Abstract 339: Importance of retinoic acid receptors alpha, beta and gamma on the in vivo tumor growth of a murine mammary bi-cellular tumor cell line: Differential response of luminal and myoepithelial components to retinoids treatment
Authors: Campodónico Paola B. , Motter Andrea , Farias Eduardo F. , de Kier Joffé Elisa D. Bal , Todaro Laura B. 
Published: February 2011

The incidence of breast cancer has apparently increased throughout the world during the last century. Caucasian women in the western world have a considerably higher breast cancer risk. Due to their role in the regulation of cell growth and differentiation through binding to RAR/RXR receptors, retinoids (Rds) are being evaluated in clinical trials of cancer prevention. We focus our studies in LM38-LP, a murine mammary tumor cell line composed by luminal (LEP) and myoepithelial (MEP) cells that express all functional retinoic acid receptors RAR/RXR.

We studied the mechanisms involved in Rds effects in LEP/MEP interactions. By using anti E-cadherin/immunobeads we managed to temporarily separate LEP and MEP compartments in LM38-LP cell line. We determined that ATRA (all trans retinoic acid) induced a significant increase of RARb2 and RARg2 only in LEP cells (RT-PCR). Rds treatment increased the adhesive capacity of the bicellular LM38-LP cell line to fibronectin (FN). When the two components of the cell line were analyzed separately, Rds only induced an increase in the adhesion to FN. of MEP cells

Also to understand the importance of each RAR regarding metastatic dissemination and tumor growth, we generated LM38-LP cell lines stably transfected with shRNA against RARa, RARb or RARg. These cells were inoculated into the mammary fat pad of syngeneic BALB/c mice. Ten days post-inoculation, when tumors were palpable, mice received a pellet with or without ATRA (10mg/mouse) during 3 weeks and we analyzed tumor volume and number and size of spontaneous lung metastasis. ATRA treatment markedly decreased the tumor volume and the number of spontaneous metastasis in those mice inoculated with LM38-LP control cells. Tumors and/or metastases grown from cells transfected with shRNA against RARa, b or g did not respond to ATRA. Interestingly the absence of RARb led to a total loss of the metastatic capacity, suggesting the involvement of RARb in malignant progression. On the contrary, loss of RARa and RARg decreased orthotopic growth but promoted lung metastases formation, suggesting that these RARs could participate in the prevention of metastatic dissemination.

In sum our in vitro studies allow us to conclude that the LEP compartment of LM38-LP cell line responds to ATRA by a classical pathway through RARb. On the other hand, ATRA modulation of the adhesiveness of LM38-LP cells to FN could be mediated by the MEP component. In vivo experiments demonstrated that the three retinoic receptors are implicated, but in a different manner, both in tumor growth and in the colonization of the target organ.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 339.