Contrasting levels of p21ras activation and expression of neurofibromin in peripheral primitive neuroectodermal tumour and neuroblastoma cells, and their response to retinoic acid

Abstract Ras protooncogenes encode small guanine nucleotide binding proteins (p21 ras ) activated by phosphorylation. Phosphorylation of p21 ras is predominantly regulated by the GTPase activating proteins type 1 GAP 120 and neurofibromin. Increased levels of p21 ras -GTP (active) have been associated with increased cell growth and malignant transformation. In this study the relationship between p21 ras , type 1 GAP 120 and neurofibromin with growth and differentiation has been examined in neuroblastoma and peripheral primitive neuroectodermal tumour (pPNET) cell lines. The level of p21 ras protein in neuroblastoma and pPNET cells was the same. However, the amount of p21 ras -GTP bound was higher in pPNET than in neuroblastoma cells. This most likely reflects the absence of neurofibromin. Retinoic acid (RA)-induced differentiation and growth inhibition of neuroblastoma cells was associated with an increase in type 1 GAP 120 and neurofibromin mRNA, and a decrease in p21 ras -GTP. In pPNET cells levels of type 1 GAP 120 but not neurofibromin mRNA were increased to similar levels to those in neuroblastoma cells. This was not associated with decreased p21 ras -GTP, modulation of growth or change in morphology. In summary, constitutive activation of p21 ras may have a role in the biology of pPNET cells. This may reflect abnormalities in neurofibromin expression, and could inpart explain why RA did not induce morphological differentiation and growth inhibition in pPNETs.