Endometrial cancer is the most common invasive gynecologic malignancy, yet molecular mechanisms and signaling pathways underlying its etiology and pathophysiology remain poorly characterized. We sought to define a functional role for the protein kinase C (PKC) isoform, PKCa, in an established cell model of endometrial adenocarcinoma. Ishikawa cells depleted of PKCa protein grew slower, formed fewer colonies in anchorage-independent growth assays and exhibited impaired xenograft tumor formation in nude mice. Consistent with impaired growth, PKCa knockdown increased levels of the cyclin-dependent kinase (CDK) inhibitors p21 Cip1/WAF1 (p21) and p27 Kip1 (p27). Despite the absence of functional phosphatase and tensin homolog (PTEN) protein in Ishikawa cells, PKCa knockdown reduced Akt phosphorylation at serine 473 and concomitantly inhibited phosphorylation of the Akt target, glycogen synthase kinase-3b (GSK-3b). PKCa knockdown also resulted in decreased basal ERK phosphorylation and attenuated ERK activation following EGF stimulation. p21 and p27 expression was not increased by treatment of Ishikawa cells with ERK and Akt inhibitors, suggesting that PKCa regulates CDK expression independently of Akt and ERK. Immunohistochemical analysis of Grade 1 endometrioid adenocarcinoma revealed aberrant PKCa expression, with foci of elevated PKCa staining, not observed in normal endometrium. These studies demonstrate a critical role for PKCa signaling in endometrial tumorigenesis by regulating expression of CDK inhibitors p21 and p27 and activation of Akt and ERK-dependent proliferative pathways. Thus, targeting PKCa may provide novel therapeutic options in endometrial tumors. ' 2009 UICC

Endometrial cancer is the most common gynecologic malignancy in the United States. However, its underlying molecular mechanisms are poorly understood; and few prognostic indicators have been identified. The protein kinase C (PKC) family has been shown to regulate pathways critical to malignant transformation; and in endometrial tumors, changes in PKC expression and activity have been linked to a more aggressive phenotype and poor prognosis. We have recently shown that PKC delta is a critical regulator of apoptosis and cell survival in endometrial cancer cells; however, PKC delta levels in endometrial tumors had not been determined. We used immunohistochemistry to examine PKC delta protein levels in normal endometrium and endometrioid carcinomas of increasing grade. Normal endometrium exhibited abundant nuclear and cytoplasmic staining of PKC delta confined to glandular epithelium. In endometrial tumors, decreased PKC delta expression, both in intensity and fraction of epithelial cells stained, was observed with increasing tumor grade, with PKC delta being preferentially lost from the nucleus. Consistent with these observations, endometrial cancer cell lines derived from poorly differentiated tumors exhibited reduced PKC delta levels relative to well-differentiated lines. Treatment of endometrial cancer cells with etoposide resulted in a translocation of PKC delta from cytoplasm to nucleus concomitant with induction of apoptosis. Decreased PKC delta expression, particularly in the nucleus, may compromise the ability of cells to undergo apoptosis, perhaps conferring resistance to chemotherapy. Our results indicate that loss of PKC delta is an indicator of endometrial malignancy and increasing grade of cancer. Thus, PKC delta may function as a tumor suppressor in endometrial cancer.

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