Summary We selected two drug resistant variants of the MCF7 human breast cancer cell line by chronic in vitro exposure to doxorubicin (MCF7/D40 cell line) and mitoxantrone (MCF7/Mitox cell line), respectively. (Dalton, 1990). In such patients, clinical drug resistance (failure to respond to drugs which were initially effective) is a common phenomenon. Clinical drug resistance is likely due to a number of factors such as tumour growth kinetics, development of pharmacologic sanctuaries due to loss of vascular supply, development of hypoxia in large tumours, and development of structural and metabolic changes in individual tumour cells. Doxorubicin (DOX) is the most active single agent currently available for the treatment of breast cancer (Tormey, 1975). For previously treated and untreated patients the response to DOX as a single agent varies between 28 and 43% respectively. In addition, DOX is an integral component of many combination chemotherapy regimens for breast cancer.

Recent evidence has demonstrated an important immunoregulatory role for pituitary PRL. Moreover, PRLs have been identified as products of transformed human lymphocyte cell lines and normal murine lymphocytes, and implicated as regulators of their proliferative responses. However, PRL synthesis by normal human lymphocytes has not yet been reported. Here we demonstrate that human thymocytes and peripheral blood lymphocytes (PBL) synthesize PRL in primary culture. The principal form produced by thymocytes is 24 kilodaltons (kDa), essentially the same size as pituitary PRL, while PBL produced a 27-kDa variant. Size heterogeneity was evident, with products detected ranging from 21-29 kDa in various tissue samples, a phenomenon also found to occur in human pituitary and decidual PRL. Thymocytes and PBLs also synthesized a low mol wt form (11 kDa) that was released into culture supernatants concurrently with the larger PRL. The 24- and 11-kDa forms expressed PRL-like bioactivity in the Nb2 node lymphoma bioassay, further supporting their PRL-like nature. Expression of these PRLs was regulated by mitogen stimulation in thymocytes, but was constitutively produced in PBL. Northern blot analysis of thymocyte RNA using a human PRL cDNA probe detected a single PRL-like mRNA, which was significantly larger than human pituitary PRL mRNA. This was constitutively present in unstimulated thymocytes. Taken together, these data demonstrate that normal human lymphocytes synthesize bioactive PRLs similar in size to those produced by the pituitary. The presence of a single PRL mRNA suggests that the size variation observed in these proteins is probably due to posttranslational modification, such as proteolysis and glycosylation.