All-trans retinoic acid (ATRA) is currently used in clinical trials for breast cancer, in virtue of its ability to inhibit cell growth and to promote cell differentiation. Elucidation of the molecular mechanism(s) underlying the pleiotropic pharmacological activity of ATRA is of fundamental relevance for an effective use of the compound in clinics. This paper reports on the effects of ATRA treatment on the cell surface expression of a panel of adhesion molecules known to regulate the interactions between the effectors of the immune system and tumor targets. Results indicate that breast cancer (BC) cell lines exposed to ATRA selectively up-modulate the surface expression of ICAM-1/CD54, a molecule regulating cell/cell contacts. Such effect could be reproduced in all the BC cell lines analyzed, independently of their hormone receptor status, indicating that estrogens and progesterone are irrelevant in this process. The regulatory effects on ICAM-1 expression are time- and dose-dependent and reversible. Moreover, other differentiating and proliferating agents comparatively tested, e.g. dimethyl sulfoxide, estradiol or dexamethas one, are ineffective, indicating that ICAM-1 up-modulation is uniquely featured by ATRA. A second observation is that ATRA treated cells are, only apparently, less sensitive to lysis by lymphocytes activated by IL-2, as determined by means of a standard 51Cr release assay. In fact, notwithstanding this effect, a marked reduction in the ability to form colonies was highlighted in ATRA treated versus control lines after incubation with LAK. Finally, the clonogenic killing effect could be reversed using anti-CD54 mAbs as blocking tools, indicating that ICAM-1 plays a key role in the phenomena.
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