Abstract B57: Microtubule dynamics regulates mammary gland morphogenesis and tumorigenesis via stathmin
Authors: Segatto Ilenia , Zompit Mara De Marco , Vinciguerra Gian Luca Rampioni , Mungo Giorgia , D’Andrea Sara , Perin Tiziana , Baldassarre Gustavo , Belletti Barbara 
Published: August 2018

Abstract
Microtubule (MT) dynamics is essential to provide a correct apico-basal polarization of epithelial cells, by regulating the movement of vesicles and proteins within the cell and by ensuring a correct orientation of the mitotic spindle during cell division. The dynamics of MT is governed by the activity of MT-stabilizing and destabilizing proteins, generally known as MT-acting proteins. Little is known about the role of the MT-dynamics and of MT-acting proteins during development of the mammary gland, a dynamic organ that displays a complex and finely organized apico-basal architecture.
Stathmin, a MT-destabilizing protein, is often overexpressed in breast cancer (BC). By studying knockout (STM KO) mice we observed that STM KO females were unable to nurse their offspring. This phenotype was accompanied by defective development of the mammary gland, particularly evident during pregnancy and postpartum phases. Our results showed that stathmin absence mainly impinged on two key features of the mammary epithelial organization, polarity and proliferation. In vitro, normal mammary epithelial cells silenced for stathmin were not able to form organized acini in 3D-matrices. In vivo, when stathmin was KO, many molecules involved in mammary gland functions were not properly localized, indicating a disruption of the apico-basal polarity. Furthermore, loss of stathmin strongly decreased proliferation and induced disoriented positioning of the mitotic spindle in dividing epithelial cells of the developing alveolus, preventing the correct orientation of the daughter cells.
Loss of polarity and deregulated proliferation are tightly linked to tumorigenesis. We therefore decided to examine the role of stathmin also in the context of HER2-driven tumorigenesis. To this end, we used a transgenic mouse model expressing a constitutively active form of HER2 (Δ16HER2), which is an alternatively spliced, very aggressive form of HER2, and intercrossed them with STM WT and KO mice. We analyzed the role of stathmin in both early and late stages of tumorigenesis. STM KO mice displayed a significantly reduced number of preneoplastic foci and this phenotype was maintained also in late stages of tumorigenesis. In WT mice, preneoplastic lesions expressed high level of stathmin compared with normal tissue, supporting the idea that stathmin played an active role in tumor initiation. Analysis of proliferation in tumors revealed that loss of stathmin strongly decreased the mitotic rate of Δ16HER2-transformed cells. Furthermore, by syngeneic injection of tumor cells in recipient mice of the two different genotypes, we could assess that stathmin plays a cell-autonomous function in tumor initiation.
Our study has shed new light on the field of mammary gland development and of breast cancer biology. Collectively, our data indicate that loss of stathmin by altering the dynamics of microtubules causes a profound disorganization of the normal mammary gland architecture and a delay in initiation of the tumorigenic process. We have uncovered new functions of stathmin that could have important implications in BC, also considering that several microtubule-acting drugs are currently employed in the therapy of BC patients.
Citation Format: Ilenia Segatto, Mara De Marco Zompit, Gian Luca Rampioni Vinciguerra, Giorgia Mungo, Sara D’Andrea, Tiziana Perin, Gustavo Baldassarre, Barbara Belletti. Microtubule dynamics regulates mammary gland morphogenesis and tumorigenesis via stathmin [abstract]. In: Proceedings of the AACR Special Conference: Advances in Breast Cancer Research; 2017 Oct 7-10; Hollywood, CA. Philadelphia (PA): AACR; Mol Cancer Res 2018;16(8_Suppl):Abstract nr B57.