Abstract:
The objective of this dissertation was to investigate the dose-dependent effects of combretastatin A-4 (CA-4) on the growth of human aortic smooth muscle cells (HASMCs). HASMCs play a critical role in the mechanism of restenosis, particularly following percutaneous transluminal angioplasty (PTA). CA-4 belongs to the combretastatin class of tubulin-binding agents. Although structurally similar to colchicine, combretastatins exhibit a higher affinity for colchicine-binding sites on tubulin. By inhibiting microtubule dynamics, they effectively prevent cell growth and proliferation.
To evaluate the anti-proliferative effects of CA-4, we employed proliferation kinetics (PK) analysis, colony formation assays (CFA, assessing clonogenic activity), and flow cytometry. For the PK analysis, HASMCs were divided into two treatment groups: a single-dose group (transient exposure) and a continuous-treatment group (sustained exposure). Continuous treatment involved regular media changes (MCs) every 4 days, followed by the re-addition of CA-4.
Compared to untreated controls, the lowest dose (0.5 × 10⁻⁹ M; see Fig. 3, Tables 5 and 6) exerted virtually no inhibitory effects on HASMC proliferation in either the single or continuous treatment groups. However, a distinct inhibitory effect emerged at a concentration of 2.5 × 10⁻⁹ M (Fig. 3, Tables 5 and 6). At this dose, cell proliferation decreased markedly, resulting in a flatter growth slope and significantly lower cell counts compared to the control group under both treatment regimens. Both groups reached a plateau, representing a steady-state where cell proliferation balanced cell death, at a reduced cell count level (see Tables 5 and 6).
The highest dose of 5 × 10⁻⁹ M (see Fig. 3, Tables 5 and 6) severely suppressed proliferation. Following the scheduled media changes, the single-exposure cells demonstrated a slow recovery trend, though cell counts remained substantially reduced. In contrast, continuous exposure induced a complete growth arrest (see Table 6), maintaining a flat curve at the baseline value of day 0. The growth curve for the single-treatment group followed a significantly flatter, exponential trajectory (see Fig. 3, 5 × 10⁻⁹ M) before transitioning into a plateau phase. Based on these results and the proliferation model (see formula, Sec. 2.5.2), we derived a dose-dependent decrease in cell count per 1 M of CA-4. For both treatment modalities, every 1 M of the CA-4 agent reduced the population by 319,000 cells.
The CFA, which evaluates reproductive capacity, revealed that cells treated with 5 × 10⁻¹⁰ M CA-4 (see Figs. 4 and 5, green curve) exhibited a growth trajectory nearly identical to the control group. This was further confirmed by the mean clonogenic scores (see Tables 7 and 8) of both treatment cohorts. The 2.5 × 10⁻⁹ M single-treatment group initially showed a slightly lower curve baseline (see Fig. 4, blue curve 2.5), indicating reduced clonogenic activity. With regular MCs, this curve gradually converged with the control group, reaching baseline clonogenic capacity toward the end of the monitoring period. This trend was also reflected in the absolute colony counts (see Table 7). This recovery in the 2.5 × 10⁻⁹ M single-treatment group is driven by the washout of the substance during routine MCs, allowing clonogenic activity to slowly recover. Conversely, the suppressive effect persisted throughout continuous exposure to 2.5 × 10⁻⁹ M CA-4. At the 5 × 10⁻⁹ M dose, clonogenic activity was completely abolished in both treatment groups after just the first day of exposure (day 4; see Tables 4 and 5). While a single MC was sufficient to restore clonogenic activity in the single-treatment group, complete suppression was maintained under continuous exposure.
To elucidate the specific mechanism of action of the cytostatic agent CA-4 during the proliferation phase, we analyzed cell cycle distribution using flow cytometry (see 2.7 Cell Cycle Analysis). At baseline, the majority of cells in both the control and lower-dose groups (0.5 × 10⁻⁹ M and 2.5 × 10⁻⁹ M CA-4) resided in the G1 phase. Over time, however, these cells exhibited a progressive phase shift toward the G2/M phase (see Table 10). In contrast, cells treated with the maximum dose of 5 × 10⁻⁹ M (see Table 10) were predominantly trapped in the G2/M phase from the beginning of the observation window. Both treatment groups eventually showed a delayed shift back toward the G1 phase. The majority of single-treatment cells returned to the G1 phase by day 6 (see Table 10), whereas this phase shift was delayed until day 14 in the continuous-exposure group. Furthermore, while the phase shift in the single-treatment group was accompanied by an increased transition of cells into the S phase—driven by media changes and subsequent recovery—continuously treated cells showed no such increase in S-phase entry (see Table 10).
In conclusion, these data demonstrate that CA-4 exerts a concentration-dependent effect on HASMC kinetics. An intermediate dose (2.5 × 10⁻⁹ M) significantly reduces proliferation and clonogenic activity; while transiently exposed cells slowly recover to control levels, continuously exposed cells stabilize at a lower plateau. At the highest dose (5 × 10⁻⁹ M), transient exposure causes severe initial suppression followed by highly decelerated kinetic and clonogenic recovery after regular MCs. Continuous exposure at this dose completely halts both proliferative and clonogenic capacity. This CA-4-mediated cellular inhibition is driven by cell cycle arrest in the G2/M phase.
