Rhabdomyosarcomas are a heterogeneous group of tumors with respect to their molecular basis, degree of differentiation, histology, and clinical behavior. Because of the wide variation of tumor morphology, it is often difficult to distinguish between the distinct subtypes of rhabdomyosarcomas. By using cryosections of tumor specimens and immunohistochemistry, in the present study we show that strong expression of myogenin in rhabdomyosarcoma is associated with alveolar histology (P ‫؍‬ <0.0001, Fisher's exact test). Although staining for myogenin was observed in 22 of 26 rhabdomyosarcomas, all alveolar rhabdomyosarcomas (nine of nine) showed high levels of staining for myogenin, as defined by the frequency and intensity of staining of the tumor cells. The staining pattern suggests that the tumor cells are clonally derived from myogenin-positive progenitor cells. In contrast, most embryonal rhabdomyosarcomas (13 of 15) were either negative or showed a low level of staining for myogenin. In these tumors a larger proportion of tumor cells were distinctly negative for myogenin. Six of seven alveolar rhabdomyosarcomas that strongly stained for myogenin were also positive for Pax3-7/Forkhead (FKHR) by polymerase chain reaction/reverse transcriptase-polymerase chain reaction. One of two embryonal rhabdomyosarcomas that strongly stained for myogenin was retrospectively found to be positive for Pax3/FKHR transcripts. Quantitative analysis for myogenin by Western blotting using a smaller subset of rhabdomyosarcomas revealed that in general there was a good correlation between immunohistochemical staining and Western blotting (P ‫؍‬ 0.01, Pearson Correlation), although the former technique was more sensitive for detecting tumors with low levels of the protein. On average, alveolar rhabdomyosarcomas expressed at least threefold more myogenin than embryonal rhabdomyosarcomas. Our data show that staining for myogenin will be a simple, rapid, and accurate adjunct for distinguishing between alveolar and embryonal rhabdomyosarcomas. We propose that embryonal rhabdomyosarcomas result from an early block in myogenesis, before the expression of myogenin. In contrast, we propose that alveolar rhabdomyosarcomas either originate from a late block in myogenesis (after expression of myogenin) or that the pathological mechanisms involved in these neoplasms also induce strong expression of this protein. Rhabdomyosarcomas are malignant myogenous tumors that can occur in any part of the body, including both skeletal muscle tissue and sites that are devoid of muscle. 1 These tumors are highly heterogeneous, not only with respect to their clinical behavior, but also with respect to the tumor morphology and spectrum of differen-

The expression of the insulin-like growth factors (IGFs) and their receptors has been linked to cellular proliferation and tumorigenicity in a number of model systems. Since rhabdomyosarcoma cells express IGF-I receptors, an autocrine or paracrine loop involving this receptor and its ligands could be responsible in part for the growth characteristics of this tumor. To assess directly the role of the IGF-I receptor in rhabdomyosarcoma cell growth and tumorigenicity, a human alveolar rhabdomyosarcoma cell line with high IGF-I receptor expression was transfected with an amplifiable IGF-I receptor antisense expression vector. Four unique, transfected clones were analyzed and found to have reduced IGF-I receptor expression relative to the parental line. Integration of the antisense sequence was demonstrated by Southern blot analysis, and expression of antisense message in these clones was shown by S1 nuclease protection assay. Reduced IGF-I receptor surface expression in the transfectants was shown by decreased immunofluorescence with an IGF-I receptor monoclonal antibody and by decreased IGF-I binding as measured by Scatchard analysis. These clones had markedly reduced growth rates in vitro, impaired colony formation in soft agar, and failed to form tumors in immunodeficient mice when compared with vector-transfected clones. These results demonstrate that reduction of IGF-I receptor expression can inhibit both the in vitro and in vivo growth of a human rhabdomyosarcoma cell line and suggest a role for the IGF-I receptor in mediating neoplastic growth in this mesenchymally derived tumor.

SUMMARY Serotonin (5-hydroxytryptamine, 5-HT) mediates many functions of the central and peripheral nervous systems by its interaction with specific neuronal and glial receptors. Fourteen serotonin receptors belonging to seven families have been identified through physiological, pharmacological, and molecular cloning studies. Monoclonal antibodies (MAbs) specific for each of these receptor subtypes are needed to characterize their expression, distribution, and function in embryonic, adult, and pathological tissues. In this article we report the development and characterization of MAbs specific to the serotonin 5-HT 2A receptor. To generate MAbs against 5-HT 2A R, mice were immunized with the N-terminal domain of the receptor. The antigens were produced as glutathionine S-transferase (GST) fusion proteins in insect cells using a Baculovirus expression system. The hybridomas were initially screened by ELISA against the GST-5-HT 2A R recombinant proteins and subsequently against GST control proteins to eliminate clones with unwanted reactivity. They were further tested by Western blotting against recombinant GST-5-HT 2A R, rat and human brain lysate, and lysate from cell lines transfected with 5-HT 2A R cDNA. One of the MAbs G186-1117, which recognizes a portion of the 5-HT 2A R N-terminus, was selected for further characterization. G186-1117 reacted with a band of molecular size 55 kD corresponding to the predicted size of 5-HT 2A R in lysates from rat brain and a 5-HT 2A R-transfected cell line. Its specificity was further confirmed by adsorption of immunoreactivity with recombinant 5-HT 2A R but not with recombinant 5-HT 2B R and 5-HT 2C R. Rat brain sections and Schwann cell cultures were immunohistochemically labeled with this MAb. G186-1117 showed differential staining in various regions of the rat brain, varying from regions with no staining to regions of intense reactivity. In particular, staining of cell bodies and dendrites of the pyramidal neurons in the cortex was observed, which is in agreement with observations of electrophysiological studies. (J Histochem Cytochem 46:811-824, 1998)

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DNA from 13 (6 alveolar and 7 embryonal) childhood rhabdomyosarcomas (RMS) was examined to determine the incidence and prognostic relevance of N- and c-myc genes. Southern analysis showed 5- to 20-fold amplification of N-myc gene in 4 of 6 alveolar but in none of 7 embryonal RMS (p less than 0.04; Fisher's exact test). The number of children who died with multiple- and single-copy N-myc gene was 4/4 and 5/9 respectively (p greater than 0.05; Chi-squared test). There was no statistically significant correlation between N-myc amplification and age, gender, site, stage or survival time. There was no amplification or gross rearrangement of c-myc in any of the 13 RMS.

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