AP2 Protein Expression as a Diagnostic Marker in Soft Tissue Tumours

T T Yang,A C C Reed,N A Athanasou

doi:10.1080/13577140120048593

Abstract

Purpose/Methods: The aP2 gene product (aP2 protein) is known to be expressed by preadipocytes and other immature fat cells in vitro. A mouse monoclonal antibody raised against an 18 amino acid segment of the aP2 protein was found to react with lipoblasts and fetal fat cells in paraffin sections of soft tissue tumours of adipose differentiation. In this immunohistochemical study, we have further examined the diagnostic utility of aP2 expression in distinguishing tumours of adipose differentiation from other benign and malignant soft tissue tumours. Result and discussion aP2 was strongly expressed by lipoblasts in lipoblastomas and all types of liposarcoma as well as brown fat cells in hibernomas. Optimal conditions for immunohistochemical identification of lipoblasts in tumours of adipose differentiation was noted when the antibody was diluted 1:30 to 1:50. Small lipoblast-like fat cells in pleomorphic lipoma and spindle cell lipoma also showed variable staining for aP2 at this dilution of the antibody. Most benign and malignant soft tissue tumours were distinguished by their absence of staining for aP2 protein, but some cases of myxoma, malignant fibrous histiocytoma, synovial sarcoma and leiomyosarcoma contained tumour cells which reacted for aP2. aP2 protein expression is likely to prove a useful means of distinguishing lipoblasts in liposarcoma but it should be used as part of a tumour panel to exclude expression in other forms of mesenchymal tumour.

Highlights

Benign and malignant tumours of adipose differentiation are amongst the most common of soft tissue tumours.[1]
We have previously shown that both polyclonal and monoclonal antibodies raised against the aP2 protein can be used to identify lipoblasts and fetal fat cells in tumours of adipose differentiation immunohistochemically.[2,3] aP2 expression was noted in liposarcomas, lipoblastomas and hibernomas
Fewer than 20% of lipoblasts reacted for aP2 in well-differentiated liposarcomas; as indicated above, the proportion of lipoblasts stained in this subtype of liposarcoma was much increased when the antibody was used at a lower dilution

Summary

Introduction

Benign and malignant tumours of adipose differentiation are amongst the most common of soft tissue tumours.[1]. With the exception of lipoblastoma, and to a lesser extent pleomorphic lipoma and spindle cell lipoma, lipoblasts are absent from benign tumours of adipose differentiation. Morphological identification of lipoblasts may be problematic, as these cells can be few in number or difficult to identify in well-differentiated liposarcomas. They may be difficult to distinguish morphologically from vacuolated tumour cells in non-adipose benign and malignant mesenchymal soft tissue tumours. We have previously shown that both polyclonal and monoclonal antibodies raised against the aP2 protein can be used to identify lipoblasts and fetal fat cells in tumours of adipose differentiation immunohistochemically.[2,3] aP2 expression was noted in liposarcomas, lipoblastomas and hibernomas. The aP2 gene ( known as P422 and adipocyte lipid binding protein) codes for a 132 amino acid, single chain 14.6kd polypeptide which shows considerable homology with myelin P2 protein.[4,5] aP2 expression is coincident with acquisition of the morphological features of the adipocyte phenotype 4 and aP2 is expressed in preadipocytes late in adipogenesis.[6]

Methods

Results

Discussion

Conclusion