Invasive micropapillary carcinoma (IMPC), a rare distinct histopathological subtype, is characterized histopathologically by the presence of small morula-like clusters of carcinoma cells displaying reverse polarity (so-called inside-out pattern), wherein the apical pole of the neoplastic cells faces the stroma side surrounded by clear stromal space (1). This rare variant was originally described in the breast in 1980 (2), and has been increasingly recognized as a distinct histopathological subtype in a variety of organs, including the salivary gland (3), lung (4), stomach (5), colon (5), and urinary bladder (3–6). Recognition of this variant is important because previous reports demonstrated that IMPC shows significantly more frequent lymphovascular invasion and lymph node metastasis (1,3–7). Although rare, IMPC in the pancreas has been described (8,9), but only one case series of IMPC of the pancreas has been reported (8). Khayyata et al analyzed IMPC occurring in the ampullo-pancreatobiliary region, and revealed that 2.6% of pancreatic carcinomas have an IMPC component (>20% of the tumor) and these patients show a poor prognosis (8). However, only limited information regarding the clinicopathological features of IMPC is available.

In order to differentiate IMPC from carcinoma with retraction artifacts, several markers, such as epithelial membrane antigen (EMA), mucin core protein 1 (MUC1), and sialyl-Lewis X, have been suggested for identifying reverse polarity in the neoplastic nests of IMPC (10–12). MUC1 is a type I transmembrane glycoprotein that belongs to the family of mucin proteins and is expressed in various epithelial cells, EMA is also expressed in various epithelial cells. The protein kinase C (PKC) isoforms are protein kinases involved in multiple signal transduction cascades (13). In particular, the PKCζ isoform is reported that it is required for growth, invasion and tumorigenesis (14), and plays an important role in the maintenance of cell polarity and is expressed in the apical portion of epithelial cells (13,15). However, there are no studies using PKCζ immunostaining to detect reverse polarity in the neoplastic nests of IMPC.

The aim of the present study is to clarify the clinicopathological features of IMPC of the pancreas. Moreover, we examine the usefulness of PKCζ immunostaining to detect reverse polarity in IMPC.

In the present study, we demonstrated that the frequency of IMPC in pancreatic ductal adenocarcinoma patients is 5.8%, although the extent of the IMPC component was less than 20% in all of the patients with IMPC component, and the presence of IMPC component (less than 20%) did not indicate a worse prognosis. Moreover, we also clearly showed that PKCζ is a useful immunohistochemical marker for detecting reverse polarity of IMPC and this is the first report for usefulness.

The clinicopathological significance of IMPC has been reported in several organs, including the breast (1,7) and urinary bladder (6). However, probably due to lack of recognition of this entity in the pancreas, only one case series of IMPC (8) and one case report of pure IMPC (9) in the pancreas have been described in the English literature. Khayyata et al reported IMPC in the pancreas for the first time (8). They defined IMPC as invasive micropapillary architecture in more than 20% of the tumor, and the incidence of pancreatic IMPC was 2.6%. All IMPC components were located in the pancreas head, and the characteristic histopathological finding was abundant infiltration of neutrophils around the tumor nests of IMPC. In the present series, we examined pancreatic ductal adenocarcinoma with IMPC component regardless of the extent, because the clinicopathological features of pancreatic ductal adenocarcinoma with less than 20% IMPC component have not been clarified. The frequency of pancreatic IMPC (5.8%) in this study was higher than the results of the previous report (2.6%) (8), although we included cases where the extent of IMPC component was less than 20%. Neutrophilic infiltration was observed in 85.7% of patients in the present series, which may be a characteristic feature of pancreatic IMPCs, because this has rarely been described in IMPCs occurring in other organs (1,3–7). Moreover, this study revealed that pancreatic IMPC can develop in the pancreas body and tail (4/14 patients).

Most IMPCs found in various organs have a conventional carcinoma component, and pure IMPC is rare. Even in the breast, where IMPC most frequently develops, pure IMPC accounts for approximately 0.9–2% of carcinomas (1,17). The amount of micropapillary carcinoma component required for diagnosis of IMPC has not been established yet, although the clinical significance of the extent of IMPC component has been discussed (1). Chen et al analyzed the association of the amount of IMPC component and the clinicopathological parameters, and they concluded that the presence of IMPC component suggested higher incidence of lymph node metastasis. However, there was no significant association between the amount of IMPC component and lymph node and distant metastases, although a trend was noted (18). Khayyata et al reported that the prognosis of pancreatic IMPC (all patients had >20% IMPC component) was poor, similar to that of poorly differentiated pancreatic adenocarcinoma (8). In the present series, there was no significant difference in overall survival between the IMPC and non-IMPC groups (P=0.38). The results of the previous report (8) and the present study do not correspond to the above-mentioned clinical significance of the presence of IMPC component in the breast (18). In this study, the limitation is that the number of target patients is small. Therefore, additional large studies are needed to clarify the clinical significance of the amount of IMPC component in pancreatic ductal adenocarcinoma.

The PKC family is composed of 12 members, and is grouped into three subclasses: Classical, novel, and atypical (13). PKCζ is an atypical PKC. Atypical PKCs play an important role in maintaining cell polarity because they are present in the tight junctions, which physically separate the apical and basolateral portions of the cell (13). PKCζ has also been reported to play a role in the reorientation of polarity and lumen formation (inside-out pattern) in Madin-Darby canine kidney cells (18). Accordingly, we speculated that PKCζ could be used as a marker of reverse polarity in IMPC. We demonstrated that PKCζ was expressed in the apical pole of the neoplastic cells in conventional adenocarcinoma; in contrast, its expression was noted in the stroma-facing surface of the neoplastic cell clusters in the IMPC component, clearly showing reverse polarity in IMPC. Several markers for demonstrating reverse polarity in IMPC, including EMA, MUC1, and sialyl-Lewis X, have been reported (10,11). However, immunostaining and evaluation of these markers is not always easy due to the background cytoplasmic staining, as shown in Fig. 2E and F. Staining with PKCζ was clearer than with EMA and MUC1, and reverse polarity was easier to determine in all cases. The present study demonstrated the usefulness of PKCζ, an apical marker, to detect reverse polarity in IMPC; therefore, PKCζ must be added as an immunohistochemical marker for detecting reverse polarity, and this marker may be available for IMPC occurring in other organs.

In summary, the present study demonstrated that IMPC component (less than 20%) did not suggest a worse prognosis (P=0.38) and there were no significant differences in tumor location, T category, lymph node metastatic status, preoperative CA19-9 level, or resection status between the IMPC and non-IMPC groups. Moreover, immunostaining for PKCζ is useful for demonstrating reverse polarity of neoplastic cells of IMPC.