TGFβ1 induces a cell-cycle-dependent increase in motility of epithelial cells

We have previously shown that addition of type 1 transforming growth factor-β (TGFβ1) to an exponentially growing population of mink lung CCl64 cells increases their average intermitotic time from 14.4 to 20.3 hours, predominantly by extending G1 from 7.5 to 13.5 hours. Here we have used the DRIMAPS system (digitally recorded interference microscopy with automatic phase-shifting) for obtaining data on cellular mass distribution, cell motility and morphology. We found no significant change in the cells’ rate of mass increase following TGFβ1 treatment, which implies that the treated cells attained a higher mass during their extended cell cycle and this was confirmed by direct measurement of cell size. However, the cells showed a dramatic motile response to treatment: TGFβ1-treated cells had a significantly higher time-averaged speed of 36.2 µm hour−1 compared to 14.5 µm hour−1 for the control cells. The time course of the response was gradual, reaching a maximum mean speed of 52.6 µm hour−1 after 15 hours exposure. We found that the gradual onset of the response was probably not due to a slow accumulation of a secondary factor but because cells were dividing throughout the experiment and most of the response to TGFβ1 occurred only after the first cell division in its presence. Thus, taking only those cells that had not yet divided, the time-averaged speed of treated cells (26.1 µm hour−1) was only moderately higher than that of untreated cells (14.9 µm hour−1) whereas, for those cells that had divided, the difference in speed between treated cells (45.1 µm hour−1) and untreated cells (14.1 µm hour−1) was much greater. Increased speed was a consequence of enhanced protrusion and retraction of the cell margin coupled with an increase in cell polarity. TGFβ1 also increased the mean spreading of the cells, measured as area-to-mass ratio, from 3.2 to 4.4 µm2 pg−1, and the intracellular mass distribution became more asymmetric. The observations indicate that a G2 signal may be necessary to reach maximal motility in the presence of TGFβ1.