis, it seems to be regulated by a jet unknown factor in the ARMS cell line RMS-13. Additionally the effects of active RAS signaling on the proliferative capacity and metabolic activity seem to vary depending on the RMS subtype. While oncRAS suppresses the proliferative capacity and metabolic activity of RUCH-2 cells, it enhances the proliferative capacity and metabolic activity of RMS-13 cells. Together, these data indicate that HH and RAS signaling interact with each other in RMS. The outcome, however, depends on the RMS subtype and the RAS isoform. Thus, depending on the oncRas subtype the prognosis can either worsen or improve.