Flow cytometry analysis of pituitary adenomas

Cell proliferation parameters and apoptosis indices in pituitary macroadenomas

BACKGROUND

Pituitary adenomas are usually well-differentiated tumors but may show locally aggressive behavior.

AIM

To investigate the relationship between proliferation and apoptosis parameters and tumor recurrence in a series of 20 radically resected pituitary macroadenomas (11 functioning, 9 non-functioning).

MATERIALS AND METHODS

Proliferative activity and DNA ploidy were analyzed by flow cytometry (FCM) on fresh surgical specimens. Immunohistochemistry for Ki-67/MIB-1 and for the anti-apoptotic protein Bcl-2 was performed on paraffin-embedded specimens from the same tumors. Tumor regrowth was evaluated by magnetic resonance imaging (MRI).

RESULTS

Six adenomas recurred after surgery, regardless of hormonal hypersecretion. Pre-surgical tumor size was significantly higher in recurrent than in non-recurrent adenomas (p=0.003). Pre-surgical MRI demonstrated cavernous sinus (CS) invasiveness in all recurrent tumors, while none of the non-invasive adenomas recurred (p=0.042, by Fisher's exact test). The DNA content was aneuploid in 5/20 adenomas, one of which recurred. Cell percentages in the S (%SPF) and G2+M (%G2-M) phases and proliferative index (PI) (PI=%SPF+%G2-M) were significantly higher in aneuploid than in diploid adenomas (p<0.05), but no significant differences concerning all FCM parameters were observed between recurrent and non-recurrent adenomas. Similarly, MIB-1 did not show a significant difference of expression between recurrent and non-recurrent adenomas (p=0.33). Bcl-2 immunoreactivity was detected in 12/15 pituitary adenomas, involving 63±35% of tumor cells, regardless of tumor recurrence.

CONCLUSIONS

In this group of radically resected pituitary macroadenomas, neuroradiological finding of CS invasiveness--but not FCM parameters nor MIB-1 and Bcl-2 expression--is useful for predicting tumor recurrence.

Genesis of pituitary adenomas: state of the art

In recent years, remarkable progress has been made in the understanding of the pathogenesis of pituitary tumors. Pituitary tumors originate from the uncontrolled proliferation of a single transformed cell in which an initiating event has caused a gain of proliferative function. After the initiation, promoting factors cooperate in the clonal expansion. Common oncogenes, such as ras, are only exceptionally involved. The only activating mutations identified so far are gsp mutations causing the constitutive activation of cAMP pathway. However, gsp-positive adenomas are not associated to a more aggressive tumoral phenotype. The oncogenic potential of gsp mutations is limited by a more rapid degradation of the mutant Gs(alpha) with respect to the wild-type protein, and by a faster removal of cAMP due to increased phosphodiesterase activity. Estrogen-inducible gene sequences with transforming properties (pituitary tumor-transforming gene (PTTG)) have been identified in human pituitary tumors. Human pituitary tumor-transforming gene (hPTTG) is involved both in early pituitary tumorigenesis, as it causes in vitro and in vivo transformation acting as a transcription activator, and in tumor progression, as it regulates the production of basic fibroblast growth factor (bFGF), a potent activator of angiogenesis and mitogenesis. Moreover, a role of cyclin D1 in pituitary tumorigenesis is emerging. The allelic loss of loci for unknown oncosuppressor genes are currently under investigation, while an exceedingly limited role for menin gene and RB1 has been demonstrated for sporadic pituitary tumors. Abnormal methylation that predisposing toward genetic instability may favor the allelic loss or the reduced expression of oncosuppressor genes, is also an emerging field of investigation. Several promoting factors, including the excessive action of physiological stimulators, the defective action of inhibitors, the susceptibility to respond to inappropriate stimuli and the locally produced growth factors, help in tumor progression. The study of homeobox genes that intervene in pituitary cell differentiation may help in expanding our knowledge in pituitary tumor cell genealogy.

Are markers of proliferation valuable in the histological assessment of pituitary tumours?

The prediction of tumour behaviour and response to treatment has led to interest in the assessment of the proliferative potential of tumours. Pituitary tumours are usually histologically benign but are capable of aggressive growth and local invasion, although distant metastasis is limited to the very rare pituitary carcinoma. These differences in tumour behaviour may determine both the prognosis and also the effectiveness of treatment whether it be surgery, drugs or radiotherapy. Immunohistochemistry using antibodies to Ki-67 and proliferating cell nuclear antigen (PCNA) which are expressed in cells that have entered the cell cycle, can be used to assess the proportion of the cells from a tumour that are proliferating. The percentage of positively stained nuclei (labelling index (LI)) may be helpful in predicting appropriate management, as there is a relationship in many tumours between labelling index, invasiveness and tumour recurrence. This has been shown to be true for pituitary tumours, although there may be significant overlap such that low LI may be seen in the rare, aggressive, metastatic pituitary carcinomas, and high LI in indolent tumours. Thus although assessment of proliferation may be helpful in arousing suspicion as to subsequent tumour recurrence or invasiveness, this technique also demonstrates that there are other important and as yet unidentified processes that determine pituitary tumour behaviour.

Progression of a Nelson's adenoma to pituitary carcinoma; a case report and review of the literature

A 25-year-old woman developed Nelson's syndrome, 3 years after successful bilateral adrenalectomy for Cushing's disease. Despite pituitary surgery and radiotherapy the tumour showed invasive growth, leading to visual disturbance, paresis of the oculomotor nerve and, 34 years after adrenalectomy, to death by widespread purulent leptomeningitis. Autopsy revealed a large adenohypophyseal carcinoma with a metastasis attached to the dura, both tumours showing immunocytochemical staining for ACTH and TSH. We review the literature on metastatic adenohypophyseal carcinoma in Cushing's disease and Nelson's syndrome and discuss the role of proliferation markers as indicators of malignant progression.