Thyroid Cancers Exhibit Oncogene-Enhanced Macropinocytosis that Is Restrained by IGF1R and Promote Albumin–Drug Conjugate Response

Abstract Purpose: Oncogene-driven macropinocytosis fuels nutrient scavenging in some cancer types, yet whether this occurs in thyroid cancers with prominent MAPK–ERK and PI3K pathway mutations remains unclear. We hypothesized that understanding links between thyroid cancer signaling and macropinocytosis might uncover new therapeutic strategies. Experimental Design: Macropinocytosis was assessed across cells derived from papillary thyroid cancer (PTC), follicular thyroid cancer (FTC), non-malignant follicular thyroid, and aggressive anaplastic thyroid cancer (ATC), by imaging fluorescent dextran and serum albumin. The impacts of ectopic BRAFV600E and mutant RAS, genetic PTEN silencing, and inhibitors targeting RET, BRAF, and MEK kinases were quantified. BrafV600E p53−/− ATC tumors in immunocompetent mice were used to measure efficacy of an albumin–drug conjugate comprising microtubule-destabilizing monomethyl auristatin E (MMAE) linked to serum albumin via a cathepsin-cleavable peptide (Alb-vc-MMAE). Results: FTC and ATC cells showed greater macropinocytosis than non-malignant and PTC cells. ATC tumors accumulated albumin at 8.8% injected dose per gram tissue. Alb-vc-MMAE, but not MMAE alone, reduced tumor size by >90% (P < 0.01). ATC macropinocytosis depended on MAPK/ERK activity and nutrient signaling, and increased by up to 230% with metformin, phenformin, or inhibition of IGF1Ri in monoculture but not in vivo. Macrophages also accumulated albumin and express the cognate IGF1R ligand, IGF1, which reduced ATC responsiveness to IGF1Ri. Conclusions: These findings identify regulated oncogene-driven macropinocytosis in thyroid cancers and demonstrate the potential of designing albumin-bound drugs to efficiently treat them.