aPKC expression has been extensively analyzed in primary human NSCLC and ovarian tumors. PKCι mRNA and protein is over-expressed in the vast majority of primary NSCLC tumors (~70%) whereas PKCζ mRNA and protein is extremely low or undetectable in both normal and cancerous lung tissues (58
). Immunohistochemistry reveals that PKCι overexpression is predominantly confined to lung tumor cells, with little or no expression in tumor-associated stroma (58
). PKCι expression is highly prognostic of poor clinical outcome; NSCLC patients with elevated tumor PKCι are 2.6 times more likely to die from cancer than patients without elevated PKCι (58
). PKCι is as good of a prognostic indicator as tumor stage, which is currently the best prognostic indicator in NSCLC (61
). Importantly however, PKCι expression does not correlate with tumor stage in NSCLC, since PKCι levels are comparable in tumors from NSCLC patients with early and late stage disease (58
). Strikingly, patients with early stage lung cancer and high PKCι are almost 11 times more likely to die from their disease than those with low PKCι (58
). Therefore, PKCι expression profiling can identify patients with early stage lung cancer at elevated risk of relapse (58
). Since ~40-50% of patients diagnosed with early stage lung cancer will eventually relapse, PKCι expression profiling can be used to identify high risk patients who would be candidates for more aggressive clinical management, perhaps with PKCι-targeted therapy.
Elevated PKCι is also frequently observed in ovarian cancer patients (59
). Just as in NSCLC, PKCι expression correlates with poor clinical outcome of ovarian cancer patients, however in ovarian cancer, PKCι expression correlates with tumor stage, suggesting that PKCι may contribute to ovarian tumor progression and aggressiveness (59
). High PKCι correlates with increased cyclin E expression and together these two markers predict poor clinical outcome, increased proliferation and defects in polarity in nonserous epithelial ovarian cancers (62
). It is unclear at present whether there is a direct functional link between PKC and cyclin E expression in these tumors. It is interesting that PKCι correlates with stage in ovarian cancer but not lung cancer. This observation suggests that PKCι over-expression is a later event in ovarian carcinogenesis than in lung carcinogenesis. Thus, PKCι may play a distinct role in the development and progression of these two tumor types. Alternatively, this apparent discrepancy may relate to differences in the staging and/or natural progression of these diseases. The relatively high relapse and metastasis rates observed in lung cancer patients diagnosed with early stage disease suggest that a significant number of these patients may be inaccurately staged using current methods. In this regard, PKCι expression profiling may be useful not only as a prognostic tool in NSCLC, but also may assist in the development of more accurate staging procedures for this disease.
The frequent over-expression of PKCι in lung and ovarian tumors prompted an investigation into potential molecular mechanisms that could account for elevated PKCι expression in these tumors. In 36% of the NSCLC tumors examined, the PKCι gene, which resides on chromosome 3q26, is amplified in a tumor-specific fashion (58
). PKCι gene amplification drives PKCι mRNA and protein expression and correlates with poor outcome in NSCLC tumors (58
). Interestingly, PKCι gene amplification was frequently found in lung squamous cell carcinonoma (SCC) (~70%) but rarely in lung adenomcarcinoma (LAC) (58
), consistent with the distribution of chromosome 3q26 amplification in NSCLC which is also confined to SCC (64
). PKCι is required for the transformed growth of SCC cells harboring PKCι gene amplification, indicating that PKCι is an important target of the chromosome 3q26 amplicon (58
). Similar tumor-specific PKCι gene amplification has been observed in ovarian cancers, particularly those of the serous sub-type (~70%) (59
). PKCι expression and PKCι gene copy number correlates with chromosome 3q26 gains in these tumors (59
) indicating that, as in the lung, PKCι is a relevant gene target for tumor-specific chromosome 3q26 amplification. Since chromosome 3q26 amplification is one of the most common chromosomal changes in human cancers, and is frequently observed in SCC of the head and neck (66
), esophagus (67
) and cervix (69
), it is likely that PKCι expression and gene copy number is of prognostic significance in these tumors as well. It remains to be determined whether PKCι also plays a critical promotive role in these human tumor types, as it does in lung and ovarian cancers.
PKCι gene amplification is not the sole mechanism by which PKCι expression is elevated in human tumors. PKCι expression is elevated to the same degree, and just as frequently, in lung SCC and LAC despite the fact that PKCι gene amplification is largely confined to SCC tumors (58
). Furthermore, PKCι is frequently over-expressed in other tumor types, including colon cancers (42
), pancreatic cancers (70
), and CML (71
) that do not harbor frequent chromosome 3q26 amplification. Though little is known about the transcriptional regulation of PKCι in either normal or transformed cells, we recently demonstrated that Bcr-Abl transcriptionally activates PKCι through a specific Elk1 element within the proximal PKCι promoter in CML cells (71
). Whether this transcriptional mechanism plays a role in the control of PKCι expression in other tumor types remains an important area for future investigation. Other potential mechanisms for oncogenic activation of PKCι, such as post-transcriptional regulation, post-translational modifications and/or somatic mutation, have not been exhaustively analyzed and warrant further investigation. In this regard, we have conducted sequence analysis of all 18 exons of the PKCι gene in 20 LAC cases and 20 SCC cases that do not harbor PKCι gene amplification and have failed to detect any mutations, suggesting that somatic mutation of PKCι does not occur or is extremely rare in NSCLC (Regala and Fields, unpublished observation). In summary, PKCι is the first PKC isozyme shown to be a bonafide human oncogene by virtue of the fact that it is activated in human NSCLC and ovarian tumors via a tumor-specific genetic alteration (gene amplification) and it is required for the transformed phenotype of these cells.