Many BH3 domain inhibitor drugs are currently being explored within the clinic which include the drug obatoclax ) that inhibits the protective function of BCL-2, BCL-XL and MCL-1 in terms of the abilities of those proteins to sequester toxic BH3 domain proteins just like BAX and BAK. Obatoclax enhanced lapatinib toxicity in a greater than additive trend in short term and long lasting viability assays . In BT474 breast cancer cells the lethal results of obatoclax + lapatinib publicity correlated with loss of mTOR and AKT phosphorylation and increased expression of LC3, PUMA and NOXA. In transformed fibroblasts deletion of BAX+BAK or of ERBB1 suppressed the toxic interaction among lapatinib and obatoclax . Knock down of MCL-1 and BCL-XL expression enhanced lapatinib lethality in breast cancer cells and impact that was suppressed by concomitant knock down of BAK .
This correlated with lapatinib + knock down marketing BAK activation more hints . As lapatinib + obatoclax exposure was expanding the ranges with the autophagy regulator LC3 in breast cancer cells and since we had previously noted a related result in colon cancer cells, we investigated in breast cancer cells the position of autophagy inside the lethality of this drug combination. Lapatinib + obatoclax exposure of BT474 cells elevated the numbers of autophagic vesicles per cell . Elevated autophagy was dependent on expression of Beclin1, ATG5 or of BAK. Lapatinib + obatoclax exposure promoted greater association of Beclin1 with Vps34 and decreased association of the protein with BCL-XL and MCL-1 . Knock down of both ATG5 or Beclin1 protected BT474 cells from the lethal results from the drug combination .
In agreement with lapatinib acting in an ontarget style to inhibit ERBB receptor signaling, knock down of ERBB1 and ERBB2 enhanced obatoclax toxicity in MCF7 cells; toxicity in the absence BMS-354825 of ERBB1 + ERBB2 was not even more enhanced by lapatinib exposure . Pre-treatment of MCF7 cells with lapatinib or with obatoclax enhanced basal amounts of BAX and BAK action and pre-treatment decreased expression of protective BCL-2 family proteins . Mixed exposure to both medication promoted PKR-like endoplasmic reticulum kinase activation, indicative of an elevated ER pressure response with concomitant suppression of translation. Pre-treatment of MCF7 cells with lapatinib or with obatoclax considerably enhanced the toxicity in the drug combination in contrast to a simple constant exposure to both medicines without having any drug pre-treatment .
Fulvestrant resistant MCF7 cells have been more delicate to lapatinib and obatoclax toxicity than parental estrogen sensitive MCF7 cells .