Lycopene inhibition of IGF-induced cancer cell growth depends on the level of cyclin D1

Insulin-like growth factors (IGFs) play an important role in normal and cancerous cell proliferation. Moreover, in recent studies IGF-I has been implicated as a major cancer risk factor. The tomato carotenoid lycopene and all-trans retinoic acid (atRA) have been shown to inhibit growth factor-induced proliferation of different types of cancer cells. This action is associated with inhibition of cell cycle progression in G₀/G₁ phase. Cyclin D1 acts as a growth factor sensor in G₁ phase and is overexpressed in many breast cancer tumors. We have previously demonstrated that slowdown of serum-stimulated cell cycle progression from G₁ to S phase by lycopene correlates with reduction in cyclin D1 levels, suggesting that the expression of this protein is a main target for lycopene’s action.

To determine whether the reported reduction in cyclin D1 level is the key mechanism for lycopene and atRA inhibitory action on IGF-I-induced cell cycle progression.

Human breast (MCF-7) and endometrial (ECC-1) cancer cells were synchronized in G₀/G₁ phase by serum deprivation followed by stimulation with IGF-I. Cell treatment with lycopene and atRA inhibited IGF-I-stimulated cell cycle progression from G₁ to S phase and decreased retinoblastoma protein (pRb) phosphorylation. These events were associated with a reduction in cyclin D1 and p21^CIP1/WAF1 level, but not that of p27^KIP1. To test the hypothesis that the decrease in cyclin D1 has a major role in the inhibitory effects of lycopene and atRA, we examined the ability of these two agents to suppress cell cycle progression in MCF-7.7D1.13 cells which are capable of expressing cyclin D1 under the control of the Zn-inducible metallothionein promoter. Our results showed that ectopic expression of cyclin D1 can overcome cell cycle inhibition caused by lycopene and atRA.

Our findings suggest that attenuation of cyclin Dl levels by lycopene and atRA is an important mechanism for the reduction of the mitogenic action of IGF-I.