Caldesmon is a major calmodulin binding protein that also interacts with actin filaments.¹ Two molecular weight forms of caldesmon—high molecular weight caldesmon (HCD) and low molecular weight caldesmon—have been purified.² In particular, HCD, with a molecular weight 120–150 kDa, is associated with smooth muscle contraction and is localised exclusively to smooth muscle cells.² In contrast to α smooth muscle actin, HCD is not detected in myofibroblasts,³ indicating that HCD could be used as a highly specific marker for smooth muscle cells and smooth muscle tumours.⁴ In the colorectum, pericryptal fibroblasts are positive for HCD.⁵

α Smooth muscle actin is an excellent marker for myofibroblasts and transformed hepatic stellate cells,⁶ but is not specific for smooth muscle cells.³ To investigate the presence of smooth muscle cells in hepatic stroma, immunostaining for HCD should be performed. There are no reports regarding immunohistochemical analysis for HCD positive stromal cells in liver neoplasms.

“α Smooth muscle actin is an excellent marker for myofibroblasts and transformed hepatic stellate cells, but is not specific for smooth muscle cells”

To investigate the smooth muscle nature of the stromal cells in the capsule of hepatocellular carcinomas, we examined the expression of HCD in the capsule of hepatocellular carcinomas and adjacent hepatic tissues.

MATERIALS AND METHODS

We examined 33 completely or partially encapsulated hepatocellular carcinomas and their adjacent hepatic tissues from 33 postmortem cases (eight patients with hepatitis B virus (HBV) associated hepatic cirrhosis, 12 with hepatitis C virus (HCV) associated hepatic cirrhosis, and the remaining 13 with hepatic cirrhosis of unknown aetiology) in the department of molecular pathology, graduate school of biomedical sciences, Hiroshima University, Japan, from 1981 to 1999. All of the 13 postmortem cases with unknown aetiology were before 1989, and laboratory examinations of HCV infection were not performed. Immunohistochemical studies were performed using a Histofine SAM-PO(multi) kit (Nichirei, Tokyo, Japan). A monoclonal antibody against HCD (clone h-CD; Dakopatts, Glostrup, Denmark; 1/50 dilution; microwave treatment) was used. Vascular smooth muscle cells served as positive controls for HCD. We regarded HCD positive cells in the capsule to be HCD positive stromal cells.

RESULTS

HCD positive stromal cells were detected in the capsule of 21 of the 33 hepatocellular carcinomas, whereas no HCD positive cells were found in the fibrous septa of the adjacent hepatic tissues examined (fig 1). Of the 21 hepatocellular carcinomas with HCD positive stromal cells, four were HBV associated lesions, nine were HCV associated lesions, and the remaining eight had an unknown aetiology.

• Download figure
• Open in new tab
• Download powerpoint

Figure 1

 Immunostaining for high molecular weight caldesmon (HCD) in hepatocellular carcinoma. HCD positive stromal cells were detected in the tumour capsule; HCD was also positive in the vessel walls (vascular smooth muscle cells; lower right hand corner).

DISCUSSION

The presence of intermediately differentiated smooth muscle cells in the capsule of hepatocellular carcinomas is suggested by immunohistochemical analysis using various smooth muscle markers (desmin, smooth muscle actin (clone 1A4, HHF35, CGA7), and smooth muscle myosin heavy chain (SM1, SM2)) and electron microscopy.⁷ Unfortunately, the antibodies used in the above mentioned study⁷ are not specific for smooth muscle cells, but are also positive in myofibroblasts.³ Therefore, to confirm the presence of smooth muscle cells, immunostaining for HCD should be performed. Recently, HCD positive stromal cells were detected in the capsule of thyroid follicular tumours and tumour-like lesions.⁸

In conclusion, HCD positive stromal cells, namely smooth muscle cells, were detected in the capsule of hepatocellular carcinomas. To elucidate the origin of the capsular smooth muscle cells, further molecular and biological investigations are needed.