Aberrant activation of Rho/ROCK signaling in impaired polarity switching of colorectal micropapillary carcinoma.

Micropapillary carcinoma (MPC) is a morphologically distinctive form of carcinoma, composed of small nests of cancer cells surrounded by lacunar spaces. Invasive MPC is associated with poor prognosis. The nests of tumor cells in MPC reportedly exhibit reverse polarity, although the molecular mechanisms underlying MPC patterns are poorly understood. Using the cancer tissue-originated spheroid (CTOS) method, we previously reported polarity switching in colorectal cancer (CRC). When cultured in suspension, the apical membrane promptly switches from the outside surface of the CTOSs to the surface of the lumen inside the CTOSs under extracellular matrix (ECM)-embedded conditions, and vice versa. Here, we investigated two CTOS lines from CRC patient tumors with MPC lesions. Xenograft tumors from the CTOSs exhibited the MPC phenotype. The MPC-CTOSs did not switch polarity in vitro. Time-course analysis of polarity switching using real-time imaging of the apical membrane revealed that local switching was continually propagated in non-MPC-CTOSs, while MPC-CTOSs were unable to complete the process. Integrin β4 translocated to the outer membrane when embedded in ECM in both MPC and non-MPC-CTOSs. Protein levels, as well as the active form of RhoA, were higher in MPC-CTOSs. The suppression of RhoA activity by GAP overexpression enabled MPC-CTOSs to complete polarity switching both in vitro and in vivo, while overexpression of active RhoA did not affect polarity switching in non-MPC-CTOSs. Pretreatment with a ROCK inhibitor enabled MPC-CTOSs to complete polarity switching both in vitro and in vivo, although delayed treatment after becoming embedded in ECM failed to do so. Thus, the inability to switch polarity might be a cause of MPC, in which the aberrant activation of RhoA plays a critical role.