In this study, we examined the prognostic significance of p27 alterations in stage I-IV colon cancer, particularly in relation to patient body mass index (BMI). We have found that tumoral p27 alterations (regardless of cytoplasmic localization or loss of expression) were associated with longer survival, independent of other clinical, pathologic or molecular characteristics. We have also found that the effect of p27 alteration was most evident among obese patients (BMI ≥30 kg/m2) whereas the effect of p27 is largely absent among normal-weight subjects (BMI <25 kg/m2). Moreover, the adverse effect of obesity on patient survival was principally limited to tumors with nuclear p27 expression. Our results support the role of tumoral p27 status and its interaction with patient BMI (i.e., a tumor-host interaction) in determining clinical outcome in colon cancer patients.
A possible host-tumor interaction between obesity (BMI ≥30 kg/m2
) and tumoral p27 alteration is intriguing. Cellular proliferation, senescence and apoptosis have been known to be influenced by cellular energy balance status. Indeed, obesity and physical inactivity have been consistently shown to be risk factors for colon cancer development and mortality (3
). One of the possible mechanisms of energy balance-mediated carcinogenesis is through growth factor receptors and their downstream signaling pathways (6
). Indeed, obesity, insulin and insulin-like growth factor-1 (IGF1) influence cellular proliferation and growth through the phosphatidylinositol 3-kinase (PI3K)/AKT1 signaling pathway (6
). Activated AKT1 inhibits transcription of p27 and promotes its degradation (7
), and in vitro
studies have shown that insulin and IGF1 result in down-regulation of p27 (8
). In animal experiments, p27 expression increases in a dose-dependent manner in response to energy restriction and/or physical activity (9
). Our current study suggests that obesity confers an increase in patient mortality if tumor cells express intact nuclear p27. Specifically, tumors with functional p27 would be most susceptible to the tumor promoting effects of excess energy balance through phosphatidylinositol 3-kinase (PI3K)/AKT1 signaling and subsequent p27 degradation. This hypothesis and our findings on the potential host-tumor interaction need to be tested and confirmed by additional studies, since we recognize a limitation of sample size in our subanalysis of BMI strara.
Examining molecular changes or prognostic factors is important in colon cancer research (51
). Previous studies have examined the relationship between tumoral p27 loss and clinical outcome in colon cancer (10
). However, none of the previous studies (11
) have examined modifying effect of patient BMI. In addition, none of the previous studies (11
) has comprehensively examined confounding effects of other related molecular events including p53, p21, cyclin D1, MSI, CIMP, LINE-1 methylation, KRAS
. In colon cancer, loss of p27 expression and cytoplasmic localization of p27 have been reported as aberrant p27 expression patterns (15
). In addition, loss of nuclear p27 in colon cancer has been associated with microsatellite instability (MSI) and the CpG island methylator phenotype (CIMP) (32
), while cytoplasmic p27 localization has been inversely associated with MSI and CIMP (33
). However, most previous studies (11
) have simply classified tumors according to nuclear p27 status, without consideration of its cytoplasmic localization. Furthermore, with the presence of cytoplasmic p27 localization, it was often difficult to precisely measure how much fraction of tumor cells expressing nuclear p27 due to obscuring effect of cytoplasmic p27 expression. Only one study (29
) examined prognostic role of cytoplasmic p27 localization in 418 colorectal cancers, and showed that cytoplasmic p27 localization was associated with good prognosis, which is consistent with our current results. Nonetheless, that study (29
) did not examine prognostic role of cytoplasmic p27 localization, separately from p27 nuclear expression and loss of p27 expression.
Previous data on p27 status in colorectal cancer and patient outcome are somewhat conflicting. Inconsistent methods of p27 immunohistochemistry and interpretation, a variation in patient cohorts, and inconsistent statistical methods in terms of covariates in multivariate analysis models probably contributed to a large variation in patient outcome data. Another concern is publication bias where small studies with null results have higher likelihood of being unpublished than small studies with “significant results” that are consistent with preconceptional hypothesis (e.g., “loss of p27 must be bad for patients”). Loss of p27 expression was associated with poor prognosis in some studies (11
), while other studies showed no significant prognostic role in p27 expression status (20
), and others showed that p27 loss was associated with superior outcome (27
). In one study analyzing 223 MSI-high cancers (30
), and another study analyzing 1274 MSS cancers (21
), tumoral p27 status did not show significant prognostic value, while another study analyzing 587 stage II MSS cancer, p27 loss was associated with poor prognosis (31
). These data collectively support the use of standardized comprehensive assessment of p27 expression in both nuclear and cytoplasmic compartments in clinical research, and the examination of other relevant tumoral molecular events (including MSI, CIMP, p53, p21 and cyclin D1), which are potential confounders.
There are currently no standardized methods to assess p27 alterations in colon cancer. Non-uniform methods to evaluate tumoral p27 alterations likely contributed to the somewhat inconsistent results in the previous studies. Nonetheless, our method yielded highly significant associations between p27 loss and other related molecular variables (including p53, MSI, CIMP and BRAF mutation) as well as between cytoplasmic p27 localization and other related molecular variables. Moreover, any random misclassification of tumors in terms of p27 status would drive our results on patient outcome towards the null hypothesis.
There are advantages in utilizing the database of the two independent prospective cohort studies, the Nurses’ Health Study and Health Professionals Follow-up Study to examine tumor-host interactions. Anthropometric measurements and other clinical information were prospectively collected, and entered into the database blinded to patient diagnosis, tumoral molecular features and outcome. Data were updated every 2 years. Cohort participants who developed colon cancer were treated at hospitals throughout the United States. Tumor specimen procurement rate has been approximately 60%, and there were no demographic difference between cases with tumor tissue analyzed and those without tumor tissue analyzed (35
). However, a limitation of this study is that data on cancer treatment were limited. Nonetheless, it is unlikely that chemotherapy use differed according to tumoral p27 status, since such data were not available to patients or treating physicians. In addition, beyond cause of mortality, data on cancer recurrences were not available in these cohorts. Nonetheless, given the median survival for metastatic colon cancer was approximately 10 to 12 months during much of the time period of this study (46
), colon cancer-specific survival should be a reasonable surrogate for cancer-specific outcomes.
In summary, our large cohort study suggests that p27 alterations mark colon cancer with superior prognosis, independent of patient and other tumoral characteristics. The effect of p27 alterations is particularly strong among obese individuals, suggesting the presence of tumor (p27)-host (BMI) interaction in determining clinical behavior of colon cancer. Moreover, the adverse effect of obesity on patient survival appears to be limited to colon cancer with nuclear p27 expression. These findings may have considerable clinical implications. Future studies are needed to confirm these results as well as to elucidate exact mechanisms by which p27 alterations and cellular energy balance status affect tumor behavior.