The phosphorylation-specific prolyl isomerase Pin1 is frequently overexpressed in human breast cancers, and its expression levels correlate with tumor grade. Pin1 catalyzes the isomerization of specific pSer/Thr-Pro motifs that have been phosphorylated in response to mitogenic signaling. In pre-clinical tissue culture and mouse models, this post-phosphorylational modification of signal transduction molecules by Pin1 is of critical importance for the oncogenic pathways downstream from c-neu or activated ras. Consistently, we have found that Pin1 null mice are largely protected from breast cancers induced by c-neu. In addition, Her2-positive breast cancer cell lines exhibit high levels of Pin1, likely through a positive feedback loop in which Her2/neu activation leads to an increase in Pin1 levels via E2F activation. Based on our pre-clinical data, we examined whether Pin1 levels correlate with Her2 status in clinical breast cancer specimens. Immunohistochemistry for Her2 and Pin1 was performed in 243 human breast cancers, with 144 (59%) of the specimen from primary cancers and 99 (41%) from metastatic sites. Consistent with previous data, we found that sixty-seven samples (28%) stained positive for Her2 (IHC 3+). One hundred forty-one (58%) were positive for Pin1, based on strong nuclear and cytoplasmic staining (2+ and 3+). Sixty-one percent (41 cases) of the Her2 3+ samples were also strongly Pin1-positive. Given that a majority of Her2-positive breast cancers overexpress Pin1, and based on our previous observation that Pin1 null mice were protected from breast cancers induced by c-neu, we examined whether Pin1 inhibition attenuates the growth of Her2-overexpressing breast cancer cells. As there are currently no pharmacologic inhibitors for Pin1 available, we used Pin1 RNAi inhibition in the Her2-overexpressing cell lines T47D, SKBR3, AU565, and BT474, as well as a Her2-negative cell line, MCF7. RNAi inhibition of Pin1 was achieved over a period of 7-10 days as determined by immunoblotting with anti-Pin1 antibodies. Pin1 RNAi inhibition alone attenuated the growth of these breast cancer cells only modestly, 4-11%, as determined by MTT assays. However, Pin-inhibition enhanced the efficacy of Herceptin by 17-47%. Therefore, we are currently exploring whether Pin1 inhibition is synergistic with trastuzumab and with other inhibitors of signal transduction. We conclude that the prolyl isomerase Pin1 is overexpressed in 61% of metastatic or primary Her2-positive breast cancers. While Pin1 inhibition alone causes only modest growth inhibition in Her2-overexpressing human cell lines, it enhances the efficacy of Herceptin in these cell lines. Given its pivotal role in signaling downstream from Her2, Pin1 inhibition may have potential in combination targeted agents such as trastuzumab and mTOR, EGFR, and VEGF inhibitors.

[Proc Amer Assoc Cancer Res, Volume 47, 2006]