AMPK signaling: a targetable tumor suppressor pathway?

Functional organic cation transporters mediate osteogenic response to metformin in human umbilical cord mesenchymal stromal cells

BACKGROUND

Compelling evidence indicates that metformin, a low-cost and safe orally administered biguanide prescribed to millions of type 2 diabetics worldwide, induces the osteoblastic differentiation of mesenchymal stromal cells (MSCs) through the 5' adenosine monophosphate (AMP)-activated protein kinase (AMPK) pathway. As a highly hydrophilic cationic compound, metformin uptake is facilitated by cell membrane organic cation transporters (OCTs) of the solute carrier 22A gene family. We hypothesized that to effectively enhance osteogenic differentiation, and ultimately bone regeneration, metformin must gain access into functional OCT-expressing MSCs.

METHODS

Data was obtained through immunoblotting, cellular uptake, mineralization and gene expression assays.

RESULTS

We demonstrate for the first time that functional OCTs are expressed in human-derived MSCs from umbilical cord Wharton's jelly, an inexhaustible source of nonembryonic MSCs with proven osteogenic potential. A clinically relevant concentration of metformin led to AMPK activation, enhanced mineralized nodule formation and increased expression of the osteogenic transcription factor Runt-related transcription factor 2 (RUNX2). Indeed, targeting OCT function through pharmacological and genetic approaches markedly blunted these responses.

CONCLUSIONS

Our findings indicate that functional OCT expression in UC-MSCs is a biological prerequisite that facilitates the intracellular uptake of metformin to induce an osteogenic effect. Future pre-clinical studies are warranted to investigate whether the expression of functional OCTs may serve as a potential biomarker to predict osteogenic responses to metformin.

8-Chloroadenosine Sensitivity in Renal Cell Carcinoma Is Associated with AMPK Activation and mTOR Pathway Inhibition

The adenosine analog 8-chloroadenosine has been shown to deplete ATP and inhibit tumor growth in hematological malignancies as well as in lung and breast cancer cell lines. We investigated effects of 8-chloroadenosine on clear cell (cc) renal cell carcinoma (RCC) cell lines. 8-chloroadenosine was effective against ccRCC cell viability in vitro, with IC₅₀ ranging from 2 μM in the most sensitive CAKI-1 to 36 μM in the most resistant RXF-393. Proteomic analysis by reverse-phase protein array revealed that 8-chloroadenosine treatment leads to inhibition of the mTOR pathway. In time-course experiments, 8-chloroadenosine treatment rapidly activated AMPK, measured by AMPK and ACC phosphorylation, and subsequently caused dephosphorylation of p70S6K and ribosomal protein RPS6 in the sensitive cell lines. However, in the resistant cell lines, AMPK activity and the mTOR pathway were unaffected by the treatment. We also noted that the resistant cell lines had elevated basal levels of phospho RPS6 and AKT. Inhibition of PI3K pathway enhanced the efficacy of 8-chloroadenosine across all cell lines. Our observations indicate that 8-chloroadenosine activity is associated with inhibition of the mTOR pathway, and that phospho RPS6 and PI3K pathway activation status may determine resistance. Among solid tumors, RCC is one of the few susceptible to mTOR inhibition. We thus infer that 8-chloroadenosine may be effective in RCC by activating AMPK and inhibiting the mTOR pathway.

Differential expression of organic cation transporter OCT-3 in oral premalignant and malignant lesions: potential implications in the antineoplastic effects of metformin

BACKGROUND

Recent evidence indicates that metformin, a biguanide used as first-line treatment for type 2 diabetes, prevents the conversion of carcinogen-induced oral dysplasias into head and neck squamous cell carcinomas (HNSCC), most likely by inhibiting mammalian target of rapamycin complex 1 (mTORC1) oncogenic signaling. Whether metformin acts directly at the primary tumor site or indirectly by modulating hormonal secretion from extratumoral organs remains unknown. As organic cation transporters (OCT) belonging to the solute carrier 22A gene family, including OCT-1, OCT-2, and OCT-3, mediate metformin uptake and activity, it is critical to define what role they play in the antineoplastic activity of metformin.

METHODS

Immunohistochemical and immunoblotting techniques were used in normal, dysplastic and HNSCC tissues, and HNSCC cell lines, respectively, to determine OCTs expression levels.

RESULTS

We report that only OCT-3 was highly expressed in a number of HNSCC cell lines, oral epithelial dysplasias, and well to moderately differentiated HNSCC. Indeed, inhibition of OCT-3 expression and activity in HNSCC cells prevented metformin-induced AMP-activated protein kinase activation and mTORC1 pathway inhibition. Moreover, in oral dysplasias, high OCT-3 expression localized to epithelial compartments where mTORC1 signaling was also upregulated suggestive of a potential local effect of metformin.

CONCLUSIONS

The concept of using metformin as a chemopreventive agent to control head and neck carcinogenesis is promising. Further work is warranted to elucidate largely unexplored mechanisms of metformin uptake and pharmacologic action that may ultimately influence the selection of the most suitable patients who can benefit from metformin in head and neck cancer chemoprevention.