Role of HIF as a Growth and Survival Factor in Multiple Myeloma.

Given that hypoxia and insulin-like growth factor-1 (IGF-1) induce hypoxia inducible factor (HIF-1)-dependent vascular endothelial growth factor (VEGF) expression in different cancer models, we began to explore the possibility that HIF-1α may act as a proliferative and survival factor in multiple myeloma cells. In this study, we demonstrate that MM cells express constitutive HIF-1α protein levels and that hypoxia and cobalt chloride, a hypoxia mimetic, increase HIF-1α and VEGF expression in several MM cell lines (ARP-1, MM1.S, RPMI8266). In the presence of IGF-1, maximal induction of HIF-1α expression in MM cells under normoxic conditions occurred after 12 hr of IGF-1 and declined thereafter. In contrast to hypoxia which inhibits the proteasomal degradation of HIF-1α, IGF-1 requires de novo protein synthesis of HIF-1α. Pharmacologic inhibition of PI3K/Akt or MAPK signaling pathways prevented IGF-1 induced HIF-1α and VEGF expression in ARP-1 cells. Since 2-methoxyestradiol (2-ME) has been shown to inhibit MM growth and associated angiogenesis, we examined whether 2-ME would inhibit the accumulation of IGF-1 induced HIF-1α expression. 2-ME markedly reduced constitutive and IGF-1-induced HIF-1α protein levels in ARP-1 cells. To further demonstrate that HIF-1α plays a role in the growth and survival of MM cells, RNA interference was used to inhibit HIF-1α expression. in ARP-1 cells. Nucleofection of ARP-1 cells with small interfering RNA (siRNA) targeting HIF-1α inhibited constitutive HIF-1α as well as IGF-1-induced HIF-1α protein levels. HIF-1β protein levels were unaffected. Consequently, the suppression of HIF-1α inhibited the proliferation of untreated and IGF-1 treated ARP-1 cells. More interestingly, IGF-1 did not fully overcome the growth inhibitory effect of dexamethasone in ARP-1 cells that were nucleofected with HIF-1 siRNA.

Taken together, our results suggest that HIF-1 plays a role in the pathophysiology of MM.