Targeted cancer therapies, which act on specific cancer-associated molecular targets, are predominantly inhibitors of oncogenic kinases. While these drugs have achieved some clinical success, the inactivation of kinase signaling via stimulation of endogenous phosphatases has received minimal attention as an alternative targeted approach. Here, we have demonstrated that activation of the tumor suppressor protein phosphatase 2A (PP2A), a negative regulator of multiple oncogenic signaling proteins, is a promising therapeutic approach for the treatment of cancers. Our group previously developed a series of orally bioavailable small molecule activators of PP2A, termed SMAPs. We now report that SMAP treatment inhibited the growth of KRAS-mutant lung cancers in mouse xenografts and transgenic models. Mechanistically, we found that SMAPs act by binding to the PP2A Aα scaffold subunit to drive conformational changes in PP2A. These results show that PP2A can be activated in cancer cells to inhibit proliferation. Our strategy of reactivating endogenous PP2A may be applicable to the treatment of other diseases and represents an advancement toward the development of small molecule activators of tumor suppressor proteins.

Prostate cancer (PCa) preferentially metastasizes to bone, leading to complications including severe pain, fractures, spinal cord compression, bone marrow suppression, and a mortality of ∼70%. In spite of recent advances in chemo-, hormonal, and radiation therapies, bone-metastatic, castrate-resistant PCa is incurable. PCa is somewhat unique among the solid tumors in its tendency to produce osteoblastic lesions composed of hypermineralized bone with multiple layers of poorly organized type I collagen fibrils that have reduced mechanical strength. Many of the signaling pathways that control normal bone homeostasis are at play in pathologic PCa bone metastases, including the receptor activator of nuclear factor-κB/receptor activator of nuclear factor-κB ligand/osteoprotegerin system. A number of PCa-derived soluble factors have been shown to induce the dysfunctional osteoblastic phenotype. However, therapies directed at these osteoblastic-stimulating proteins have yielded disappointing clinical results to date. One of the soluble factors expressed by PCa cells, particularly in bone metastases, is prostatic acid phosphatase (PAP). Human PAP is a prostate epithelium-specific secretory protein that was the first tumor marker ever described. Biologically, PAP exhibits both phosphatase activity and ecto-5′-nucleotidase activity, generating extracellular phosphate and adenosine as the final products. Accumulating evidence indicates that PAP plays a causal role in the osteoblastic phenotype and aberrant bone mineralization seen in bone-metastatic, castrate-resistant PCa. Targeting PAP may represent a therapeutic approach to improve morbidity and mortality from PCa osteoblastic bone metastases.

The first rate-limiting step in the conversion of arachidonic acid to PGs is catalyzed by cyclooxygenase (Cox). Two isoforms of Cox have been identified, Cox-1 (constitutively expressed) and Cox-2 (inducible form), which are the products of two different genes. In this study we describe the immunohistochemical localization of Cox-1 and -2 in the human male fetal and adult reproductive tracts. There was no Cox-1 expression in fetal samples (prostate, seminal vesicles, or ejaculatory ducts), and only minimal expression in adult tissues. There was no expression of Cox-2 in the fetal prostate. In a prepubertal prostate there was some Cox-2 expression that localized exclusively to the smooth muscle cells of the transition zone. In adult hyperplastic prostates, Cox-2 was strongly expressed in smooth muscle cells, with no expression in the luminal epithelial cells. Cox-2 was strongly expressed in epithelial cells of both fetal and adult seminal vesicles and ejaculatory ducts. The Cox-2 staining intensity in the fetal ejaculatory ducts during various times of gestation correlated with previously reported testosterone production rates by the fetal testis. These data indicate that Cox-2 is the predominant isoform expressed in the fetal male reproductive tract, and its expression may be regulated by androgens. The distinct cell type-specific expression patterns of Cox-2 in the prostate (smooth muscle) vs. the seminal vesicles and ejaculatory ducts (epithelium) may reflect the different roles of PGs in these tissues.